Breaking the symmetry to suppress the Plateau-Rayleigh instability and optimize hydropower utilization

Patterned Surface Energy for Modulating Solid-Liquid Interfacial Properties

Surface energy, as an intrinsic property of solids, plays a crucial role in modulating the characteristics of solid surfaces, especially of the solid-liquid interface. Due to inevitable processes such as surface adsorption or contamination, the surface energy of practical solids is usually nonuniform. However, if this nonuniformity is rationally designed and effectively utilized, it is capable of endowing great potential for liquid manipulation. With the rapid development of microfabrication and surface modification techniques, a variety of artificial patterned surface energy surfaces (PSESs) have been fabricated, which extend the diversity, tunability, and precision of liquid-based applications. In this review, we discuss the regulation of solid-liquid interface properties with PSESs from a relatively macroscopic perspective, particularly focusing on how to control matter and energy through rational design. First, we provide a brief introduction about the definition and significance of PSESs. Then, matter selective adhesion by PSESs is summarized, including liquid dynamics regulation, crystallization inducement, and biosample self-distribution. In the following, we discuss how PSESs regulate physical fields, including the thermal field, electric field, and acoustic field, with an explanation centered on discontinuous solid-liquid contact on PSESs. Finally, associated challenges of surface energy regulation for liquid-based scenarios are included.

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.

Drop splitting on hydrophobic wedge-shaped tips after central impact: effect of sharpness and wetting properties

Drop impact on a wedged structure is a common phenomenon in daily life and industry. Although drop impact has been studied extensively since high-speed cameras have become available, little is known about drop impact on wedge tips of these structures. Here, we combine experiments and volume-of-fluid simulations to determine how velocity, the sharpness of the structure, and the surface wettability influence the outcome. The central impact of water drops onto wedge tips coated with superhydrophobic nanofilaments or with hydrophobic polystyrene (PS) was imaged. On superhydrophobic surfaces, drops fully rebound or split after impact. On hydrophobic PS surfaces, drops are deposited or split. A critical Weber number (We) was used to describe the transition between deposition/rebounding and splitting. It increases with the top width of the wedge tip and its top angle. The critical We and drop behavior is also affected by wetting properties which determine the drop adhesion and lateral drop friction. Our investigations may help to design new structures to prevent icing or produce tiny drops efficiently in applications.

Advancement in surfactant-enhanced droplet deposition on the hydrophobic surfaces

Droplets impacting solid surfaces are encountered in nature and industry, from rain to agricultural spraying and inkjet printing. Surfactants are an important factor that affects the impact behavior of droplets. An in-depth knowledge of the influence and mechanisms of surfactants on the dynamics of droplet impact can enhance the precise control of droplets in industrial processes. Herein, recent insights into surfactant-enhanced droplet deposition on hydrophobic surfaces are reviewed. First, the mechanisms of surfactant-enhanced droplet deposition are summarized. Second, the factors that influence droplet deposition, such as molecular diffusion, convective diffusion of surfactants, characteristics of hydrophobic surfaces, and interaction between the surfactant-laden droplets and the hydrophobic surfaces, are explored. Additionally, the influences of surfactants on the spreading and retraction processes of impacting droplets, maximum spreading factor, and oscillation dynamics are reviewed. Finally, typical applications of surfactants in different fields, such as inkjet printing, supercooled surface, and agricultural spray, are summarized, along with challenges and prospects in future research, to provide suggestions for subsequent studies.

Hermetic hydrovoltaic cell sustained by internal water circulation

Numerous efforts have been devoted to harvesting sustainable energy from environment. Among the promising renewable resources, ambient heat exhibits attractive prospects due to its ubiquity and inexhaustibility, and has been converted into electricity through water evaporation-induced hydrovoltaic approaches. However, current hydrovoltaic approaches function only in low-humidity environments and continuously consume water. Herein, we fabricate a hermetic hydrovoltaic cell (HHC) to harvest ambient heat, and have fully addressed the limitations posed by environmental conditions. Meanwhile, for the first time we develop an internal circulation hydrovoltaic mechanism. Taking advantage of the heterogeneous wicking bilayer structure, we verify that inside the hermetic cell, the ambient temperature fluctuation-induced evaporation and further the water circulation can persist, which sustains the hydrovoltaic effect to convert ambient heat into electricity. More importantly, the hermetic design enables the cell to work continuously and reliably for 160 h with negligible water consumption, unaffected by external influences such as wind and light, making it an excellent candidate for extreme situations such as water-scarce deserts, highly humid tropical rain forests, rainy days, and dark underground engineering. These findings provide an easily accessible and widely applicable route for stably harnessing renewable energy, and more notably, offer a novel paradigm toward leveraging low-grade ambient heat energy via circulation design.

'Rewritable' and 'liquid-specific' recognizable wettability pattern

Bio-inspired surfaces with wettability patterns display a unique ability for liquid manipulations. Sacrificing anti-wetting property for confining liquids irrespective of their surface tension (γLV), remains a widely accepted basis for developing wettability patterns. In contrast, we introduce a 'liquid-specific' wettability pattern through selectively sacrificing the slippery property against only low γLV (<30 mN m-1) liquids. This design includes a chemically reactive crystalline network of phase-transitioning polymer, which displays an effortless sliding of both low and high γLV liquids. Upon its strategic chemical modification, droplets of low γLV liquids fail to slide, rather spill arbitrarily on the tilted interface. In contrast, droplets of high γLV liquids continue to slide on the same modified interface. Interestingly, the phase-transition driven rearrangement of crystalline network allows to revert the slippery property against low γLV liquids. Here, we report a 'rewritable' and 'liquid-specific' wettability pattern for high throughput screening, separating, and remoulding non-aqueous liquids.

A Human Friendly Self-Assembled Triboelectric Sensor for Multifunctional Tactile Sensing

Obtaining bioenergy from human movement is not only a prospective complementation to electrochemical power supply such as batteries in portable electronics but also a decipherable process for developing self-powered sensors that can simultaneously monitor the physiological movement. In this study, a low-cost, robust, and environmentally friendly triboelectric nanogenerator (TENG) was prepared with enhanced mechanical stability and tunneling conductivity on the base of cotton fabric. The as-designed TENG may produce energy sustainably by physical movements, and it can yield an amazing 417 V open-circuit voltage, 11.7 μA short-circuit current, and 237.60 mW/m2 excellent power density, showcasing its potential for efficient energy conversion in the single-electrode mode. Besides, such a design also shows real-time tactile perception ability toward human physiological signal and body motion where intelligent application of these environmental benign TENGs in sports and writing training were demonstrated, thus providing vital instruction for the creation of versatile and sustainable TENGs in the Internet of Things era.

Droplet-based mechanical transducers modulated by the symmetry of wettability patterns

Asymmetric mechanical transducers have important applications in energy harvesting, signal transmission, and micro-mechanics. To achieve asymmetric transformation of mechanical motion or energy, active robotic metamaterials, as well as materials with asymmetric microstructures or internal orientation, are usually employed. However, these strategies usually require continuous energy supplement and laborious fabrication, and limited transformation modes are achieved. Herein, utilizing wettability patterned surfaces for precise control of the droplet contact line and inner flow, we demonstrate a droplet-based mechanical transducer system, and achieve multimodal responses to specific vibrations. By virtue of the synergistic effect of surface tension and solid-liquid adhesion on the liquid dynamics, the droplet on the patterned substrate can exhibit symmetric/asymmetric vibration transformation when the substrate vibrates horizontally. Based on this, we construct arrayed patterns with distinct arrangements on the substrate, and employ the swarm effect of the arrayed droplets to achieve three-dimensional and multimodal actuation of the target plate under a fixed input vibration. Further, we demonstrate the utilization of the mechanical transducers for vibration management, object transport, and laser modulation. These findings provide a simple yet efficient strategy to realize a multimodal mechanical transducer, which shows significant potential for aseismic design, optical molding, as well as micro-electromechanical systems (MEMS).

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.

Understanding and Utilizing Droplet Impact on Superhydrophobic Surfaces: Phenomena, Mechanisms, Regulations, Applications, and Beyond

Droplet impact is a ubiquitous liquid behavior that closely tied to human life and production, making indispensable impacts on the big world. Nature-inspired superhydrophobic surfaces provide a powerful platform for regulating droplet impact dynamics. The collision between classic phenomena of droplet impact and the advanced manufacture of superhydrophobic surfaces is lighting up the future. Accurately understanding, predicting, and tailoring droplet dynamic behaviors on superhydrophobic surfaces are progressive steps to integrate the droplet impact into versatile applications and further improve the efficiency. In this review, the progress on phenomena, mechanisms, regulations, and applications of droplet impact on superhydrophobic surfaces, bridging the gap between droplet impact, superhydrophobic surfaces, and engineering applications are comprehensively summarized. It is highlighted that droplet contact and rebound are two focal points, and their fundamentals and dynamic regulations on elaborately designed superhydrophobic surfaces are discussed in detail. For the first time, diverse applications are classified into four categories according to the requirements for droplet contact and rebound. The remaining challenges are also pointed out and future directions to trigger subsequent research on droplet impact from both scientific and applied perspectives are outlined. The review is expected to provide a general framework for understanding and utilizing droplet impact.

Self-Adaptive Droplet Bouncing on a Dual Gradient Surface

Rapid detachment of impacting droplets from underlying substrate is highly preferred for mass, momentum, and energy exchange in many practical applications. Driven by this, the past several years have witnessed a surge in engineering macrotexture to reduce solid-liquid contact time. Despite these advances, these strategies in reducing contact time necessitate the elegant control of either the spatial location for droplet contact or the range of impacting velocity. Here, this work circumvents these limitations by designing a dual gradient surface consisting of a vertical spacing gradient made of tapered pillar arrays and a lateral curvature gradient characterized as macroscopic convex. This design enables the impacting droplets to self-adapt to asymmetric or pancake bouncing mode accordingly, which renders significant contact time reduction (up to ≈70%) for a broad range of impacting velocities (≈0.4-1.4 m s-1 ) irrespective of the spatial impacting location. This new design provides a new insight for designing liquid-repellent surfaces, and offers opportunities for applications including dropwise condensation, energy conversion, and anti-icing.

A UV-Resistant Heterogeneous Wettability-Patterned Surface

Preparing UV-resistant heterogeneous wettability patterns is critical for the practical application of surfaces with heterogeneous wettability. However, combining UV-resistant superhydrophobic and superhydrophilic materials on heterogeneous surfaces is challenging. Inspired by the structure of cell membranes, a UV-resistant heterogeneous wettability-patterned surface (UPS) is designed via laser ablation of the coating of multilayer structures. UV-resistant superhydrophobic silica patterns can be created in situ on surfaces covered with superhydrophilic TiO2 nanoparticles. The UV resistance time of the UPS with a TiO2 -based surface is more than two orders of magnitude higher than that obtained with other surface molecular modification methods that require a mask. The cell-membrane-like structure of the UPS regulates the migration of internal siloxane chain segments in the hydrophilic and hydrophobic regions of the surface. The UPS enables efficient patterning of functional materials under UV irradiation, controlling the wetting behavior of liquids in open-air systems. Furthermore, its heterogeneous wettability remains stable even after 50 h of intense UV irradiation (365 nm, 500 mW cm-2 ). These UV-resistant heterogeneous wettability patterned surfaces will likely be applied in microfluidics, cell culture, energy conversion, and water collection in the future.

Advancements in Passive Wireless Sensors, Materials, Devices, and Applications

In recent years, passive wireless sensors have been studied for various infrastructure sectors, making them a research and development focus. While substantial evidence already supports their viability, further effort is needed to understand their dependability and applicability. As a result, issues related to the theory and implementation of wireless sensors still need to be resolved. This paper aims to review and summarize the progress of the different materials used in different passive sensors, the current status of the passive wireless sensor readout devices, and the latest peripheral devices. It will also cover other related aspects such as the system equipment of passive wireless sensors and the nanogenerators for the energy harvesting for self-powered sensors for applications in contemporary life scenarios. At the same time, the challenges for future developments and applications of passive wireless are discussed.

Rotating Surfaces Promote the Shedding of Droplets

Achieving rapid shedding of droplets from solid surfaces has received substantial attention because of its diverse applications. Previous studies have focused on minimizing contact times of liquid droplets interacting with stationary surfaces, yet little consideration has been given to that of moving surfaces. Here, we report a different scenario: A water droplet rapidly detaches from micro/nanotextured rotating surfaces in an intriguing doughnut shape, contributing to about 40% contact time reduction compared with that on stationary surfaces. The doughnut-shaped bouncing droplet fragments into satellites and spontaneously scatters, thus avoiding further collision with the substrate. In particular, the contact time is highly dependent on impact velocities of droplets, beyond previous descriptions of classical inertial-capillary scaling law. Our results not only deepen the fundamental understanding of droplet dynamics on moving surfaces but also suggest a synergistic mechanism to actively regulate the contact time by coupling the kinematics of droplet impingement and surface rotation.

Ink-Drop Dynamics on Chemically Modified Surfaces

Inkjet printing provides an efficient routine for distributing functional materials into locations with well-designed arrangements. As one of the most critical factors in determining the printing quality, the impacting and depositing behaviors of ink drops largely depend on the wettability of the target surface. In addition to printing on solids with intrinsic wettability, various ink-drop impact dynamics and deposition morphologies have been reported through modifying the surface wettability including both homogeneous and heterogeneous, which opens up possibilities for applications such as advanced optic/electric device fabrication and highly sensitive detection. In this Perspective, we summarize recent progress in the modification methods of solid surface wettability and their capability in modulating the ink-drop impacting and depositing dynamics. The challenges facing ink-drop regulation by chemical modification methodologies are also envisaged at the end of the Perspective.

Floating Hydrogel Beads Made by Droplet Impact

Droplet impact is a ubiquitous natural phenomenon that has been widely utilized to inspire and facilitate many industrial applications. Compared to the widely studied water droplet impact onto identical liquid surfaces, the water droplet impact onto an oil layer floating on a water bath (OLW) receives far less attention and its potential application has never been exploited. Herein, the process of water droplet impact onto the OLW is investigated with emphasis on the metastable states and potential applications. It is found that the dramatic deformation of the oil-water interface caused by the water droplet impact leads to two metastable states: oil in water in oil in water (O/W/O/W) and oil in water in oil (O/W/O). Through the subsequent introduction of gelation process, the metastable states can be frozen into floating hydrogel beads with similar shape to the roly-poly toys, which are attempted in gastric retentive drug delivery and algae bloom control. Specifically, the floating hydrogel beads perform well in gastric retentive drug delivery in vitro due to their inherent slow-release properties and floating capability. In addition, the floating hydrogel beads loading photocatalysts can capture more sunshine, and exhibit high photocatalytic efficiency, which is thus responsible for efficient algae bloom control.

Droplet Bouncing: Fundamentals, Regulations, and Applications

Droplet impact is a ubiquitous phenomenon in nature, daily life, and industrial processes. It is thus crucial to tune the impact outcomes for various applications. As a special outcome of droplet impact, the bouncing of droplets keeps the form of the droplets after the impact and minimizes the energy loss during the impact, being beneficial in many applications. A unified understanding of droplet bouncing is in high demand for effective development of new techniques to serve applications. This review shows the fundamentals, regulations, and applications of millimeter-sized droplet bouncing on solid surfaces and same/miscible liquids (liquid pool and another droplet). Regulation methods and current applications are summarized, and potential directions are proposed.

Non-Hookean Droplet Spring for Enhancing Hydropower Harvest

Nonlinear elastic materials are significant for engineering and micromechanics. Droplets with the merits of easy-accessibility, diversity, and energy-absorption capability exhibit a variety of non-Hookean elastic behaviors. Herein, benefiting from the confinement of heterogeneous-wettable parallel plates, the non-Hookean mechanics of the droplet-based spring are systematically investigated. Experimental results and theoretical analysis reveal that the force generated by the spring varies nonlinearly with its deformation, and a force model is accordingly built to depict the mechanics of springs with different sized/numbered droplets and confined by different wettability patterns. Importantly, for the droplet-based spring, the droplet-plate contact area expands nonlinearly with the pressing force, which is employed to optimize the output performance of the droplet-based triboelectric nanogenerator to 226% compared with the control test. This finding deepens the understanding of the non-Hookean behavior of droplet-based springs, and sheds light on applications in energy harvesting, micromechanics, and miniature optic/electric devices.