Bionic dual-scale structured films for efficient passive radiative cooling accompanied by robust durability.

Passive radiative cooling (PRC), as an energy-free cooling approach, is ingeniously harnessed for certain natural organisms to withstand extreme high-temperature climates, which has inspired numerous bionic designs. However, it is a great challenge to enhance the durability of the designed materials in practical scenarios while inheriting the natural biological principles. We demonstrate bionic dual-scale structured (BDSS) films for efficient passive radiative cooling accompanied by robust durability after discovering the excellent thermoregulatory properties of the inner surface of Hawaiian scallop shell. We found that the inner surface of the shell consists of large-scale triangular ridges scattered with small-scale terrace steps. This dual-scale structure can enhance the reflectivity of sunlight by efficient Mie scattering and increase the emissivity in the mid-infrared range by lengthening the propagation of photons, thereby decreasing the surface temperature. Underpinned by this finding, we developed a BDSS film that features a strong solar spectrum reflectivity of 0.95 and a high mid-infrared emissivity of 0.98, achieving a sub-ambient cooling of 10.8 °C under direct sunlight. Additionally, the designed films possess robust durability including excellent self-cleaning, flexibility, mechanical strength, chemical stability, and anti-ultraviolet radiation, which is promising for thermal thermoregulation in various harsh scenarios.

Versatile bubble maneuvering on photopyroelectric slippery surfaces.

Contactless bubble manipulation with a high spatiotemporal resolution brings a qualitative leap forward in a variety of applications. Despite considerable advances, light-induced bubble maneuvering remains challenging in terms of robust transportation, splitting and detachment. Here, a photopyroelectric slippery surface (PESS) with a sandwich structure is constructed to achieve the versatile bubble manipulation. Due to the generated dielectric wetting and nonuniform electric field under the irradiation of near infrared (NIR) light, a bubble is subject to both the Laplace force and dielectrophoresis force, enabling a high-efficiency bubble steering. We demonstrate that the splitting, merging and detachment of underwater bubbles can be achieved with high flexibility and precision, high velocity and agile direction maneuverability. We further extend the capability of bubble control to microrobots for cargo transportation, micropart assembly and transmission of gear structures. We envision this robust bubble manipulation strategy on the PESS would provide a valuable platform for various bubble-involved processes, ranging from microfluidic devices to soft robotics.

Efficient Passive Daytime Radiative Cooling by Hierarchically Designed Films Integrating Robust Durability.

Surfaces with efficient passive daytime radiative cooling (PDRC) are underpinned by maximizing both solar reflection and thermal radiation to the outer space at no additional energy cost. Despite notable progress, their practical applications are of great challenge due to their complicated fabrication processes, easy contamination and damage, and high costs. Herein, we fabricate a hierarchically designed passive daytime radiative cooling film (HPRF) comprising cost-effective Al2O3 particles and poly(dimethylsiloxane) (PDMS) via a simple phase separation method. The designed film possesses a high solar spectrum reflectance of ∼0.96 and a mid-infrared emittance of ∼0.95, achieving a ∼12.4 °C subambient cooling under direct solar irradiation. This excellent PDRC is due to the efficient Mie scattering of sunlight by hierarchical micro-/nanostructures and selected molecular vibrations of PDMS combined with the phonon polariton resonance of Al2O3 particles, respectively. Moreover, the designed HPRF is accompanied with robust durability endowed by superior self-cleaning, flexibility, and anti-ultraviolet radiation that can present substantial application promises of thermal management in various electronic devices and wearable products.

Ultrasonic tweezer for multifunctional droplet manipulation.

Spatiotemporally controllable droplet manipulation is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite considerable advances, droplet manipulation without surface or droplet pretreatment is still challenging in terms of response and functional adaptability. Here, a droplet ultrasonic tweezer (DUT) based on phased array is proposed for versatile droplet manipulation. The DUT can generate a twin trap ultrasonic field at the focal point for trapping and maneuvering the droplet by changing the position of the focal point, which enables a highly flexible and precise programmable control. By leveraging the acoustic radiation force resulting from the twin trap, the droplet can pass through a confined slit 2.5 times smaller than its own size, cross a slope with an inclination up to 80°, and even reciprocate in the vertical direction. These findings provide a satisfactory paradigm for robust contactless droplet manipulation in various practical settings including droplet ballistic ejection, droplet dispensing, and surface cleaning.

Bidirectional Underwater Drag Reduction on Bionic Flounder Two-Tier Structural Surfaces.

Engineering marvels found throughout the exclusive structural features of biological surfaces have given rise to the progressive development of skin friction drag reduction. However, despite many previous works reporting forward drag reduction where the bio-inspired surface features are aligned with the flow direction, it is still challenging to achieve bidirectional drag reduction for non-morphable surface structures. Inspired by the flounder ctenoid scales characterized by tilted, millimeter-sized oval fins embedded with sub-millimeter spikes, we fabricate a bionic flounder two-tier structural surface (BFTSS) that can remarkably reduce the forward skin friction drag by ηdr = 19%. Even in the backwards direction, where the flow is completely against the tilting direction of surface structures, BFTSS still exhibits a considerable drag reduction of ηdr = 4.2%. Experiments and numerical simulations reveal that this unique bidirectional drag reduction is attributed to synergistic effects of the two-tier structures of BFTSS. The array of oval fins can distort the boundary layer flow and mitigate the viscous shear, whilst the microscale spikes act to promote the flow separation to relieve the pressure gradient in the viscous sublayer. Notably, the pressure gradient relief effect of microscale spikes remains invariant to the flow direction and is responsible for the backward drag reduction as well. The bidirectional drag reduction of BFTSS can be extensively applied in minimizing the energy consumption of ships and underwater vessels, as well as in pipeline transport.

Electrically switched underwater capillary adhesion.

Developing underwater adhesives that can rapidly and reversibly switch the adhesion in wet conditions is important in various industrial and biomedical applications. Despite extensive progresses, the manifestation of underwater adhesion with rapid reversibility remains a big challenge. Here, we report a simple strategy that achieves strong underwater adhesion between two surfaces as well as rapid and reversible detachment in on-demand manner. Our approach leverages on the design of patterned hybrid wettability on surfaces that selectively creates a spatially confined integral air shell to preserve the water bridge in underwater environment. The overall adhesion strength can be multiplied by introducing multiple air shells and rapidly broken by disturbing the integrity of the protective air shell in response to the applied voltage on two surfaces. Our design can be constructed on the flexible substrate with hybrid wettability, which can be applied to non-conductive substrates and adapted to more complicated morphologies, extending the choice of underlying materials.

One-Step Fabrication of Hot-Water-Repellent Surfaces.

Hot-water repellency is of great challenge on traditional superhydrophobic surfaces due to the condensation of tiny droplets within the cavities of surface textures, which builds liquid bridges to connect the substrate and hot water and thus destroys the surface water-repellence performance. For the unique structural features and scales, current approaches to fabricate surfaces with hot-water repellency are always complicated and modified by fluorocarbon. Here, we propose a facile and fluorine-free one-step vapor-deposition method for fabricating excellent hot-water-repellent surfaces, which at room temperature even repel water droplets of temperature up to 90 °C as well as other normal-temperature droplets with surface tension higher than 48.4 mN/m. We show that whether the unique hot-water repellency is achieved depends on a trade-off between the solid-liquid contact time and hot-vapor condensation time, which determines the probability of formation of liquid bridges between the substrate and hot-water. Moreover, the designed surfaces exhibit excellent self-cleaning performance in some specific situations, such as oil medium, hot water and condensation environments. We envision that this facile and fluorine-free strategy for fabricating excellent hot-water-repellent surfaces could be valuable in popularizing their practical applications.

Highly Tough, Stretchable, and Solvent-Resistant Cellulose Nanocrystal Photonic Films for Mechanochromism and Actuator Properties.

Cellulose nanocrystals (CNCs)-derived photonic materials have confirmed great potential in producing renewable optical and engineering areas. However, it remains challenging to simultaneously possess toughness, strength, and multiple responses for developing high-performance sensors, intelligent coatings, flexible textiles, and multifunctional devices. Herein, the authors report a facile and robust strategy that poly(ethylene glycol) dimethacrylate (PEGDMA) can be converged into the chiral nematic structure of CNCs by ultraviolet-triggered free radical polymerization in an N,N-dimethylformamide solvent system. The resulting CNC-poly(PEGDMA) composite exhibits impressive strength (42 MPa), stretchability (104%), toughness (31 MJ m-3 ), and solvent resistance. Notably, it preserves vivid optical iridescence, displaying stretchable variation from red, yellow, to green responding to the applied mechanical stimuli. More interestingly, upon exposure to spraying moisture, it executes sensitive actuation (4.6° s-1 ) and multiple complex 3D deformation behaviors, accompanied by synergistic iridescent appearances. Due to its structural anisotropy of CNC with typical left-handedness, the actuation shows the capability to generate a high probability (63%) of right-handed helical shapes, mimicking a coiled tendril. The authors envision that this versatile system with sustainability, robustness, mechanochromism, and specific actuating ability will open a sustainable avenue in mechanical sensors, stretchable optics, intelligent actuators, and soft robots.

Three-dimensional capillary ratchet-induced liquid directional steering.

Conventional understanding has it that a liquid deposited on a surface tends to move along directions that reduce surface energy, which is mainly dictated by surface properties rather than liquid properties, such as surface tension. Achieving well-controlled directional steering remains challenging because the liquid-solid interaction mainly occurs in the two-dimensional (2D) domain. We show that the spreading direction of liquids with different surface tensions can be tailored by designing 3D capillary ratchets that create an asymmetric and 3D spreading profile both in and out of the surface plane. Such directional steering is also accompanied by self-propulsion and high flow velocity, all of which are preferred in liquid transport.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part II: Theory of Effect.

In the first part of this research, we reported the experimental study of the drop impact on the superhydrophobic circular groove arrays, which resulted in a directional droplet transport. In the second part, we further explored the influence of the Weber number (We), ridge height (H), and the deviation distance (r) between the impacting point and the center of curvature on the lateral offset distance (ΔL) of bouncing drops. The suggested theoretical analysis is in reasonable agreement with the experimental observations. We demonstrate that a Cassie-Wenzel wetting transition occurred within the microstructures of the relief under the threshold Weber number, for example, We ≅ 19-25, which switched the nature of drop bouncing. The dynamic pressure plays a decisive role in the directional droplet transport. The reported investigation may shed light on the solid-liquid interactions occurring on the patterned hierarchical surfaces and open up new opportunities for directional droplet transportation.

Synchronous oil/water separation and wastewater treatment on a copper-oxide-coated mesh.

Despite remarkable progress in oil/water separation and wastewater treatment, the ability to carry out the two processes in a synchronous manner has remained difficult. Here, synchronous oil/water separation and wastewater treatment were proposed on mesh surfaces coated with copper-oxide particles, which possess superwetting and catalytic properties. The superwetting performance generates additional pressure to achieve the permselectivity of the designed mesh, on which the oil phase is selectively repelled while the water phase passes though easily. Moreover, the catalytic performance of the copper oxide forms reactive oxygen species to purify the water during oil/water separation process. We show that the oil/water separation and catalytic degradation efficiencies for organic pollutants can reach more than 99% by adjusting the content of copper oxide on the mesh surfaces. Such a unique design for integrating multifunctionality on single mesh surfaces strongly underpins the synchronization of oil/water separation and wastewater treatment, which will provide a new insight for separating pure water from industrial oil/water mixtures.

Counterintuitive Ballistic and Directional Liquid Transport on a Flexible Droplet Rectifier.

Achieving the directional and long-range droplet transport on solid surfaces is widely preferred for many practical applications but has proven to be challenging. Particularly, directionality and transport distance of droplets on hydrophobic surfaces are mutually exclusive. Here, we report that drain fly, a ubiquitous insect maintaining nonwetting property even in very high humidity, develops a unique ballistic droplet transport mechanism to meet these demanding challenges. The drain fly serves as a flexible rectifier to allow for a directional and long-range propagation as well as self-removal of a droplet, thus suppressing unwanted liquid flooding. Further investigation reveals that this phenomenon is owing to the synergistic conjunction of multiscale roughness, structural periodicity, and flexibility, which rectifies the random and localized droplet nucleation (nanoscale and microscale) into a directed and global migration (millimeter-scale). The mechanism we have identified opens up a new approach toward the design of artificial rectifiers for broad applications.

Tip-induced flipping of droplets on Janus pillars: From local reconfiguration to global transport.

Despite their simplicity, water droplets manifest a wide spectrum of forms and dynamics, which can be actuated using special texture at solid surfaces to achieve desired functions. Along this vein, natural or synthetic materials can be rendered water repellent, oleophobic, antifogging, anisotropic, etc.-all properties arising from an original design of the substrate and/or from the use of special materials promoting capillary or elastic forces at the droplet scale. Here, we report an original phenomenon occurring at the tip of asymmetric (half-flat, half-curved) pillars: Droplets reconfigure and get oriented on the curved side of these Janus tips. This local, geometry-driven effect, namely, tip-induced flipping of droplets, is found to be generic and have spectacular global consequences: Vast assemblies of Janus pillars enable a continuous, long-range, and fast self-transport of water harvested from fogs, which makes it possible to collect and concentrate droplets at different scales.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part I: Experimental Findings.

Directional transport of liquid droplets is crucial for various applications including water harvesting, anti-icing, and condensation heat transfer. Here, bouncing of water droplets with patterned superhydrophobic surfaces composed of circular equidistant grooves was studied. The directional transport of droplets toward the pole of the grooves was observed. The impact of the Weber number, initial polar distance r, and geometrical parameters of the surface on the directional droplet bouncing was experimentally explored. The nature of bouncing was switched when the Weber numbers exceeded We ≅ 20-25. The rebouncing height was slightly dependent on the initial polar coordinate of the impact point for a fixed We, whereas it grew for We > 20. The weak dependence of the droplet spreading time on the Weber number was close to the dependence predicted by the Hertz bouncing, thus evidencing the negligible influence of viscosity in the process. Change in the scaling exponent describing the dependence of the normalized spreading time on the Weber number was registered for We ≅ 25. The universal dependence of the offset distance ΔL of the droplets on the Weber number ΔL/D0 ∼ We1.5 was established. The normalized offset distance decreased with the normalized initial polar distance as ΔL/D0 ∼ (r/S)-1, where D0 and S are the droplet diameter and groove width, respectively. This research may yield more insights into droplet bouncing on patterned surfaces and offer more options in directed droplet transportation.

Robust Slippery Liquid-Infused Porous Network Surfaces for Enhanced Anti-icing/Deicing Performance.

Slippery liquid-infused porous surfaces (SLIPSs) have recently been intensively investigated because of promising potential in various applications that require water repellency. However, the use of SLIPS is limited by its unsatisfactory oil-storage and -replacement capabilities. Here we designed network surface structures with interconnected microchannels and cross-linked nanosheets, which acted as natural oil reservoirs and vessels. A lubricant can be firmly locked and stored into the networks, leading to an efficient water repellency as well as improved mechanical durability and stability. We further show the surface structures can be applied to anti-icing/deicing, demonstrated by its improved icing-delaying, anti-icing, and deicing properties even after multiple cycles, compared to those on superhydrophobic surfaces (SHSs) and the conventional SLIPSs. We envision that this unique design of the slippery liquid-infused porous network surface (SLIPNS) with robust stability and durability may expand its application in extreme environments.

Inhibiting Random Droplet Motion on Hot Surfaces by Engineering Symmetry-Breaking Janus-Mushroom Structure.

Concentrating impacting droplets onto a localized hotspot and inducing them to remain in a preferential heat transfer mode is essential for efficient thermal management such as spray cooling. Conventionally, droplets impacting on hot surfaces can randomly bounce off without becoming fully evaporated, resulting in low heat transfer efficiency. Although the directional and guided transport of impacting droplets to a preferential location can be achieved through the introduction of a structural gradient, the manifestation of such a motion requires the meticulous control of the spatial location where the droplet is released. Here, a novel surface consisting of regularly patterned posts with Janus-mushroom structure (JMS) is designed, in which the sidewalls of the individual posts are decorated with straight and curved morphologies. It is revealed that such structural symmetry-breaking in the individual posts leads to directional liquid penetration and vapor flow toward the straight sidewall, and also reduces the work of adhesion, altogether triggering collective and preferential droplet transport at a high temperature. By surrounding a conventional surface with JMS endowed with favorable directionality, it is possible to concentrate small impacting droplets preferentially onto a localized hotspot to achieve enhanced cooling efficiency.

Designing biomimetic liquid diodes.

Just as the innovation of electronic diodes that allow the current to flow in one direction provides a foundation for the development of digital technologies, the engineering of surfaces or devices that allow the directional and spontaneous transport of fluids, termed liquid diodes, is highly desired in a wide spectrum of applications ranging from medical microfluidics, advanced printing, heat management and water collection to oil-water separation. Recent advances in manufacturing, visualization techniques, and biomimetics have led to exciting progress in the design of various liquid diodes. In spite of exciting progress, formulating a general framework broad enough to guide the design, optimization and fabrication of engineered liquid diodes remains a challenging task to date. In this review, we first present an overview of the development of biological and engineered liquid diodes to elucidate how to control the surface chemistry and topography to regulate the transport of liquids without the need for external energy. Then the latest design strategies allowing for the creation of longitudinal and transverse liquid diodes are discussed and compared. We also define some figures of merit such as the rectification coefficient and the transport velocity and distance to quantify the performance of liquid diodes. Finally, we highlight perspectives on the development of engineered liquid diodes that transcend nature and adapt to various practical applications.

Integrative Bioinspired Surface with Wettable Patterns and Gradient for Enhancement of Fog Collection.

A novel integrative bioinspired surface with wettable patterns and gradient (WPGS) is proposed for fog collection via a novel anodic oxidation strategy. We study the water collection behaviors on WPGS with different parameters. Quantitative force analysis is presented, providing evidence for the underlying mechanism leading to the directional motion of the droplet, which is consistent with the experimental results. Such a surface can not only improve the fog droplet capture performance effectively owing to wettable patterns but also accelerate surface regeneration by taking full advantage of the cooperation of multidriving forces, leading to a further fog collection enhancement.

Biological and Engineered Topological Droplet Rectifiers.

The power of the directional and spontaneous transport of liquid droplets is revealed through ubiquitous biological processes and numerous practical applications, where droplets are rectified to achieve preferential functions. Despite extensive progress, the fundamental understanding and the ability to exploit new strategies to rectify droplet transport remain elusive. Here, the latest progress in the fundamental understanding as well as the development of engineered droplet rectifiers that impart superior performance in a wide variety of working conditions, ranging from low temperature, ambient temperature, to high temperature, is discussed. For the first time, a phase diagram is formulated that naturally connects the droplet dynamics, including droplet formation modes, length scales, and phase states, with environmental conditions. Parallel approaches are then taken to discuss the basic physical mechanisms underlying biological droplet rectifiers, and a variety of strategies and manufacturing routes for the development of robust artificial droplet rectifiers. Finally, perspectives on how to create novel man-made rectifiers with functionalities beyond natural counterparts are presented.

Controlled droplet transport to target on a high adhesion surface with multi-gradients.

We introduce multi-gradients including Laplace pressure gradient, wettable gradient and wettable different gradient on a high adhesion surface via special wedge-pattern and improved anodic oxidation method. As a result of the cooperative effect mentioned above, controlled directional motion of a droplet on a high adhesion surface is realized, even when the surface is turned upside down. The droplet motion can be predicted and the movement distances can be controlled by simply adjusting the wedge angle and droplet volume. More interestingly, when Laplace pressure gradient is introduced on a V-shaped wettable gradient surface, two droplets can move toward one another as designed.