Magnetic-actuated "capillary container" for versatile three-dimensional fluid interface manipulation

Fluid Control on Bionics-Energized Surfaces

Engineered surfaces play a vital role in various fluid applications, serving specific functions such as self-cleaning, anti-icing, thermal management, and water energy harvesting. In nature, biological surfaces, particularly those displaying physiochemical heterogeneity, showcase remarkable fluid behaviors and functionalities, offering valuable insights for artificial designs. In this Review, we focus on exploring the fascinating fluid phenomena observed on natural biological surfaces and the manipulation of fluids on bioengineered surfaces, with a particular emphasis on droplets, liquid flows, and vapor flows. We delve into the fundamental principles governing symmetric fluid motion on homogeneous surfaces and directed fluid motion on heterogeneous surfaces. We discuss surface design strategies tailored to different fluid scenarios, outlining the strengths and limitations of engineered surfaces for specific applications. Additionally, the challenges faced by engineered surfaces in real-world fluid applications are put forward. By highlighting promising research directions, we hope to stimulate advancements in bioinspired engineering and fluid science, paving the way for future developments.

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.

Making Interfacial Solar Evaporation of Seawater Faster than Fresh Water

Interfacial solar evaporation-based seawater desalination is regarded as one of the most promising strategies to alleviate freshwater scarcity. However, the solar evaporation rate of real seawater is significantly constricted by the ubiquitous salts present in seawater. In addition to the common issue of salt accumulation on the evaporation surface during solar evaporation, strong hydration between salt ions and water molecules leads to a lower evaporation rate for real seawater compared to pure water. Here a facile and general strategy is developed to reverse this occurrence, that is, making real seawater evaporation faster than pure water. By simply introducing specific mineral materials into the floating photothermal evaporator, ion exchange at air-water interfaces directly results in a decrease in seawater evaporation enthalpy, and consequently achieves much higher seawater evaporation rates compared to pure water. This process is spontaneously realized during seawater solar evaporation. Considering the current enormous clean water production from evaporation-based desalination plants, such an evaporation performance improvement can remarkably increase annual clean water production, benefiting millions of people worldwide.

Biomimetic Transparent Slippery Surface for the Locomotion of Photocontrol Droplets and Bubbles

Directed transportation and collection of liquids and bubbles play a vital role in the survival of ecosystems. Among them, the optical response control is widely used in the fields of microfluidic chips and chemical synthesis because of its high remote operation and fast response speed. However, due to poor light transmission, the development direction of traditional near-infrared (NIR) absorbing materials in the field of visualization is limited, and there are few reports of manufacturing an operating platform that can realize the directional movement of droplets/bubbles on a single platform. Here, a transparent photo-responsive PBFS platform is prepared for droplet and bubble manipulation by coating the etched glass substrate with Prussian blue (PB) nanocubes. When near-infrared (NIR) irradiation on the PBFS platform, PB nanocubes trigger heat production by photothermal means, due to the action of Marangoni force, the surface tension on the left and right sides of the droplets and bubbles is not uniform, forming a surface tension gradient, thereby driving the movement of the droplets and bubbles. The control platform has good application potential in the field of microchemical reaction and biomedical engineering and brings new solutions to the field of transparent photothermal materials.

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.

Building Functional Liquid-Based Interfaces: From Mechanism to Application

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.

Liquid-liquid reactions performed by cellular reactors

Liquid-liquid reactions play a significant role in organic synthesis. However, control of the phase interface between incompatible two-phase liquids remains challenging. Moreover, separating liquid acid, base and oxidants from the reactor takes a long time and high cost. To address these issues, we draw inspiration from the structure and function of cells in living organisms and develop a biomimetic 3D-printed cellular reactor. The cellular reactor houses an aqueous phase containing the catalyst or oxidant while immersed in the organic phase reactant. This setup controls the distribution of the phase interface within the organic phase and increases the interface area by 2.3 times. Notably, the cellular reactor and the aqueous phase are removed from the organic phase upon completing the reaction, eliminating additional separation steps and preventing direct contact between the reactor and acidic, alkaline, or oxidizing substances. Furthermore, the cellular reactor offers the advantages of digital design feasibility and cost-effective manufacturing.

A bioinspired surface tension-driven route toward programmed cellular ceramics

The intriguing biomineralization process in nature endows the mineralized biological materials with intricate microarchitected structures in a facile and orderly way, which provides an inspiration for processing ceramics. Here, we propose a simple and efficient manufacturing process to fabricate cellular ceramics in programmed cell-based 3D configurations, inspired by the biomineralization process of the diatom frustule. Our approach separates the ingredient synthesis from architecture building, enabling the programmable manufacturing of cellular ceramics with various cell sizes, geometries, densities, metastructures, and constituent elements. Our approach exploits surface tension to capture precursor solutions in the architected cellular lattices, allowing us to control the liquid geometry and manufacture cellular ceramics with high precision. We investigate the geometry parameters for the architected lattices assembled by unit cells and unit columns, both theoretically and experimentally, to guide the 3D fluid interface creation in arranged configurations. We manufacture a series of globally cellular and locally compact piezoceramics, obtaining an enhanced piezoelectric constant and a designed piezoelectric anisotropy. This bioinspired, surface tension-assisted approach has the potential to revolutionize the design and processing of multifarious ceramic materials for structural and functional applications in energy, electronics and biomedicine.

Magnetic Lateral Ladder for Unidirectional Transport of Microrobots: Design Principles and Potential Applications of Cells-on-Chip

Functionalized microrobots, which are directionally manipulated in a controlled and precise manner for specific tasks, face challenges. However, magnetic field-based controls constrain all microrobots to move in a coordinated manner, limiting their functions and independent behaviors. This article presents a design principle for achieving unidirectional microrobot transport using an asymmetric magnetic texture in the shape of a lateral ladder, which the authors call the "railway track." An asymmetric magnetic energy distribution along the axis allows for the continuous movement of microrobots in a fixed direction regardless of the direction of the magnetic field rotation. The authors demonstrated precise control and simple utilization of this method. Specifically, by placing magnetic textures with different directionalities, an integrated cell/particle collector can collect microrobots distributed in a large area and move them along a complex trajectory to a predetermined location.  The authors can leverage the versatile capabilities offered by this texture concept, including hierarchical isolation, switchable collection, programmable pairing, selective drug-response test, and local fluid mixing for target objects. The results demonstrate the importance of microrobot directionality in achieving complex individual control. This novel concept represents significant advancement over conventional magnetic field-based control technology and paves the way for further research in biofunctionalized microrobotics.

Liquid marbles: review of recent progress in physical properties, formation techniques, and lab-in-a-marble applications in microreactors and biosensors

Liquid marbles (LMs) are nonsticking droplets whose surfaces are covered with low-wettability particles. Owing to their high mobility, shape reconfigurability, and widely accessible liquid/particle possibilities, the research on LMs has flourished since 2001. Their physical properties, fabrication mechanisms, and functionalisation capabilities indicate their potential for various applications. This review summarises the fundamental properties of LMs, the recent advances (mainly works published in 2020-2023) in the concept of LMs, physical properties, formation methods, LM-templated material design, and biochemical applications. Finally, the potential development and variations of LMs are discussed.

Magnetic Janus origami robot for cross-scale droplet omni-manipulation

The versatile manipulation of cross-scale droplets is essential in many fields. Magnetic excitation is widely used for droplet manipulation due to its distinguishing merits. However, facile magnetic actuation strategies are still lacked to realize versatile multiscale droplet manipulation. Here, a type of magnetically actuated Janus origami robot is readily fabricated for versatile cross-scale droplet manipulation including three-dimensional transport, merging, splitting, dispensing and release of daughter droplets, stirring and remote heating. The robot allows untethered droplet manipulation from ~3.2 nL to ~51.14 μL. It enables splitting of droplet, precise dispensing (minimum of ~3.2 nL) and release (minimum of ~30.2 nL) of daughter droplets. The combination of magnetically controlled rotation and photothermal properties further endows the robot with the ability to stir and heat droplets remotely. Finally, the application of the robot in polymerase chain reaction (PCR) is explored. The extraction and purification of nucleic acids can be successfully achieved.

Designing Versatile Superhydrophilic Structures via an Alginate-Based Hydrophilic Plasticene

The rational design of superhydrophilic materials with a controllable structure is a critical component in various applications, including solar steam generation, liquid spontaneous transport, etc. The arbitrary manipulation of the 2D, 3D, and hierarchical structures of superhydrophilic substrates is highly desirable for smart liquid manipulation in both research and application fields. To design versatile superhydrophilic interfaces with various structures, here we introduce a hydrophilic plasticene that possesses high flexibility, deformability, water absorption, and crosslinking capabilities. Through a pattern-pressing process with a specific template, 2D prior fast spreading of liquids at speeds up to 600 mm/s was achieved on the superhydrophilic surface with designed channels. Additionally, 3D superhydrophilic structures can be facilely designed by combining the hydrophilic plasticene with a 3D-printed template. The assembly of 3D superhydrophilic microstructure arrays were explored, providing a promising route to facilitate the continuous and spontaneous liquid transport. The further modification of superhydrophilic 3D structures with pyrrole can promote the applications of solar steam generation. The optimal evaporation rate of an as-prepared superhydrophilic evaporator reached ~1.60 kg·m^-2·h^-1 with a conversion efficiency of approximately 92.96%. Overall, we envision that the hydrophilic plasticene should satisfy a wide range of requirements for superhydrophilic structures and update our understanding of superhydrophilic materials in both fabrication and application.

Optically Triggered Nanoscale Plasmonic Dynamite

Photons as energy carriers are clean and abundant, which can be conveniently applied for nanoactuation but the response is usually slow with very low energy efficiency/density. Here, we underpin the concept of robust nanoscale plasmonic dynamite by incorporating fullerene (C₆₀). The Au@C₆₀ core-shell nanoparticles can be triggered to explode in nanoscale with synergy of plasmon-enhanced photochemical and photothermal effects. It is suggested that a sensible amount of CO₂ was generated and pressurized in nanometric volume in an extremely short time scale (∼ns), which triggers the nanoexplosion, rendering the ejection of Au NPs at the speed over 300 m/s. The ejection generates extremely large local forces (∼1 μN) with thermomechanical energy efficiency up to ∼30%, which is demonstrated as a powerful nanoengine for controlled mobilization of micro-objects on solid surfaces. Such nanoscale plasmonic dynamite is highly exploitable for different types of nanomachines, which provides a powerful energy source for nanoactuation and nanomigration.

"Motile-targeting" drug delivery platforms based on micro/nanorobots for tumor therapy

Traditional drug delivery systems opened the gate for tumor-targeted therapy, but they generally took advantage of enhanced permeability and retention or ligand-receptor mediated interaction, and thus suffered from limited recognition range (<0.5 nm) and low targeting efficiency (0.7%, median). Alternatively, micro/nanorobots (MNRs) may act as emerging "motile-targeting" drug delivery platforms to deliver therapeutic payloads, thereby making a giant step toward effective and safe cancer treatment due to their autonomous movement and navigation in biological media. This review focuses on the most recent developments of MNRs in "motile-targeting" drug delivery. After a brief introduction to traditional tumor-targeted drug delivery strategies and various MNRs, the representative applications of MNRs in "motile-targeting" drug delivery are systematically streamlined in terms of the propelling mechanisms. Following a discussion of the current challenges of each type of MNR in biomedical applications, as well as future prospects, several promising designs for MNRs that could benefit in "motile-targeting" drug delivery are proposed. This work is expected to attract and motivate researchers from different communities to advance the creation and practical application of the "motile-targeting" drug delivery platforms.

A review on control of droplet motion based on wettability modulation: principles, design strategies, recent progress, and applications

The transport of liquid droplets plays an essential role in various applications. Modulating the wettability of the material surface is crucial in transporting droplets without external energy, adhesion loss, or intense controllability requirements. Although several studies have investigated droplet manipulation, its design principles have not been categorized considering the mechanical perspective. This review categorizes liquid droplet transport strategies based on wettability modulation into those involving (i) application of driving force to a droplet on non-sticking surfaces, (ii) formation of gradient surface chemistry/structure, and (iii) formation of anisotropic surface chemistry/structure. Accordingly, reported biological and artificial examples, cutting-edge applications, and future perspectives are summarized.

Liquid-Repellent Surfaces

Surfaces are vibrant sites for various activities with environments, especially as the transfer station for mass and energy exchange. In nature, natural creatures exhibit special wetting and interfacial properties such as water repellency and water affinity to adapt to various environmental challenges by taking advantage of air or liquid infusion media. Inspired by natural surfaces, various engineered liquid-repellent surfaces have been developed with a wide range of applications in both open and closed underwater environments. In particular, underwater conditions are characterized by high viscosity, high pressure, and complex compositions, which pose more challenges for the design of robust and functional repellent surfaces. In this Perspective, we take a parallel approach to introduce two classical liquid-repellent surfaces: an air-infused repellent surface and a lubricated liquid-repellent surface. Then we highlight fundamental challenges and design configurations of robust liquid-repellent surfaces both in air and underwater. We summarize the advantages and drawbacks of two kinds of repellent surfaces and list several applications of liquid-repellent surfaces for use in the ocean, medical care, and energy harvesting. Finally, we provide an outlook of research directions for robust liquid-repellent surfaces.

Reconfigurable Magnetic Liquid Metal Robot for High-Performance Droplet Manipulation

Droplet manipulation is crucial for diverse applications ranging from bioassay to medical diagnosis. Current magnetic-field-driven manipulation strategies are mainly based on fixed or partially tunable structures, which limits their flexibility and versatility. Here, a reconfigurable magnetic liquid metal robot (MLMR) is proposed to address these challenges. Diverse droplet manipulation behaviors including steady transport, oscillatory transport, and release can be achieved by the MLMR, and their underlying physical mechanisms are revealed. Moreover, benefiting from the magnetic-field-induced active deformability and temperature-induced phase transition characteristics, its droplet-loading capacity and shape-locking/unlocking switching can be flexibly adjusted. Because of the fluidity-based adaptive deformability, MLMR can manipulate droplets in challenging confined environments. Significantly, MLMR can accomplish cooperative manipulation of multiple droplets efficiently through on-demand self-splitting and merging. The high-performance droplet manipulation using the reconfigurable and multifunctional MLMR unfolds new potential in microfluidics, biochemistry, and other interdisciplinary fields.