Electrostatic tweezer for droplet manipulation.

Various physical tweezers for manipulating liquid droplets based on optical, electrical, magnetic, acoustic, or other external fields have emerged and revolutionized research and application in medical, biological, and environmental fields. Despite notable progress, the existing modalities for droplet control and manipulation are still limited by the extra responsive additives and relatively poor controllability in terms of droplet motion behaviors, such as distance, velocity, and direction. Herein, we report a versatile droplet electrostatic tweezer (DEST) for remotely and programmatically trapping or guiding the liquid droplets under diverse conditions, such as in open and closed spaces and on flat and tilted surfaces as well as in oil medium. DEST, leveraging on the coulomb attraction force resulting from its electrostatic induction to a droplet, could manipulate droplets of various compositions, volumes, and arrays on various substrates, offering a potential platform for a series of applications, such as high-throughput surface-enhanced Raman spectroscopy detection with single measuring time less than 20 s.

Surface Nanostructures Promote Droplet Splash on Hot Substrates.

Splash, one of the most visually apparent droplet dynamics, can manifest on any surface above a certain impact velocity, regardless of surface wettability. Previous studies demonstrate that elevating the substrate temperature can suppress droplet splash, which is unfavorable for many practical applications, such as spray cooling and combustion. Here, we report that the suppression effect of substrate temperature on splash is nullified by utilizing surfaces with nanostructures. By manipulating air evacuation time through surface nanostructures, we have identified a pathway for precise control over the splash threshold and the ability to tailor the dependence of the splash onset on surface temperature. We further propose a theoretical criterion to determine different splash regimes by considering the competition between air evacuation and the development of flow instabilities. Our findings underscore the crucial role of nanostructures in splash dynamics, offering valuable insights for the control of splash in various industrial scenarios.

Heterogenous Slippery Surfaces: Enabling Spontaneous and Rapid Transport of Viscous Liquids with Viscosities Exceeding 10 000 mPa s.

Superhydrophobic and slippery lubricant-infused surfaces have garnered significant attention for their potential to passively transport low-viscosity liquids like water (1 mPa s). Despite exciting progress, these designs have proven ineffective for transporting high-viscosity liquids such as polydimethylsiloxane (5500 mPa s) due to their inherent limitations imposed by the homogenous surface design, resulting in high viscous drags and compromised capillary forces. Here, a heterogenous water-infused divergent surface (WIDS) is proposed that achieves spontaneous, rapid, and long-distance transport of viscous liquids. WIDS reduces viscous drag by spatially isolating the viscous liquids and surface roughness through its heterogenous, slippery topological design, and generates capillary forces through its heterogenous wetting distributions. The essential role of surface heterogeneity in viscous liquid transport is theoretically and experimentally verified. Remarkably, such a heterogenous paradigm enables transporting liquids with viscosities exceeding 12 500 mPa s, which is two orders of magnitude higher than state-of-the-art techniques. Furthermore, this heterogenous design is generic for various viscous liquids and can be made flexible, making it promising for various systems that require viscous liquid management, such as micropatterning.

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.

Rapid and Persistent Suction Condensation on Hydrophilic Surfaces for High-Efficiency Water Collection.

Water collection by dew condensation emerges as a sustainable solution to water scarcity. However, the transient condensation process that involves droplet nucleation, growth, and transport imposes conflicting requirements on surface properties. It is challenging to satisfy all benefits for different condensation stages simultaneously. By mimicking the structures and functions of moss Rhacocarpus, here, we report the attainment of dropwise condensation for efficient water collection even on a hydrophilic surface gated by a liquid suction mechanism. The Rhacocarpus-inspired porous surface (RIPS), which possesses a three-level wettability gradient, facilitates a rapid, directional, and persistent droplet suction. Such suction condensation enables a low nucleation barrier, frequent surface refreshing, and well-defined maximum droplet shedding radius simultaneously. Thus, a maximum ∼160% enhancement in water collection performance compared to the hydrophobic surface is achieved. Our work provides new insights and a design route for developing engineered materials for a wide range of water-harvesting and phase-change heat-transfer applications.

Hydrophilic Slippery Surface Promotes Efficient Defrosting.

Frost accretion occurs ubiquitously in various industrial applications and causes tremendous energy and economic loss, as manifested by the Texas power crisis that impacted millions of people over a vast area in 2021. To date, extensive efforts have been made on frost removal by micro-engineering surfaces with superhydrophobicity or lubricity. On such surfaces, air or oil cushions are introduced to suspend the frost layer and promote the rapid frost sliding off, which, although promising, faces the instability of the cushions under extreme frosting conditions. Most existing hydrophilic surfaces, characterized by large interfacial adhesion, have long been deemed unfavorable for frost shedding. Here, we demonstrated that a hydrophilic and slippery surface can achieve efficient defrosting. On such a surface, the hydrophilicity gave rise to a highly interconnected basal frost layer that boosted the substrate-to-frost heat transfer; then, the resulting melted frost readily slid off the surface due to the superb slipperiness. Notably, on our surface, the retained meltwater coverage after frost sliding off was only 2%. In comparison to two control surfaces, for example, surfaces lacking either hydrophilicity or slipperiness, the defrosting efficiency was 13 and 19 times higher and the energy consumption was 2.3 and 6.2 times lower, respectively. Our study highlights the use of a hydrophilic surface for the pronounced defrosting in a broad range of industrial applications.

Tuning Ice Nucleation and Propagation with Counterions on Multilayer Hydrogels.

Ice formation on solid surfaces includes heterogeneous ice nucleation and ice propagation processes. However, no study has been focused on tuning of both ice nucleation and ice propagation via a simple anti-icing coating method. In this work, we have prepared multilayer hydrogels based on simple layer-by-layer (LBL) deposition approach and discover the ion-specific effect on both ice nucleation and ice propagation. A large ice nucleation temperature window of 11 °C is controlled via changing different counterions; meanwhile, the differences in ice propagation time can be tuned up to 4 orders of magnitude. Through synergistically controlling of ice nucleation and propagation delay times, we can tune the freezing delay time of water droplets on multilayer hydrogel surfaces up to 3 orders of magnitude via changing various counterions. Considering the application requirements, these multilayer hydrogels are stable under different conditions and can be coated on various materials without destroying the existing surface. This new insight can inspire the design of anti-icing surfaces based on regulating both ice nucleation and ice propagation.

Control of Ice Propagation by Using Polyelectrolyte Multilayer Coatings.

Ice propagation is of great importance to the accumulation of ice/frost on solid surfaces. However, no investigation has been reported on the tuning of ice propagation through a simple coating process. Herein, we study the ice propagation behavior on polyelectrolyte multilayer (PEM) surfaces coated with the layer-by-layer (LBL) deposition approach. We discover that ice propagation is strongly dependent on the amount of water in the outermost layer of PEMs, that is, the ice propagation rate increases with the amount of water in the outermost layer. The ice propagation rate can be tuned by up to three orders of magnitude by changing the polyelectrolyte pairs, counterions of the outermost polymer layer, or the salt concentration during the preparation of PEMs. Because the simple, versatile, and inexpensive LBL deposition approach is generally applicable to almost all available surfaces, the PEM coatings can tune ice propagation on a wide range of substrates.

How liquids charge the superhydrophobic surfaces.

Liquid-solid contact electrification (CE) is essential to diverse applications. Exploiting its full implementation requires an in-depth understanding and fine-grained control of charge carriers (electrons and/or ions) during CE. Here, we decouple the electrons and ions during liquid-solid CE by designing binary superhydrophobic surfaces that eliminate liquid and ion residues on the surfaces and simultaneously enable us to regulate surface properties, namely work function, to control electron transfers. We find the existence of a linear relationship between the work function of superhydrophobic surfaces and the as-generated charges in liquids, implying that liquid-solid CE arises from electron transfer due to the work function difference between two contacting surfaces. We also rule out the possibility of ion transfer during CE occurring on superhydrophobic surfaces by proving the absence of ions on superhydrophobic surfaces after contact with ion-enriched acidic, alkaline, and salt liquids. Our findings stand in contrast to existing liquid-solid CE studies, and the new insights learned offer the potential to explore more applications.

Charge-Powered Electrotaxis for Versatile Droplet Manipulation.

Taxis is an instinctive behavior of living organisms to external dangers or benefits. Here, we report a taxis-like behavior associated with liquid droplets on charged substrates in response to the external stimuli, referred to as droplet electrotaxis. Such droplet electrotaxis enables us to use either solid or liquid (such as water) matter, even a human finger, as stimuli to spatiotemporal precisely manipulate the liquid droplets of various physicochemical properties, including water, ethanol with low surface tension, viscous oil, and so on. Droplet electrotaxis also features a flexible configuration that even can manifest in the presence of an additional layer, such as the ceramic with a thickness of ∼10 mm. More importantly, superior to existing electricity-based strategies, droplet electrotaxis can harness the charges generated from diverse manners, including pyroelectricity, triboelectricity, piezoelectricity, and so on. These properties dramatically increase the application scenarios of droplet electrotaxis, such as cell labeling and droplet information recording.

Achieving Extreme Pressure Resistance to Liquids on a Super-Omniphobic Surface with Armored Reentrants.

Static repellency and pressure resistance to liquids are essential for high-performance super-omniphobic surfaces. However, these two merits appear mutually exclusive in conventional designs because of their conflicting structural demands: Static liquid repellency necessitates minimal solid-liquid contact, which in turn inevitably undercuts the surface's ability to resist liquid invasion exerted by the elevated pressure. Here, inspired by the Springtail, these two merits can be simultaneously realized by structuring surfaces at two size scales, with a micrometric reentrant structure providing static liquid repellency and a nanometric reentrant structure providing pressure resistance, which dexterously avoids the dilemma of their structural conflicts. The nanometric reentrants are densely packed on the micrometric ones, serving as "armor" that prevents liquids invasion by generating multilevel energy barriers, thus naming the surface as the armored reentrants (AR) surface. The AR surface could repel liquids with very low surface tensions, such as silicone oil (21 mN m-1 ), and simultaneously resist great pressure from the liquids, exemplified by enduring the impact of low-surface-tension liquids under a high weber number (>400), the highest-pressure resistance ever reported. With its scalable fabrication and enhanced performance, our design could extend the application scope of liquid-repellent surfaces toward ultimate industrial settings.

3D Conformal Fabrication of Piezoceramic Films.

Piezoceramic films are an essential class of energy-conversion materials that have been widely used in the electronics industry. Although current methods create a great freedom for fabricating high-quality piezoceramic films, it requires well-controlled synthesis conditions, including special high-cost equipment and planar substrates particularly. The limited substrate selections hinder the applications of piezoceramic films in 3D conformal structures where most objects possess complex curvilinear surfaces. To overcome such limitations, a fast, energy-efficient, and cost-effective approach, named flame treated spray (FTS) coating, is developed for preparing piezoceramic films on free-form surfaces. The flame treatment significantly enhances the hydrophilicity of a substrate, assisting in forming a uniform and continuous thin film. The followed spray coating deposits hundreds of nanometers to several micrometers thick films on 3D free-form surfaces. Given the size controllability and arbitrary surface compatibility of the FTS method, a highly conformal piezoelectric tactile sensor array (4 × 4) is assembled on a spherical surface for mimicking robot fingers and an on-site thin-film sensor on the wing of an aircraft model to monitor the vibration in real-time during flight. The FTS film deposition offers a highly promising methodology for the application of functional thin-film from micro- to marcoscale devices, regardless of conformal problems.

Triboelectric wetting for continuous droplet transport.

Manipulating liquid is of great significance in fields from life sciences to industrial applications. Owing to its advantages in manipulating liquids with high precision and flexibility, electrowetting on dielectric (EWOD) has been widely used in various applications. Despite this, its efficient operation generally needs electrode arrays and sophisticated circuit control. Here, we develop a largely unexplored triboelectric wetting (TEW) phenomenon that can directly exploit the triboelectric charges to achieve the programmed and precise water droplet control. This key feature lies in the rational design of a chemical molecular layer that can generate and store triboelectric charges through agile triboelectrification. The TEW eliminates the requirement of the electric circuit design and additional source input and allows for manipulating liquids of various compositions, volumes, and arrays on various substrates in a controllable manner. This previously unexplored wetting mechanism and control strategy will find diverse applications ranging from controllable chemical reactions to surface defogging.

Explosive Pancake Bouncing on Hot Superhydrophilic Surfaces.

The rapid detachment of liquid droplets from engineered surfaces in the form of complete rebound, pancake bouncing, or trampolining has been extensively studied over the past decade and is of practical importance in many industrial processes such as self-cleaning, anti-icing, energy conversion, and so on. The spontaneous trampolining of droplets needs an additional low-pressure environment and the manifestation of pancake bouncing on superhydrophobic surfaces requires meticulous control of macrotextures and impacting velocity. In this work, we report that the rapid pancake-like levitation of impinging droplets can be achieved on superhydrophilic surfaces through the application of heating. In particular, we discovered explosive pancake bouncing on hot superhydrophilic surfaces made of hierarchically non-interconnected honeycombs, which is in striking contrast to the partial levitation of droplets on the surface consisting of interconnected microposts. This enhanced droplet bouncing phenomenon, characterized by a significant reduction in contact time and increase in the bouncing height, is ascribed to the production and spatial confinement of pressurized vapor in non-interconnected structures. The manifestation of pancake bouncing on the superhydrophilic surface rendered by a bottom-to-up boiling process may find promising applications such as the removal of trapped solid particles.

Inhibiting Condensation Freezing on Patterned Polyelectrolyte Coatings.

Condensation freezing inhibition is of great practical importance for anti-icing applications; however, no coatings with this performance have been reported. Here, we report the inhibition of condensation freezing on patterned polyelectrolyte coatings, including polyelectrolyte brush (PB), polyelectrolyte multilayer (PEM), and polyelectrolyte hydrogel (PH) surfaces, benefiting from their feature in regulating ice nucleation and propagation via changing counterions. On the reported surfaces, ice nucleation can be initiated exclusively at the domains with the polyelectrolytes; moreover, spontaneous ice propagation can be achieved atop the patterned polyelectrolyte surface. Consequently, condensed water surrounding the frozen drops on the patterned polyelectrolyte surface evaporates due to the instantaneously released latent heat in the course of ice propagation. Afterward, ice grows specifically on polyelectrolyte surfaces via desublimation as the saturated vapor pressure of ice is smaller than that of condensed water drops. As such, an ice-free region up to 96% of the entire surface area can be accomplished. We demonstrate that various polyelectrolyte coatings can be easily introduced on almost all surfaces, revealing great promise for anti-icing applications.