Light-driven motion of liquids on a photoresponsive surface

Material selection, preparation, driving and applications of light-driven micro/nano motors: a review

As an external energy stimulus, light possesses the advantageous qualities of being reversible, wireless and remotely maneuverable while driving the motion of micro and nano motors. Despite the extensive publication of articles on light-driven micro- and nano-motors (LDMNMs) over the past two decades, reviews that address LDMNMs in general, from material selection, design, preparation, driving to applications, remain scarce. Therefore, it is necessary to highlight the superiority of light as a stimulating energy source for driving MNMs, as well as to promote the continuous development of LDMNMs and give newcomers a more basic and comprehensive knowledge in this field. This present review focuses on advanced preparation methods for LDNMNs, and provides a comprehensive comparison of the advantages and limitations of various techniques. In addition, general design strategies for building asymmetric fields around LDMNMs are introduced, as well as a variety of photoactive materials, including photocatalytic, photothermal, and photoinduced isomerization materials. The existing propulsive mechanisms and kinematic behaviours of LDMNMs are described in detail, including photocatalytic oxidation, photothermal effects and photoinduced isomerization. The principles of the various drive mechanisms are also analysed in detail and their merits and shortcomings summarized. Finally, following a comprehensive review of the potential applications in biomedicine, environmental remediation and other fields, further perspectives on future developments are presented with a view to overcoming key challenges.

A Review of Droplet/Bubble Transportation on Bionic Superwetting Surface

The controllable droplets/bubble transportation has a wide range of applications in the fields of biomedical, chemistry, energy, and material applications, and has aroused great attention for its significant scientific and technology importance. The main challenges derived from the liquid/solid or gas/solid contact strength and actuating energy input. Artificial superwetting surfaces inspired by nature creatures have triggered technology revolution in many fields relevant to droplet operation, and the applied actuating force improve the controllability to preferential direction. In this review, we highlights recent advancements in droplets/bubble transportation on the superwetting surfaces driven by passive or active stimulation methods inspired by bionic function interfaces. The three main superwetting surfaces including superhydrophobic surface, slippery liquid-infused porous surface, hybrid surface, various stimuli methods including gravity/buoyance, chemical/morphology gradient, heat, magnetism, electricity, light, adhesion force, and prosperous applications including micro-reaction, biochemical analysis, fog collection/antifog, energy transfer, bubble/liquid micro-robot, self-cleaning, light/circle switch have been systematically summarized. Finally, the challenges and future perspectives of research innovations and practical applications are discussed.

Substituent effects in N-acetylated phenylazopyrazole photoswitches

Phenylazopyrazole photoswitches proved to be valuable structural motifs for various applications ranging from materials science to medicine. Despite their potential, their structural diversity is still limited and a larger pool of substitution patterns remains to be systematically investigated. This is paramount as electronic effects play a crucial role in the behavior of photoswitches and a deeper understanding enables their straightforward development for specific applications. In this work, we synthesized novel N-acylpyrazole-based photoswitches and conducted a comparative study with 33 phenylazopyrazoles, comparing their photoswitching properties and the impact of electronic effects. Using UV-vis and NMR spectroscopy, we discovered that simple acylation of the pyrazole moiety leads to increased quantum yields of isomerization, long Z-isomer life-times, good spectral separation, and high photostability.

Filming evolution dynamics of Hg nanodroplets mediated at solid-gas and solid-liquid interfaces by in-situ TEM

Nanodroplets at multiphase interfaces are ubiquitous in nature with implications ranging from fundamental interfacial science to industrial applications including catalytic, environmental, biological and medical processes. Direct observation of full dynamic evolutions of liquid metal nanodroplets at nanoscale multiphase interfaces offers indispensable insights, however, remains challenging and unclear. Here, we fabricate gas and liquid cells containing HgS nanocrystals through electrospinning and achieve the statistical investigations of full picture of Hg nanodroplets evolving at solid-gas and solid-liquid interfaces by in-situ transmission electron microscopy. In the gas cells, the voids nucleate, grow and coalesce into the crack-like feature along the <001> direction, while Hg nanodroplets form, move rapidly on the ratchet surface and are evolved into bigger ones through the nanobridges. Distinctly, mediated by the solid-liquid interface, the liquid Hg with the ink-like feature jets in the liquid cells. Such ink-jetting behavior occurs multiple times with the intervals from several to several tens of seconds, which is modulated through the competition between reductive electrons and oxidative species derived from the radiolysis of liquids. In-depth understanding of distinct nanodroplets dynamics at nanoscale solid-gas and solid-liquid interfaces offers a feasible approach for designing liquid metal-based nanocomplexes with regulatory interfacial, morphological and rheological functionalities.

Droplet Fast Gyrating on the Anisotropic Surface

It is fundamental and practical to understand the basic hydrodynamic principles of bouncing droplets on superhydrophobic surfaces and to develop control strategies. Although recent research has focused on adjusting the contact time of the bouncing droplet, little attention has been paid to manipulating the rebound of the droplet from the perspective of energy optimization, which determines the development of long-term continuous dynamics. Here, we study the impact of water droplets on an anisotropic superhydrophobic surface at a low Weber number of 3.85, where the droplets bounce vertically after impacting the surface and gyrotron tangentially at a speed of 7200 rpm. The initial kinetic energy required can be reduced by more than double relative to the traditional bounce behavior at the same bounce distance in the horizontal direction. This study proposes a low-energy long-distance transport method for droplets, providing valuable insights for enabling energy-efficient applications, including self-cleaning surfaces, liquid transfer, and spray cooling in industrial settings.

Regulation of Liquid Self-Transport Through Architectural-Thermal Coupling: Transitioning From Free Surfaces to Open Channels

In this work, the regulation of liquid self-transport is achieved through architectural and thermal coupling, transitioning from free surfaces to open channels. Hierarchical structures inspired by the skin of a Texas horned lizard are designed, with the primary structure of wedged grooves and the secondary structure of capillary crura. This design enables advantages including long-distance self-transport, liquid storage and active reflux management on free surfaces, directional transportation, synthesis and detection of reagents in confined spaces, as well as controllable motion and enhanced heat dissipation in open channels. The regulation capacity can be precisely controlled by adjusting the secondary capillary crura and external thermal gradients. The regulation mechanism is further elucidated through microscopic flow observation and a deduced theoretical model. The proposed structures are expected to introduce a new concept for designing lubrication systems, microfluidic chips, methods for chemical synthesis, and heat transfer in the future.

Soft Materials and Devices Enabling Sensorimotor Functions in Soft Robots

Sensorimotor functions, the seamless integration of sensing, decision-making, and actuation, are fundamental for robots to interact with their environments. Inspired by biological systems, the incorporation of soft materials and devices into robotics holds significant promise for enhancing these functions. However, current robotics systems often lack the autonomy and intelligence observed in nature due to limited sensorimotor integration, particularly in flexible sensing and actuation. As the field progresses toward soft, flexible, and stretchable materials, developing such materials and devices becomes increasingly critical for advanced robotics. Despite rapid advancements individually in soft materials and flexible devices, their combined applications to enable sensorimotor capabilities in robots are emerging. This review addresses this emerging field by providing a comprehensive overview of soft materials and devices that enable sensorimotor functions in robots. We delve into the latest development in soft sensing technologies, actuation mechanism, structural designs, and fabrication techniques. Additionally, we explore strategies for sensorimotor control, the integration of artificial intelligence (AI), and practical application across various domains such as healthcare, augmented and virtual reality, and exploration. By drawing parallels with biological systems, this review aims to guide future research and development in soft robots, ultimately enhancing the autonomy and adaptability of robots in unstructured environments.

Pinning-Induced Microdroplet Self-Transport

Droplets are prone to adhere or "pin" on solid surfaces which contain unavoidable micro- and nanoscale surface defects formed through chemical and topographical heterogeneity. To initiate droplet motion, potential energy gradients, surface energy gradients, or external energy input are needed. Here, in contrast to established wisdom, we show that properly designed surface heterogeneity can promote microdroplet self-transport without any external force or anisotropy. In the presence of topological defects, microdroplets can take advantage of contact line pinning to generate contact line and corresponding contact angle asymmetry, leading to spontaneous motion over distances 10-20 times larger than the droplet radius. The outcomes of this work present an alternative pathway for taking advantage of intrinsic surface heterogeneity to achieve droplet mobility in a range of applications, where passive droplet motion is desired.

Directional Continuous Bouncing Behavior of Water Droplets on a Surface with a Chemical Gradient

Manipulation of directional bouncing behavior of liquid droplets after impacting solid surfaces is highly significant for biological, agricultural, engineering, and industrial applications. Here, we prepared a surface with a chemical gradient on a Ti-6Al-4V substrate, on which directional multiple bouncing of droplets and long-range movement has been achieved. The wetting gradient of the vapor-deposited surface reached 2.5° mm-1 by finely controlling the distribution of low surface energy functional groups. Droplet adhesion force analysis was carried out to visualize the variation of surface wettability. On this surface with inhomogeneous wettability, the droplet repeated the impacting and rebounding 8 times along the direction of the chemical gradient, displaying an interesting phenomenon of "droplet trampoline". The maximal rebound height and the horizontal jumping distance reached 8.36 and 10.19 mm, respectively. Additionally, the underlying mechanism behind this consecutive bouncing behavior of droplets was thoroughly elucidated from energy and force perspectives. This research is anticipated to advance the understanding of directional continuous bounce behavior of a droplet and provide a promising strategy to delicately manipulate the movement of droplets.

Directional Liquid Transport in Thin Fibrous Matrices: Enhancement of Advanced Applications

Directional liquid transport fibrous matrices (DLTFMs) have the unique ability to direct liquid movement in a single direction through their thickness. Beyond their inherent liquid transport function, DLTFMs can also enhance the effectiveness of additional functionalities. This review focuses on recent advances in DLTFMs, particularly the role of DLTs in enhancing secondary functions. We begin with a brief overview of the historical development and major achievements in DLTFM research, followed by an outline of the classification, fabrication techniques, and basic functions derived from their natural liquid transport properties. The integration of DLT to enhance secondary functionalities such as responsiveness, thermal regulation, and wearable technology for innovative applications in various sectors is then discussed. The review concludes with a discussion of key challenges and prospects in the field, including the durability and reliability of DLT performance, the precise regulation of fluid transport rates, the resilience and longevity of DLTFMs in harsh environments, and the impact of DLT variations on performance enhancement. The goal of this review is to stimulate further innovative studies on DLTFMs and to promote their practical implementation in a variety of industries.

Long-Range Vapor-Mediated Interactions between Adjacent Droplets

Droplet motion can occur due to interaction with the surrounding vapor phase. We examined experimentally the motion of two adjacent droplets, either pure liquid or a binary mixture, without direct contact. A droplet is repelled or attracted by the (pinned) adjacent droplet, which acts as a vapor source, depending on its initial concentration as well as the composition in the vapor, even for a pure liquid. The observation is explained by a theoretical model that combines evaporation and adsorption processes, which unifies the mechanism for both directions of motion (attraction and repulsion) and, more importantly, for both binary mixtures and pure liquid droplets. Good agreement is achieved between the theoretical model and the experimental observations. A critical concentration is proposed to determine the transition between attractive and repulsive motion, this being a criterion to predict droplet motion.

Interfacial fluid manipulation with bioinspired strategies: special wettability and asymmetric structures

The inspirations from nature always enlighten us to develop advanced science and technology. To survive in complicated and harsh environments, plants and animals have evolved remarkable capabilities to control fluid transfer via sophisticated designs such as wettability contrast, oriented micro-/nano-structures, and geometry gradients. Based on the bioinspired structures, the on-surface fluid manipulation exhibits spontaneous, continuous, smart, and integrated performances, which can promote the applications in the fields of heat transfer, microfluidics, heterogeneous catalysis, water harvesting, etc. Although fluid manipulating interfaces (FMIs) have provided plenty of ideas to optimize the current systems, a comprehensive review of history, classification, fabrication, and integration focusing on their interfacial chemistry and asymmetric structure is highly required. In this review, we systematically introduce development and highlight the state-of-the-art progress of bioinspired FMIs. Firstly, the biological prototype and development timeline are presented, and the underlying mechanism of on-surface fluid control on versatile structures is analyzed. Secondly, the definition and classification of FMIs as well as the strategy for controlling fluid/interface interaction are discussed. Thirdly, emergent applications of FMIs in practical scenarios including fog/vapor collection, fluid diodes, interfacial catalysis, etc. are presented. Furthermore, the challenges and prospects of interfacial liquid manipulation are concluded. We envision that this review should provide guidance for the incorporation of FMIs into suitable situations, which enlightens interdisciplinary research and practical applications in the fields of interface chemistry, materials design, bionic science, fluid dynamics, etc.

Solvatochromism and cis-trans isomerism in azobenzene-4-sulfonyl chloride

Solvatochromism exhibited by azobenzene-4-sulfonyl chloride (here abbreviated as Azo-SCl) has been investigated in a series of non-polar, polar-aprotic and polar-protic solvents. The UV-vis spectra of Azo-SCl exhibit two long-wavelength bands, observed at 321-330 nm (band-I) and 435-461 nm (band-II), which are ascribed to the π*-π (S2 ← S0) and π*-n (S1 ← S0) transitions, respectively. The shorter wavelength band indicates a reversal in solvatochromism, from negative to positive solvatochromism, for a solvent with a dielectric constant of 32.66 (which is characteristic of methanol), while the longer wavelength band signposts negative solvatochromism in all range of solvent's dielectric constant investigated, demonstrating different interactions with the solvents in the S2 and S1 excited states. Using Catalán and Kamlet-Taft solvation energy models, we found that the shift in the solvatochromic behavior of band-I (S2 ← S0) happens because solvent dipolarity/polarizability and hydrogen bonding affect the S2 state in opposite ways. Dipolarity/polarizability stabilizes the S2 state compared to the ground state, while hydrogen bonding destabilizes it. In contrast, for S1, both effects work together to destabilize the excited state. For all studied solvents, UV irradiation (λ ≥ 311 nm; room temperature) was found to lead to fast trans-cis azo photoisomerization. In the absence of light, the photogenerated cis form quickly converts back to the trans form. Interpretation of the experimental data is supported by quantum chemical calculations undertaken within the Density Functional Theory (DFT) framework, including Time Dependent DFT calculations for excited states.

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

Light-induced manipulation of ultra-low surface tension droplets on stable quasi-liquid surfaces

HYPOTHESIS

Perfluorocarbon is commonly used as a coolant, chemical reaction carrier solvent, medical anti-hypoxic agents and blood substitutes. The realization of non-contact complex manipulation of perfluorocarbon liquids is urgently needed in human life and industrial production. However, most liquid-repellent interfaces are ineffective for the transport of ultra-low surface tension perfluorocarbon liquids, and struggle to maintain good durability due to unstable air or oil cushions in the surface. Therefore, preparing surfaces for stable non-contact complex manipulation of ultra-low surface tension droplets remains a challenge.

EXPERIMENTS

In this paper, a novel solution, a photothermal responsive droplet manipulation surface based on polydimethylsiloxane brushes, has been reported. On this surface, droplets with different surface tensions (as low as 10 mN/m) can be efficiently manipulated through induced near-infrared light. Notably, this surface maintains its effectiveness after exposure to extreme anthropogenic conditions.

FINDINGS

The interface effect between perfluorocarbon droplets and polydimethylsiloxane brushes by near-infrared light-induced was investigated in detail. In addition, ultra-low surface tension droplets demonstrate the ability to transport solid particles. The conductive droplets exhibit sophisticated manipulation realizing the controlled switching of smart circuits. This research opens up new possibilities for advancing the capabilities and adaptability of ultralow surface tension droplets in a range of applications.

Photoswitchable Arylazopyrazole Surfactants at the Water-Air Interface: A Microscopic Perspective

Surfactants play an important role in modifying the properties of water-air interfaces. Here, we combine information from molecular dynamics simulations, surface tensiometry, and vibrational sum-frequency generation spectroscopy to study the interfacial behavior of photoswitchable arylazopyrazole (AAP) surfactants. This combination of the experimental techniques allows a direct relation between surface tension and surface concentration rather than just the bulk concentration. Specifically, we conducted a comparison between two derivatives, one with an octyl terminal group (O-AAP) and the other without this group (H-AAP), focusing on their respective E and Z isomers. From the simulations of these four systems, we see that those with a stronger cluster formation, likely resulting from higher intermolecular attractive interactions, display higher surface tensions for the intermediate surface excess. In some cases, even a small but noticeable maximum in the surface tension isotherm is observed for systems with strong cluster formation. Such a maximum is not observed in the experiments, although such an observation would be compatible with the general properties of the Frumkin isotherm. We exclude that the peak is due to the finite width of the simulation box. Apart from this effect, the general features of the surface tension are consistent between the experiment and simulation. Evidence is also provided that it is primarily the interaction of the aromatic rings that determines the strength of the surfactant interactions.

From Friction to Function: A High-Voltage Sliding Triboelectric Nanogenerator for Highly Efficient Energy Autonomous IoTs and Self-Powered Actuation

An advanced energy autonomous system that simultaneously harnesses and stores energy on the same platform offers exciting opportunities for the near-future self-powered miniature electronics. However, achieving optimal synchronization between the power output of an energy harvester and the storage unit or integrating it seamlessly with real-time microelectronics to build a highly efficient energy autonomous system remains challenging. Herein, a unique bimetallic layered double hydroxide (LDH) based tribo-positive layer is introduced for a high-voltage sliding triboelectric nanogenerator (S-TENG) with an output voltage of ≈1485 V and power output of 250 µW, respectively. To demonstrate the potential of a self-charging power system, S-TENG is integrated with on-chip micro-supercapacitors (MSCs) as a storage unit. The MSC array effectively self-charged up to 4.8 V (within 220s), providing ample power to support micro-sensory systems. In addition, by utilizing the high-voltage output of the S-TENG, the efficient operation of electrostatic actuators and digital microfluidic (DMF) systems driven directly by simple mechanical motion is further demonstrated. Overall, this work can provide a solid foundation for the advancement of next-generation energy-autonomous systems.

Highly-Strong and Highly-Tough Alginate Fibers with Photo-Modulating Mechanical Properties

The good combination of high strength and high toughness is a long-standing challenge in the design of robust biomaterials. Meanwhile, robust biomaterials hardly perform fast and significant mechanical property changes under the trigger of light at room temperature. These limit the application of biomaterials in some specific areas. Here, photoresponsive alginate fibers are fabricated by using the designed azobenzene-containing surfactant as flexible contact point for cross-linking polysaccharide chains of alginate, which gain high mechanics through reinforced plastic strain and photo-modulating mechanics through isomerization of azobenzene. By transferring molecular motion into macro-scale mechanical property changes, such alginate fibers achieve reversible photo-modulations on the mechanics. Their breaking strength and toughness can be photo-modulated from 732 MPa and 112 MJ m-3 to 299 MPa and 27 MJ m-3, respectively, leading to record high mechanical changes among the developed smart biomaterials. With merits of good tolerance to pH and temperature, fast response to light, and good biocompatibility, the reported fibers will be suitable for working in various application scenarios as new smart biomaterials. This study provides a new design strategy for gaining highly-strong and highly-tough photoresponsive biomaterials.

Phase Behaviors and Photoresponsive Thin Films of Syndiotactic Side-Chain Liquid Crystalline Polymers with High Densely Substituted Azobenzene Mesogens

Azobenzene-containing polymers (azopolymers) are a kind of fascinating stimuli-responsive materials with broad and versatile applications. In this work, a series of syndiotactic C1 type azopolymers of Pm-Azo-Cn with side-chain azobenzene mesogens of varied length alkoxy tails (n=1, 4, 8, 10) and different length alkyl spacers (m=6, 10) have been prepared via Rh-catalyzed carbene polymerization. The thermal properties and ordered assembly structures of thus synthesized side chain liquid crystalline polymers (SCLCPs) have been systematically investigated with differential scanning calorimetry (DSC), polarized optical microscopy (POM) and variable-temperature small/wide-angle X-ray scattering (SAXS/WAXS) analyses. P10-Azo-C1 and P10-Azo-C4 with shorter alkoxy tails exhibited hierarchical structures SmB/Colob and transformed into SmA/Colob at a higher temperature, while P10-Azo-C8 and P10-Azo-C10 with longer alkoxy tails only displayed side group dominated layered SmB phase and transformed into SmA phase at higher temperatures. For P6-Azo-C4 with a shorter spacer only showed a less ordered SmA phase owing to interference by partly coupling between the side chain azobenzene mesogens and the helical backbone. More importantly, the series high densely substituted syndiotactic C1 azopolymer thin films, exhibited evidently and smoothly reversible photoresponsive properties, which demonstrated promising photoresponsive device applications.

Enhancing Directional Droplet Transport via Surface Charge Gradient: Insights from Molecular Dynamics Simulations

The phenomenon of spontaneous droplet transport has a wide range of implications in water collection, microfluidic manipulation, oil-water separation, and various other fields. Achieving efficient and controllable spontaneous droplet transport is therefore of paramount importance. This study investigates the potential of surface charge manipulation to enhance spontaneous droplet transport through comprehensive molecular dynamics simulations. Our findings reveal that the surface charge of the substrate significantly influences its wettability, reducing the contact angle of the droplet and increasing both the contact area and interaction energy. Moreover, we introduce a novel approach to enhance droplet mobility by creating a surface charge gradient on the substrate. By introducing bands with varying charges along a specific direction of the substrate, the droplet experiences a force directed toward regions of increasing charge, thereby facilitating its movement. Importantly, the driving mechanism of droplet motion is well explained by combining classical electrowetting theory with the analysis of the droplet's advancing and receding contact angles, which demonstrates that a more pronounced surface charge gradient generates greater force and enhances droplet mobility. These findings offer valuable insights into the design of microfluidic systems and related applications based on electrowetting.

In Situ Multi-Directional Liquid Manipulation Enabled by 3D Asymmetric Fang-Structured Surface

Decorating surfaces with wetting gradients or topological structures is a prevailing strategy to control uni-directional spreading without energy input. However, current methods, limited by fixed design, cannot achieve multi-directional control of liquids, posing challenges to practical applications. Here, a structured surface composed of arrayed three-dimensional asymmetric fang-structured units is reported that enable in situ control of customized multi-directional spreading for different surface tension liquids, exhibiting five novel modes. This is attributed to bottom-up distributed multi-curvature features of surface units, which create varied Laplace pressure gradients to guide the spreading of different-wettability liquids along specific directions. The surface's capability to respond to liquid properties for multimodal control leads to innovative functions that are absent in conventional structured surfaces. Selective multi-path circuits can be constructed by taking advantage of rich liquid behaviors with the surface; surface tensions of wetting liquids can be portably indicated with a resolution scope of 0.3-3.4 mN m-1 using the surface; temperature-mediated change of liquid properties is utilized to smartly manipulate liquid behavior and achieve the spatiotemporal-controllable targeted cooling of the surface at its heated state. These novel applications open new avenues for developing advanced surfaces for liquid manipulation.

Smart materials for light control of droplets

Droplet manipulation plays a critical role in both fundamental research and practical applications, especially when combined with smart materials and external fields to achieve multifunctional droplet manipulation. Light control of droplets has emerged as a significant and widely used strategy, driven primarily by photochemistry, photomechanics, light-induced Marangoni effects, and light-induced electric effects. This approach allowing for droplet manipulation with high spatial and temporal resolution, all while maintaining a remote and non-contact mode of operation. This review aims to provide a comprehensive overview of the mechanisms underlying light control of droplets, the design of smart materials for this purpose, and the diverse range of applications enabled by this technique. These applications include merging, splitting, releasing, forwarding, backward movement, and rotation of droplets, as well as chemical reactions, droplet robots, and microfluidics. By presenting this information, we aim to establish a unified framework that guides the sustainable development of light control of droplets. Additionally, this review addresses the challenges associated with light control of droplets and suggests potential directions for future development.

Steering droplets on substrates with plane-wave wettability patterns and deformations

Motivated by strategies for targeted microfluidic transport of droplets, we investigate how sessile droplets can be steered toward a preferred direction using travelling waves in substrate wettability or deformations of the substrate. To perform our numerical study, we implement the boundary-element method to solve the governing Stokes equations for the fluid flow field inside the moving droplet. In both cases we find two distinct modes of droplet motion. For small wave speed the droplet surfs with a constant velocity on the wave, while beyond a critical wave speed a periodic wobbling motion occurs, the period of which diverges at the transition. These observation can be rationalized by the nonuniform oscillator model and the transition described by a SNIPER bifurcation. For the travelling waves in wettability the mean droplet velocity in the wobbling state decays with the inverse wave speed. In contrast, for travelling-wave deformations of the substrate it is proportional to the wave speed at large speed values since the droplet always has to move up and down. To rationalize this behavior, the nonuniform oscillator model has to be extended. Since the critical wave speed of the bifurcation depends on the droplet radius, this dependence can be used to sort droplets by size.

Photo-controllable azobenzene microdroplets on an open surface and their application as transporters

The control of droplet motion is a significant challenge, as there has been no simple method for effective manipulation. Utilizing light for the control of droplets offers a promising solution due to its non-contact nature and high degree of controllability. In this study, we present our findings on the translational motion of pre-photomelted droplets composed of azobenzene derivatives on a glass surface when exposed to UV and visible light sources from different directions. These droplets exhibited directional and continuous motion upon light irradiation and this motion was size-dependent. Only droplets with diameters less than 10 μm moved with a maximum velocity of 300 μm min-1. In addition, the direction of the movement was controllable by the direction of the light. The motion is driven by a change in contact angle, where UV or visible light switched the contact angle to approximately 50° or 35°, respectively. In addition, these droplets were also found to be capable carriers for fluorescent quantum dots. As such, droplets composed of photoresponsive molecules offer unique opportunities for designing novel light-driven open-surface microfluidic systems.

Steerable mass transport in a photoresponsive system for advanced anticounterfeiting

Numerous anticounterfeiting platforms using photoresponsive materials have been designed to improve information security, enabling applications in anticounterfeiting technology. However, fabricating sophisticated micro/nanostructures using bidirectional mass transport to achieve advanced anticounterfeiting remains challenging. Here, we propose one strategy to achieve steerable mass transport in a photoresponsive system with the assistance of solvent vapor at room temperature. Upon optimizing the host-guest ratio and the width of photoisomerized areas, wettability gradient is acquired just photo-patterning once, then bidirectional mass transport is realized due to the competition of mass transport induced by surface energy gradient of the material itself and flow of the solvent on the film surface with wettability gradient. Taking advantage of the interaction between solvent and film surface with wettability gradient, this bidirectional polymer flow has been successfully applied in multi-mode anticounterfeiting. This work paves a promising avenue toward high-level information storage in soft materials, demonstrating the potential applications in anticounterfeiting.

Velocity-Switched Droplet Rebound Direction on Anisotropic Superhydrophobic Surfaces

Droplet well-controlled directional motion being an essential function has attracted much interest in academic and industrial applications, such as self-cleaning, micro-/nano-electro-mechanical systems, drug delivery, and heat-transferring. Conventional understanding has it that a droplet impacted on an anisotropic surface tends to bounce along the microstructural direction, which is mainly dictated by surface properties rather than initial conditions. In contrast to previous findings, it demonstrates that the direction of a droplet's rebound on an anisotropic surface can be switched by designing the initial impacting velocity. With an increase in impacting height from 2 to 10 cm, the droplet successively shows a backward, vertical, and forward motion on anisotropic surfaces. Theoretical demonstrations establish that the transition of droplet bouncing on the anisotropic surface is related to its dynamic wettability during impacting process. Characterized by the liquid-solid interaction, it is demonstrated that the contact state at small and large impacting heights induces an opposite resultant force in microstructures. Furthermore, energy balance analysis reveals that the energy conversion efficiency of backward motion is almost three times as that of traditional bouncing. This work, including experiments, theoretical models, and energy balance analysis provides insight view in droplet motions on the anisotropic surfaces and opens a new way for the droplet transport.

Trifurcated Splitting of Water Droplets on Engineered Lithium Niobate Surfaces

Controlled splitting of liquid droplets is a key function in many microfluidic applications. In recent years, various methodologies have been used to accomplish this task. Here, we present an optofluidic technique based on an engineered surface formed by coating a z-cut iron-doped lithium niobate crystal with a lubricant-infused layer, which provides a very slippery surface. Illuminating the crystal with a light spot induces surface charges of opposite signs on the two crystal faces because of the photovoltaic effect. If the light spot is sufficiently intense, millimetric water droplets placed near the illuminated spot split into two charged fragments, one fragment being trapped by the bright spot and the other moving away from it. The latter fragment does not move randomly but rather follows one of three well-defined trajectories separated by 120°, which reflect the anisotropic crystalline structure of Fe:LiNbO3. Numerical simulations explain the behavior of water droplets in the framework of the forces induced by the interplay of pyroelectric, piezoelectric, and photovoltaic effects, which originate simultaneously inside the illuminated crystal. Such a synergetic effect can provide a valuable feature in applications that require splitting and coalescence of droplets, such as chemical microreactors and biological encapsulation and screening.

New buffer systems for photopainting of single biomolecules

We present newly developed buffer systems that significantly improve the efficiency of a photochemically induced surface modification at the single molecule level. Buffers with paramagnetic cations and radical oxygen promoting species facilitate laser-assisted protein adsorption by photobleaching (LAPAP) of single fluorescently labelled oligonucleotides or biotin onto multi-photon-lithography-structured 2D and 3D acrylate scaffolds. Single molecule fluorescence microscopy has been used to quantify photopainting efficiency. We identify specific cation interaction sites for members of the cyanine, coumarin and rhodamine classes of fluorophores using quantum mechanical calculations. We show that our buffer systems provide an up to three-fold LAPAP-efficiency increase for the cyanine fluorophore, while keeping excitation parameters constant.

Preparation, Stimulus-Response Mechanisms and Applications of Micro/Nanorobots

Micro- and nanorobots are highly intelligent and efficient. They can perform various complex tasks as per the external stimuli. These robots can adapt to the required functional form, depending on the different stimuli, thus being able to meet the requirements of various application scenarios. So far, microrobots have been widely used in the fields of targeted therapy, drug delivery, tissue engineering, environmental remediation and so on. Although microbots are promising in some fields, few reviews have yet focused on them. It is therefore necessary to outline the current status of these microbots' development to provide some new insights into the further evolution of this field. This paper critically assesses the research progress of microbots with respect to their preparation methods, stimulus-response mechanisms and applications. It highlights the suitability of different preparation methods and stimulus types, while outlining the challenges experienced by microbots. Viable solutions are also proposed for the promotion of their practical use.

Visualization of the Dynamics of Photoinduced Crawling Motion of 4-(Methylamino)Azobenzene Crystals via Diffracted X-ray Tracking

The photoinduced crawling motion of crystals is a continuous motion that azobenzene molecular crystals exhibit under light irradiation. Such motion enables object manipulation at the microscale with a simple setup of fixed LED light sources. Transportation of nano-/micromaterials using photoinduced crawling motion has recently been reported. However, the details of the motion mechanism have not been revealed so far. Herein, we report visualization of the dynamics of fine particles in 4-(methylamino)azobenzene (4-MAAB) crystals under light irradiation via diffracted X-ray tracking (DXT). Continuously repeated melting and recrystallization of 4-MAAB crystals under light irradiation results in the flow of liquid 4-MAAB. Zinc oxide (ZnO) particles were introduced inside the 4-MAAB crystals to detect diffracted X-rays. The ZnO particles rotate with the flow of liquid 4-MAAB. By using white X-rays with a wide energy width, the rotation of each zinc oxide nanoparticle was detected as the movement of a bright spot in the X-ray diffraction pattern. It was clearly shown that the ZnO particles rotated increasingly as the irradiation light intensity increased. Furthermore, we also found anisotropy in the rotational direction of ZnO particles that occurred during the crawling motion of 4-MAAB crystals. It has become clear that the flow perpendicular to the supporting film of 4-MAAB crystals is enhanced inside the crystal during the crawling motion. DXT provides a unique means to elucidate the mechanism of photoinduced crawling motion of crystals.

Live Imaging of 3D Hanging Drop Arrays through Manipulation of Light-Responsive Pyroelectric Slippery Surface and Chip Adhesion

Three-dimensional (3D) hanging drop cell culture is widely used in organoid culture because of its lack of selection pressure and rapid cell aggregation. However, current hanging drop technology has limitations, such as a dependence on complex microfluidic transport channels or specific capillary force templates for drop formation, which leads to unchangeable drop features. These methods also hinder live imaging because of space and complexity constraints. Here, we have developed a hanging drop construction method and created a flexible 3D hanging drop construction platform composed of a manipulation module and an adhesion module. Their harmonious operation allows for the easy construction of hanging drops of varying sizes, types, and patterns. Our platform produces a cell hanging drop chip with small sizes and clear fields of view, thereby making it compatible with live imaging. This platform has great potential for personalized medicine, cancer and drug discovery, tissue engineering, and stem cell research.

Long-Distance Continuous Self-Transport of a Droplet by Merging Droplets on a Graphene-Covered Multibranch Gradient Groove Surface

Although the self-transport of liquid droplets by a gradient-textured substrate can break away from the energy input, the long distance and even continuous spontaneous motion of droplets will be limited by the length in the surface-gradient direction. This article introduces a novel design with a monolayer graphene-covered multibranch gradient groove surface (GMGGS). The design aims to achieve long-distance, continuous self-transport of a mercury (Hg) droplet by merging with other mercury droplets, and the process is carried out using molecular dynamics (MD) simulation. This method achieves the merging of mercury droplets through the structure of multibranch gradient grooves, and we have observed that the merged mercury droplet can be reaccelerated in the gradient groove. The results demonstrate that droplet merging allows for control over the surface morphology variations of mercury droplets within the gradient groove. This creates a forward pressure difference, which leads to reacceleration of the mercury droplets. In light of this mechanism, the trunk droplet can achieve long-distance continuous self-transport on the GMGGS by continuously merging with branch droplets. These findings will broaden our comprehension of droplet merging and self-transport behavior, offering corresponding theoretical support for the long-distance continuous self-transport of droplets.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

Photocharging of Carbon Nitride Thin Films for Controllable Manipulation of Droplet Force Gradient Sensors

Intentional generation, amplification, and discharging of chemical gradients is central to many nano- and micromanipulative technologies. We describe a straightforward strategy to direct chemical gradients inside a solution via local photoelectric surface charging of organic semiconducting thin films. We observed that the irradiation of carbon nitride thin films with ultraviolet light generates local and sustained surface charges in illuminated regions, inducing chemical gradients in adjacent solutions via charge-selective immobilization of surfactants onto the substrate. We studied these gradients using droplet force gradient sensors, complex emulsions with simultaneous and independent responsive modalities to transduce information on transient gradients in temperature, chemistry, and concentration via tilting, morphological reconfiguration, and chemotaxis. Fine control over the interaction between local, photoelectrically patterned, semiconducting carbon nitride thin films and their environment yields a new method to design chemomechanically responsive materials, potentially applicable to micromanipulative technologies including microfluidics, lab-on-a-chip devices, soft robotics, biochemical assays, and the sorting of colloids and cells.

Light-manipulated binary droplet transport on a high-energy surface

Flexible and precise manipulation of droplet transport is of significance for scientific and engineering applications, but real-time and on-demand droplet manipulation remains a challenge. Herein, we report a strategy using light for the outstanding manipulation of binary droplet motion on a high-energy surface and reveal the underlying mechanism. Upon irradiation to a substrate by a focused light beam, the substrate can provide a localized heating source via photothermal conversion, and a binary droplet can be flexibly transported on a high-energy surface with free contact-line pinning, exhibiting light-propelled droplet transport. We theoretically showed that the surface tension gradient across the droplet interface resulting from the localized photothermal effect is responsible for actuating droplet transport. Remarkably, the high reconfigurability and flexibility of light allowed for binary droplet transport with dynamically tunable velocity and direction as well as arbitrary trajectory assisted by 2D channels on a high-energy surface. Complex droplet transportation, controllable droplet coalescence, and anti-gravity motion were realized. The promising applicability of this light-fueled droplet platform was also demonstrated by directional transport of biosample droplets containing DNA molecules and cells, as well as successional microreactions.

Time-Resolved Structural Analysis of Fast-Photoresponsive Surfactant Micelles by Stroboscopic Small-Angle Neutron Scattering

Photoresponsive materials are garnering attention because of their applications toward building a sustainable society. A recently developed fast-photoresponsive amphiphilic lophine dimer (3TEG-LPD) responds rapidly to light, making it a promising candidate for drug-delivery systems. In this study, the mechanism of structural changes induced by ultraviolet (UV) irradiation in 3TEG-LPD micelles in an aqueous solution was investigated via an in situ time-resolved small-angle neutron scattering (SANS) technique. Since subsecond resolution was necessary to observe the structural changes in the 3TEG-LPD micelles, stroboscopic SANS analysis was employed to obtain scattering profiles with a time width of 0.5 s. The structural parameters were quantitatively determined by performing a model-fitting analysis of the SANS results. The stroboscopic SANS results showed that upon UV irradiation, the axial ratio and pseudo-aggregation number of the 3TEG-LPD micelles increased by 1.8 and 1.6 times, respectively, whereas the number of water molecules per surfactant molecule decreased. This finding suggested that the change in the shape of the micelles from spherical to ellipsoidal shape was accompanied by dehydration. Under the present UV irradiation conditions, this structural change of the micelle occurred rapidly during the first 30 s after the start of UV irradiation. Each structural parameter recovered exponentially and reversibly during the recovery process after the cessation of UV irradiation. The changes in these parameters were analyzed in terms of kinetics by comparing them with the changes in the molecular structure. We found that the change of the micelles proceeds approximately twice as fast as the association of the molecule. Furthermore, from the perspective of the critical packing parameter consideration, the SANS analysis revealed that the UV-induced changes in 3TEG-LPD micelles are dominated by the enthalpy contribution. This finding is expected to be useful for developing new materials for various applications.

Laser-guided programmable construction of cell-laden hydrogel microstructures forin vitrodrug evaluation

Cell-laden hydrogel microstructures have been used in broad applications in tissue engineering, translational medicine, and cell-based assays for pharmaceutical research. However, the construction of cell-laden hydrogel microstructuresin vitroremains challenging. The technologies permitting generation of multicellular structures with different cellular compositions and spatial distributions are needed. Herein, we propose a laser-guided programmable hydrogel-microstructures-construction platform, allowing controllable and heterogeneous assembly of multiple cellular spheroids into spatially organized multicellular structures with good bioactivity. And the cell-laden hydrogel microstructures could be further leveraged forin vitrodrug evaluation. We demonstrate that cells within hydrogels exhibit significantly higher half-maximal inhibitory concentration values against doxorubicin compared with traditional 2D plate culture. Moreover, we reveal the differences in drug responses between heterogeneous and homogeneous cell-laden hydrogel microstructures, providing valuable insight intoin vitrodrug evaluation.

All-Optical Rapid Formation, Transport, and Sustenance of a Sessile Droplet in a Two-Dimensional Slit with Few-Micrometer Separation

We present a sessile droplet manipulation platform that enables the formation and transport of a droplet on a light-absorbing surface via local laser-beam irradiation. The mechanism relies on solutocapillary Marangoni flow arising from a concentration gradient in a binary mixture liquid. Because the mixture is strongly confined in a two-dimensional slit with a spacing of a few micrometers, the wetting film is stably sustained, enabling the rapid formation, deformation, and transport of a sessile droplet. In addition, to sustain the droplet in the absence of laser irradiation, we developed a method to bridge the droplet between the top and bottom walls of the slit. The bridge is stably sustained because of the hydrophilicity of the slit wall. Splitting and merging of the droplet bridges are also demonstrated.

Microfluidic platform using focused ultrasound passing through hydrophobic meshes with jump availability

Applications in chemistry, biology, medicine, and engineering require the large-scale manipulation of a wide range of chemicals, samples, and specimens. To achieve maximum efficiency, parallel control of microlitre droplets using automated techniques is essential. Electrowetting-on-dielectric (EWOD), which manipulates droplets using the imbalance of wetting on a substrate, is the most widely employed method. However, EWOD is limited in its capability to make droplets detach from the substrate (jumping), which hinders throughput and device integration. Here, we propose a novel microfluidic system based on focused ultrasound passing through a hydrophobic mesh with droplets resting on top. A phased array dynamically creates foci to manipulate droplets of up to 300 μL. This platform offers a jump height of up to 10 cm, a 27-fold improvement over conventional EWOD systems. In addition, droplets can be merged or split by pushing them against a hydrophobic knife. We demonstrate Suzuki-Miyaura cross-coupling using our platform, showing its potential for a wide range of chemical experiments. Biofouling in our system was lower than in conventional EWOD, demonstrating its high suitability for biological experiments. Focused ultrasound allows the manipulation of both solid and liquid targets. Our platform provides a foundation for the advancement of micro-robotics, additive manufacturing, and laboratory automation.

Magnetic Beads inside Droplets for Agitation and Splitting Manipulation by Utilizing a Magnetically Actuated Platform

We successfully developed a platform for the magnetic manipulation of droplets containing magnetic beads and examined the washing behaviors of the droplets, including droplet transportation, magnetic bead agitation inside droplets, and separation from parent droplets. Magnetic field gradients were produced with two layers of 6 × 1 planar coils fabricated by using printed circuit board technology. We performed theoretical analyses to understand the characteristics of the coils and successfully predicted the magnetic field and thermal temperature of a single coil. We then investigated experimentally the agitation and splitting kinetics of the magnetic beads inside droplets and experimentally observed the washing performance in different neck-shaped gaps. The performance of the washing process was evaluated by measuring both the particle loss ratio and the optical density. The findings of this work will be used to design a magnetic-actuated droplet platform, which will separate magnetic beads from their parent droplets and enhance washing performance. We hope that this study will provide digital microfluidics for application in point-of-care testing. The developed microchip will be of great benefit for genetic analysis and infectious disease detection in the future.

Dynamically tunable lamellar surface structures from magnetoactive elastomers driven by a uniform magnetic field

Stimuli responsive materials are key ingredients for any application that requires dynamically tunable or on-demand responses. In this work we report experimental and theoretical investigation of magnetic-field driven modifications of soft-magnetic elastomers whose surface was processed by laser ablation into lamellar microstructures that can be manipulated by a uniform magnetic field. We present a minimal hybrid model that elucidates the associated deflection process of the lamellae and explains the lamellar structure frustration in terms of dipolar magnetic forces arising from the neighbouring lamellae. We experimentally determine the magnitude of the deflection as a function of magnetic flux density and explore the dynamic response of lamellae to fast changes in a magnetic field. A relationship between the deflection of lamellae and modifications of the optical reflectance of the lamellar structures is resolved.

Wettability gradient-driven droplets with an applied external force

On homogeneous substrates, droplets can slide due to external driving forces, such as gravity, whereas in the presence of wettability gradients, sliding occurs without external forces since this gradient gives rise to an internal driving force. Here, we study via molecular dynamics simulations the more complex behavior when droplets are driven under the combined influence of an external and internal driving force. For comparison, the limiting cases of a single driving force are studied as well. During a large part of the sliding process over the borderline of both substrates, separating both wettabilities, the velocity is nearly constant. When expressing it as the product of the effective mobility and the effective force, the effective mobility mainly depends on the mobility of the initial substrate, experienced by the receding contact line. This observation can be reconciled with the properties of the flow pattern, indicating that the desorption of particles at the receding contact line is the time-limiting step. The effective force is the sum of the external force and a renormalized internal force. This renormalization can be interpreted as stronger dissipation effects when driving occurs via wettability gradients.

Surface Morphing of Azopolymers toward Advanced Anticounterfeiting Enabled by a Two-Step Method: Light Writing and Then Reading in Liquid

Surface morphing of organic materials is necessary for advances in semiconductor processing, optical gratings, anticounterfeiting etc., but it is still challenging, especially for its fundamental explanation and further applications like advanced anticounterfeiting. Here, we report one strategy to acquire surface deformation of the liquid-crystalline azopolymer film using a two-step method: selective photoisomerization of azopolymers and then solvent development. In the first step, surface tension of the polymer film can be patterned by the selective photoisomerization of azopolymers, and then in the second step, the flowing solvent drags the underlying polymer to transport, leading to the formation of surface deformation. Interestingly, the direction of mass transport is opposite to the traditional Marangoni flow, and the principle of solvents' choice is the matching of surface tensions between the azopolymer and the solvent. The two-step method shows characteristics of efficient surface morphing, which could be applied in advanced anticounterfeiting by the way of photomask-assistant information writing or microscale direct writing, and then reading in a specific liquid environment. This paves a new way for understanding the mechanism of mass transport toward numerous unprecedented applications using various photoresponsive materials.

On-Chip Liquid Manipulation via a Flexible Dual-Layered Channel Possessing Hydrophilic/Hydrophobic Dichotomy

The hydrophilic/hydrophobic cooperative interface provides a smart platform to control liquid distribution and delivery. Through the fusion of flexibility and complex structure, we present a manipulable, open, and dual-layered liquid channel (MODLC) for on-demand mechanical control of fluid delivery. Driven by anisotropic Laplace pressure, the mechano-controllable asymmetric channel of MODLC can propel the directional slipping of liquid located between the paired tracks. Upon a single press, the longest transport distance can reach 10 cm with an average speed of ∼3 cm/s. The liquid on the MODLC can be immediately manipulated by pressing or dragging processes, and versatile liquid-manipulating processes on hierarchical MODLC chips have been achieved, including remote droplet magneto-control, continuous liquid distributor, and gas-producing chip. The flexible hydrophilic/hydrophobic interface and its assembly can extend the function and applications of the wettability-patterned interface, which should update our understanding of complex systems for sophisticated liquid transport.

Arylazopyrazole-Modified Thiolactone Acrylate Copolymer Brushes for Tuneable and Photoresponsive Wettability of Glass Surfaces

Photoswitches have long been employed in coatings for surfaces and substrates to harness light as a versatile stimulus to induce responsive behavior. We previously demonstrated the viability of arylazopyrazole (AAP) as a photoswitch in self-assembled monolayers (SAMs) on silicon and glass surfaces for photoresponsive wetting applications. We now aim to transfer the excellent photophysical properties of AAPs to polymer brush coatings. Compared to SAMs, polymer brushes offer enhanced stability and an increase of the thickness and density of the functional organic layer. In this work, we present thiolactone acrylate copolymer brushes which can be post-modified with AAP amines as well as hydrophobic acrylates, making use of the unique chemistry of the thiolactones. This strategy enables photoresponsive wetting with a tuneable range of contact angle change on glass substrates. We show the successful synthesis of thiolactone hydroxyethyl acrylate copolymer brushes by means of surface-initiated atom-transfer radical polymerization with the option to either prepare homogeneous brushes or to prepare micrometer-sized brush patterns by microcontact printing. The polymer brushes were analyzed by atomic force microscopy, time-of-flight secondary ion spectrometry, and X-ray photoelectron spectroscopy. Photoresponsive behavior imparted to the brushes by means of post-modification with AAP is monitored by UV/vis spectroscopy, and wetting behavior of homogeneous brushes is measured by static and dynamic contact angle measurements. The brushes show an average change in static contact angle of around 13° between E and Z isomer of the AAP photoswitch for at least five cycles, while the range of contact angle change can be fine-tuned between 53.5°/66.5° (E/Z) and 81.5°/94.8° (E/Z) by post-modification with hydrophobic acrylates.

Autonomous and directional flow of water and transport of particles across a subliming dynamic crystal surface

Chemical and morphological traits of natural substrates that can propel and transport fluids over their surfaces have long provided inspiration for the engineering of artificial materials that can harvest and collect water from aerial humidity. Here we report that the gradual widening of parallel microchannels on a surface of a slowly subliming hexachlorobenzene crystal can promote the autonomous and bidirectional transduction of condensed aerial water. Driven by topology changes on the surface of the crystal and water exchange with the gas phase, droplets of condensed water migrate over the crystal. These droplets are also able to transport silver particles and other particulate matter, such as dust. The velocity of the particles was shown to be dependent on both the sublimation rate of the crystal and the relative humidity of its environment. This example of a sublimation-powered water flow demonstrates that topological surface changes accompanying crystal phase transitions can be harnessed to transport liquid and solid matter over surfaces.

Overflow Control for Sustainable Development by Superwetting Surface with Biomimetic Structure

Liquid flowing around a solid edge, i.e., overflow, is a commonly observed flow behavior. Recent research into surface wetting properties and microstructure-controlled overflow behavior has attracted much attention. Achieving controllable macroscale liquid dynamics by manipulating the micro-nanoscale liquid overflow has stimulated diverse scientific interest and fostered widespread use in practical applications. In this review, we outline the evolution of overflow and present a critical survey of the mechanism of surface wetting properties and microstructure-controlled liquid overflow in multilength scales ranging from centimeter to micro and even nanoscale. We summarize the latest progress in utilizing the mechanisms to manipulate liquid overflow and achieve macroscale liquid dynamics and in emerging applications to manipulate overflow for sustainable development in various fields, along with challenges and perspectives.

Toward droplets displaying life-like interaction behaviors

Developments in synthetic biology usually bring the conception of individual artificial cells. A key feature of living systems is, however, the interaction between individuals, in which living units can interact autonomously and display a role differentiation such as the case of entities chasing each other. On the other hand, droplets have become a very useful and exciting medium for modern microengineering and biomedical technologies. In this Perspective, we show a brief discussion-outlook of different approaches to recreate predator-prey interactions in both swimmer and crawling droplet systems toward a new generation of synthetic life with impact in both fundamental insights and relevant applications.

Phase behaviour and adsorption of deoxyribonucleic acid onto an azobenzene liquid crystalline ligand at the interfaces

Azobenzene liquid crystalline (ALC) ligand contains a cholesteryl group linked to an azobenzene moiety through a carbonyl dioxy spacer (C7) and terminated with an amine group as a polar head. The phase behaviour of the C7 ALC ligand at the air-water (A-W) interface is investigated employing surface manometry. The surface pressure-area per molecule isotherm shows that C7 ALC ligand exhibit two different phases following the phase sequence viz., liquid expanded (LE₁ and LE₂) and then collapse to three-dimensional crystallites. Further, our investigations under different pH conditions and in the presence of DNA reveal the following. Compared to the bulk, the acid dissociation constant (pK_a) of an individual amine reduces to 5 at the interfaces. For pH (3.5) < pK_a, the protonation of amine groups of C7 ALC ligand facilitates the condensation of the film and enhances the stability. For pH values > pK_a, the phase behaviour of the ligand remains the same due to the partial dissociation of the amine groups. The presence of DNA in the sub-phase result in the expansion of isotherm to the higher area per molecule and the compressional modulus extracted reveals the phase sequence; liquid expanded, liquid condensed, followed by a collapse. Further, the kinetics of adsorption of DNA to the amine groups of the ligand is investigated, suggesting the interactions are influenced by surface pressure corresponding to different phases and pH of the sub-phase. Brewster angle microscope studies are carried out at different surface densities of the ligand as well as in the presence of DNA also supports this inference. Atomic force microscope is employed to acquire the surface topography and height profile of C7 ALC ligand (1 layer) after transferring on onto a silicon substrate using Langmuir Blodgett deposition. The difference in the surface topography and thickness of the film indicates the adsorption of DNA onto the amine groups of the ligand. The characteristic UV-visible absorption bands of the ligand films (10 layers) at the air-solid interface are tracked and the hypsochromic shift of these bands is also attributed to these DNA interactions.