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.

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.

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 AlO particles and poly(dimethylsiloxane) (PDMS) via a simple phase separation method.

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.

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 two-tier structural surface (BFTSS) that can remarkably reduce the forward skin friction drag by = 19%.

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.

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.

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.

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.

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 (), ridge height (), and the deviation distance () between the impacting point and the center of curvature on the lateral offset distance (Δ) of bouncing drops. The suggested theoretical analysis is in reasonable agreement with the experimental observations.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Controlled Smart Anisotropic Unidirectional Spreading of Droplet on a Fibrous Surface.

Smart anisotropic-unidirectional spreading is displayed on a wettable-gradient-aligned fibrous surface due to a synergetic directing effect from the aligned structure and the ratio of hydrophilic components.

Dynamic Magnetic Responsive Wall Array with Droplet Shedding-off Properties.

Directional control of droplets on a surface is an important issue for tasks of long-range liquid-transport, self-cleaning and water repellency. However, it is still challenging to control the structure motions in orientations so as to control the shedding-off of droplets. Herein, we report a novel dynamic magnetic responsive wall (DMRW) array on PDMS (polydimethylsiloxane)-based surface.

Radial wettable gradient of hot surface to control droplets movement in directions.

A radial wettable gradient was fabricated on the surface of graphite plate by a simple one-step anodic oxidation process. It was found that the direction and value of the wettable gradient could be easily controlled by adjusting current and oxidation time gradient. With the increase of surface temperature, droplets on surface not only exhibited the transition of boiling mode, but also showed the controlled radial spreading, evaporation and movement behaviors.

Controlled droplet transport on a gradient adhesion surface.

A surface with continuously changed adhesion from ultrahigh to ultralow is fabricated by an integrated method of anodic oxidation combined with octafluorocyclobutane (C4F8) plasma. The control of droplet transport along the direction of the adhesion gradient in length is achieved, as the surface is submitted to either tilted angle or vibration frequency.

Controlling of water collection ability by an elasticity-regulated bioinspired fiber.

A special artificial spider silk is presented which is fabricated by using both an elastic polymer and a fiber, and the water collection behavior is investigated. Through exerting tension in varying degree, the length of the three-phase contact line (TCL) and the area of spindle knot can be regulated readily, which makes a great contribution to the improvement of collecting efficiency and water-hanging ability. The water-hanging ability can be predicted at a given stretching ratio according to the given expression of the TCL.