On‐Demand Liquid Transportation Using Bioinspired Omniphobic Lubricated Surfaces Based on Self‐Organized Honeycomb and Pincushion Films

In this work, on‐demand control of liquids is realized by using elastic, patterned omniphobic surfaces. This paves the way for novel microfluidics, as well as liquid harvesting, transportation, and manipulation technologies. Inspired by the lubricating properties of pitcher plants, microstructured 1,2‐polybutadiene honeycomb and pincushion films obtained by self‐organization are fluorinated by the ene‐thiol reaction and infused with fluorinated lubricant to obtain omniphobic liquid‐repellent surfaces. Unlike conventional bioinspired omniphobic surfaces, the liquid repellency of the fabricated surface can be programmed by changing the surface microstructures via patterning of the film. Furthermore, the elasticity of the omniphobic film is suitable for controlling the repellency through external stimuli. The method presented here for the fabrication of lubricant‐infused omniphobic microstructured surfaces is also simple, cost‐effective, and can be scaled for large area fabrication.     

External Stimuli Responsive Liquid‐Infused Surfaces Switching between Slippery and Nonslippery States: Fabrications and Applications

Surfaces with controllable liquid wettability and related functions have gained increasing attention from interfacial scientists due to the high demand of fundamental research and practical applications. Inspired by pitch plant's excellent liquid repellency, external stimuli responsive lubricant‐infused surfaces switching between slippery state and nonslippery state under external stimuli (E‐LIS) have been developed by introducing external stimuli responsive materials as substrates, lubricants, or repellent liquids. This progress report is focused on recent development of E‐LIS. First, design strategy and fabrication of E‐LIS upon external stimuli exposure, including stress, electrical field, magnetic field, and temperature, is summarized. Then, emerging interfacial applications of E‐LIS, such as microreactors, pipetting devices, fog collection devices, and so on, are highlighted. In addition, remaining challenges and future prospects are provided.     

Stable Omniphobic Anisotropic Covalently Grafted Slippery Surfaces for Directional Transportation of Drops and Bubbles

Directional transportation and collection of liquids and bubbles are highly desirable in human life and industrial production. As one of the most promising types of functional surfaces, the reported anisotropic slippery liquid‐infused porous surfaces (SLIPSs) demonstrate unique advantages in liquid directional transportation. However, anisotropic SLIPSs readily suffer from the depletion of lubricant when used to manipulate droplets and bubbles, which leads to unstable surface properties. Therefore, fabricating stable anisotropic slippery surfaces for the directional transportation of drops and bubbles remains a challenge. Here, stable anisotropic covalently grafted slippery surfaces are fabricated by grafting polydimethylsiloxane molecular brushes onto directional microgrooved surfaces. The fabricated surfaces show remarkable anisotropic omniphobic sliding behaviors towards droplets with different surface tensions ranging from 72.8 to 37.7 mN m^?1 in air and towards bubbles underwater. Impressively, the surface maintains outstanding stability for the transportation of droplets (in air) and air bubbles (underwater) even after 240 d. Furthermore, anisotropic self‐cleaning towards various dust particles in air and directional bubble collection underwater are achieved on this surface. This stable anisotropic slippery surface has great potential for applications in the directional transportation of liquids and bubbles, microfluidic devices, directional drag reduction, directional antifouling, and beyond.     

Liquid‐Infused Smooth Coating with Transparency,Super‐Durability,and Extraordinary Hydrophobicity

Liquid‐infused coatings are because of their fluidity of considerable technological importance for hydrophobic materials with multifunctional properties, such as self‐healing, transmittance, and durability. However, conventional coatings absorb viscous liquid into their sponge‐like structured surface, causing uncontrollable liquid layer formation or liquid transport. In addition, a hydrophobic‐liquid‐retained surface can cause instability and lead to limitation of the hydrophobicity, optical properties, and flexibility due to liquid layer evaporation. Here, we report a strategy for controllable liquid‐layer formation on smooth surfaces (R _rms < 1 nm) by π ‐electron interactions. Using this technology, superoleophilic wetting of decyltrimethoxysilane results in the design of a surface with π ‐interaction liquid adsorption, smoothness, and hydrophobicity (SPLASH), that shows extraordinary hydrophobicity (CAH = 0.75°), and stable repellence for various water‐based solutions including micrometer‐sized mist. The smoothness of the solid under a liquid layer enabled the SPLASH to exhibit stable hydrophobicity, transparency (>90%), structure damage durability and flexibility, regardless of the liquid layer thickness by bending or evaporation. Furthermore, the patterned π ‐electrons' localization on the smooth coating enables controlled liquid‐layer formation and liquid transport. This strategy may provide new insights into designing functional liquid surfaces and our designed surface with multifunctional properties could be developed for various applications.     

Tunable Structural Color Surfaces with Visually Self‐Reporting Wettability

Functional materials with wettability of specific surfaces are important for many areas. Here, a new lubricant‐infused elastic inverse opal is presented with tunable and visually “self‐reporting” surface wettability. The elastic inverse opal films are used to lock in the infused lubricating fluid and construct slippery surfaces to repel droplets of various liquids. The films are stretchable, and the lubricating fluid can penetrate the pores under stretching, leaving the surface layer free of lubrication; the resultant undulating morphology of the inverse opal scaffold topography can reversibly pin droplets on the fluidic film rather than the solid substrate. This mechanical stimulation process provides an effective means of dynamically tuning the surface wettability and the optical transparency of the inverse opal films. In particular, as the adjustments are accompanied by simultaneous deformation of the periodic macroporous structure, the inverse opal films can self‐report on their surface status through visible structural color changes. These features make such slippery structural color materials highly versatile for use in diverse applications.     

Slippery Liquid‐Infused Porous Surfaces that Prevent Microbial Surface Fouling and Kill Non‐Adherent Pathogens in Surrounding Media: A Controlled Release Approach

Many types of slippery liquid‐infused porous surfaces (‘SLIPS’) can resist adhesion and colonization by microorganisms. These ‘slippery’ materials thus offer approaches to prevent fouling on commercial and industrial surfaces. However, while SLIPS can prevent fouling on surfaces to which they are applied, they can currently do little to prevent the proliferation of non‐adherent organisms. Here, multi‐functional SLIPS are reported that address this issue and expand the potential utility of these materials. The approach is based on the release of antimicrobial agents from the porous matrices used to host the infused oil phases. It is demonstrated that SLIPS fabricated from nanoporous polymer multilayers can prevent colonization and biofilm formation by four common fungal and bacterial pathogens, and that the polymer and oil phases comprising these materials can be used to sustain the release of triclosan, a model antimicrobial agent, into surrounding media. This approach improves the inherent anti‐fouling properties of these materials and endows them with the ability to kill non‐adherent pathogens. This strategy has the potential to be general; the strategies and concepts reported here will enable the design of SLIPS with improved anti‐fouling properties and open the door to new applications of slippery liquid‐infused materials that host or release other active agents.     

Microtribology of Aqueous Carbon Nanotube Dispersions

The tribological behavior of carbon nanotubes (CNTs) in aqueous humic acid (HA) solutions was studied using a surface forces apparatus (SFA) and shows promising lubricant additive properties. Adding CNTs to the solution changes the friction forces between two mica surfaces from “adhesion controlled” to “load controlled” friction. The coefficient of friction with either single‐walled (SW) or multi‐walled (MW) CNT dispersions is in the range 0.30–0.55 and is independent of the load and sliding velocity. More importantly, lateral sliding promotes a redistribution or accumulation, rather than squeezing out, of nanotubes between the surfaces. This accumulation reduced the adhesion between the surfaces (which generally causes wear/damage of the surfaces), and no wear or damage was observed during continuous shearing experiments that lasted several hours even under high loads (pressures ～10 MPa). The frictional properties can be understood in terms of the Cobblestone Model where the friction force is related to the fraction of the adhesion energy dissipated during impacts of the nanoparticles. We also develop a simple generic model based on the van der Waals interactions between particles and surfaces to determine the relation between the dimensions of nanoparticles and their tribological properties when used as additives in oil‐ or water‐based lubricants.     

Magnetoresponsive Surfaces for Manipulation of Nonmagnetic Liquids: Design and Applications

Magnetic actuation provides a remote, nondestructive, and real‐time way for controllable liquid manipulation, which has promising technological applications for areas ranging from digital microfluidics, biochemical assays, and microreactors, to liquid collection. However, conventional magnetic liquid manipulation usually relies on incorporating magnetic particles into a liquid to empower its motion in response to an external magnetic field, resulting in considerable limitations of magnetic actuation in various applications. Recently, a range of magnetoresponsive surfaces (MRSs) with elaborately designed surface structures and/or compositions have enabled on‐demand liquid manipulation using magnetic fields, even when the liquids do not contain any magnetic particles. Here, the state‐of‐the‐art of MRSs capable of manipulation of nonmagnetic liquids is reviewed. Their preparation, different manipulation modes, including directed propulsion, spreading, and rebound, and the underlying working mechanisms are discussed. Based on the working principles, MRSs are classified into three categories, including surfaces with magnetic bendable microstructures, surfaces with switchable topographies, and surfaces infused with ferrofluids. Their applications in microfluidics, microreactors, liquid distributors and pumps, fog collection, and anti‐icing are presented. Finally, key challenges and the future prospects of MRSs are provided.     

Remote Photothermal Actuation of Underwater Bubble toward Arbitrary Direction on Planar Slippery Fe3O4‐Doped Surfaces

Unidirectional underwater gas bubble (UGB) transport on a surface is realized by buoyant force or wettability gradient force (F_wet‐grad) derived from a tailored geography. Unfortunately, intentional control of the UGB over transport speed, direction, and routes on horizontal planar surfaces is rarely explored. Herein reported is a light‐responsive slippery lubricant‐infused porous surface (SLIPS) composed of selective lubricants and super‐hydrophobic micropillar‐arrayed Fe₃O₄/polydimethylsiloxane film. Upon this SLIPS, the UGB can be horizontally actuated along arbitrary directions by remotely loading/discharging unilateral near‐infrared (NIR) stimuli. The underlying mechanism is that F_wet‐grad can be generated within 1 s in the presence of a NIR‐trigger due to the photothermal effect of Fe₃O₄. Once the NIR‐stimuli are discharged, F_wet‐grad vanishes to break the UGB on the SLIPS. Moreover, performed are systematic parameter studies to investigate the influence of bubble volume, lubricant rheology, and F_wet‐grad on the UGB steering performance. Fundamental physics renders the achievement of antibuoyancy manipulation of the UGBs on an inclined SLIPS. Significantly, steering UGBs by horizontal SLIPS to configurate diverse patterns, as well as facilitating light‐control‐light optical shutter, is deployed. Compared with the previous slippery surfaces, light‐responsive SLIPS is more competent for manipulating UGBs with controllable transport speed, direction, and routes independent of buoyancy or geography derivative force.     

Lubricant‐Infused Anisotropic Porous Surface Design of Reduced Graphene Oxide Toward Electrically Driven Smart Control of Conductive Droplets' Motion

The study of Nepenthes pitcher plants‐bioinspired anisotropic slippery liquid‐infused porous surfaces (SLIPS) is currently in its infancy. The factors that influence their anisotropic self‐cleaning and electric response of a drop's motion and the mechanism have not been fully elucidated. In order to address these problems, two new types of anisotropic slippery surfaces have been designed by using directional, porous, conductive reduced graphene oxide (rGO) films, and different lubricating fluids (conductive and nonconductive), which are used to study the influencing factors and the mechanism of anisotropic self‐cleaning and electric‐responsive control of a drop's motion. The results demonstrate the anisotropic self‐cleaning property of these two types of SLIPS is closely related to the interaction between liquid drops, lubricating fluids and dirt, and the conductive lubricating fluids filling the rGO porous film can reduce the response voltage of the electrically driven reversible control of a drop's slide. The uniqueness of this research lies in the use of two different lubricating fluids and graphene materials to prepare anisotropic SLIPS, identify the key factors to achieve an electrically driven system. These studies are essential for advancing the application of electronically responsive SLIPS in the fields of liquid directional transportation, microfluidics, microchips, and other related research.     

Mechanical Properties of Highly Porous Super Liquid‐Repellent Surfaces

Surfaces with self‐cleaning properties are desirable for many applications. Conceptually, super liquid‐repellent surfaces are required to be highly porous on the nano‐ or micrometer scale, which inherently makes them mechanically weak. Optimizing the balance of mechanical strength and liquid repellency is a core aspect toward applications. However, quantitative mechanical testing of porous, super liquid‐repellent surfaces is challenging due to their high surface roughness at different length scales and low stress tolerance. For this reason, mechanical testing is often performed qualitatively. Here, the mechanical responses of soot‐templated super liquid‐repellent surfaces are studied qualitatively by pencil and finger scratching and quantitatively by atomic force microscopy, colloidal probe force measurements, and nanoindentation. In particular, colloidal probe force measurements cover the relevant force and length scales. The effective elastic modulus, the plastic work W_plastic and the effective adhesive work W_adhesive are quantified. By combining quantitative information from force measurements with measurements of surface wetting properties, it is shown that mechanical strength can be balanced against low wettability by tuning the reaction parameters.     

Photoswitchable Nanomaterials Based on Hierarchically Organized Siloxane Oligomers

Materials with highly ordered molecular arrangements have the capacity to display unique properties derived from their nanoscale structure. Here, the synthesis and characterization of azobenzene (AZO)‐functionalized siloxane oligomers of discrete length that form photoswitchable supramolecular materials are described. Specifically, synergy between phase segregation and azobenzene crystallization leads to the self‐assembly of an exfoliated 2D crystal that becomes isotropic upon photoisomerization with UV light. Consequently, the material undergoes a rapid athermal solid‐to‐liquid transition which can be reversed using blue light due to the unexpectedly fast 2D crystallization that is facilitated by phase segregation. In contrast, enabling telechelic supramolecular polymerization through hydrogen bonding inhibits azobenzene crystallization, and nanostructured pastes with well‐ordered morphologies are obtained based on phase segregation alone, thus demonstrating block copolymer‐like behavior. Therefore, by tailoring the balance of self‐assembly forces in the azobenzene‐functionalized siloxane oligomers, fast and reversible phase‐changing materials can be engineered with various mechanical properties for applications in photolithography or switchable adhesion to lubricant properties.     

Magnetocontrollable Droplet and Bubble Manipulation on a Stable Amphibious Slippery Gel Surface

Smart manipulation of liquid/bubble transport has garnered widespread attention due to its potential applications in many fields. Designing a responsive surface has emerged as an effective strategy for achieving controllable transport of liquids/bubbles. However, it is still challenging to fabricate stable amphibious responsive surfaces that can be used for the smart manipulation of liquid in air and bubbles underwater. Here, amphibious slippery surfaces are fabricated using magnetically responsive soft poly(dimethylsiloxane) doped with iron powder and silicone oil. The slippery gel surface retains its magnetic responsiveness and demonstrates strong affinity for bubbles underwater but shows small and switching resistance forces with the water droplets in air and bubbles underwater, which is the key factor for achieving the controllable transport of liquids/bubbles. On the slippery gel surface, the sliding behaviors of water droplets and bubbles can be reversibly controlled by alternately applying/removing an external magnetic field. Notably, compared with slippery liquid‐infused porous surfaces, the slippery gel surface demonstrates outstanding stability, whether in air or underwater, even after 100 cycles of alternately applying/removing the magnetic field. This surface shows potential applications in gas/liquid microreactors, gas–liquid mixed fluid transportation, bubble/droplet manipulation, etc.     

Tunable Anisotropic Wettability of Rice Leaf‐Like Wavy Surfaces

Rice leaves can directionally shed water droplets along the longitudinal direction of the leaf. Inspired by the hierarchical structures of rice leaf surfaces, synthetic rice leaf‐like wavy surfaces are fabricated that display a tunable anisotropic wettability by using electrostatic layer‐by‐layer assembly on anisotropic microwrinkled substrates. The nanoscale roughness of the rice leaf‐like surfaces is controlled to yield tunable anisotropic wettability and hydrophobic properties that transitioned between the anisotropic/pinned, anisotropic/rollable, and isotropic/rollable water droplet behavior states. These remarkable changes result from discontinuities in the three‐phase (solid–liquid–gas) contact line due to the presence of air trapped beneath the liquid, which is controlled by the surface roughness of the hierarchical nanostructures. The mechanism underlying the directional water‐rolling properties of the rice leaf‐like surfaces provides insight into the development of a range of innovative applications that require control over directional flow.     

Integration of Self‐Lubrication and Near‐Infrared Photothermogenesis for Excellent Anti‐Icing/Deicing Performance

The economic and safety issues caused by ice accretion have become more and more serious. Except for traditional ways of anti‐icing, such as spraying agents, mechanical/thermal removal, etc., more economic approaches are urgently required. This work demonstrates the conceptual feasibility of using a self‐lubricated photothermal coating for both anti‐icing and deicing function. The coating is generally water repellent and infiltrated with hydrocarbon or perfluorocarbon oils as the lubricant to endow a liquid interface for preventing ice accumulation and minimizing the adhesion of ice on surfaces once it is formed. Fe₃O₄ nanoparticles are added to the film to afford high efficiency photothermal effect under near‐infrared irradiation for rapidly melting the accumulated ice. The conceptual strategy can be easily implemented as a facile method to fabricate analogous sprayed coatings. It represents a major advance to tackle the challenging icing issue that is normally seen as a disaster in everyday life.     

Dynamically Actuated Liquid‐Infused Poroelastic Film with Precise Control over Droplet Dynamics

Traditional dynamic adaptive materials rely on an atomic/molecular mechanism of phase transition to induce macroscopic switch of properties, but only a small number of these materials and a limited responsive repertoire are available. Here, liquid as the adaptive component is utilized to realize responsive functions. Paired with a porous matrix that can be put in motion by an actuated dielectric elastomer film, the uncontrolled global flow of liquid is broken down to well‐defined reconfigurable localized flow within the pores and conforms to the network deformation. A detailed theoretical and experimental study of such a dynamically actuated liquid‐infused poroelastic film is discussed. This system demonstrates its ability to generate tunable surface wettability that can precisely control droplet dynamics from complete pinning, to fast sliding, and even more complex motions such as droplet oscillation, jetting, and mixing. This system also allows for repeated and seamless switch among these different droplet manipulations. These are desired properties in many applications such as reflective display, lab‐on‐a‐chip, optical device, dynamic measurements, energy harvesting, and others.     

Lubricant‐Infused Nanoparticulate Coatings Assembled by Layer‐by‐Layer Deposition

Omniphobic coatings are designed to repel a wide range of liquids without leaving stains on the surface. A practical coating should exhibit stable repellency, show no interference with color or transparency of the underlying substrate and, ideally, be deposited in a simple process on arbitrarily shaped surfaces. We use layer‐by‐layer (LbL) deposition of negatively charged silica nanoparticles and positively charged polyelectrolytes to create nanoscale surface structures that are further surface‐functionalized with fluorinated silanes and infiltrated with fluorinated oil, forming a smooth, highly repellent coating on surfaces of different materials and shapes. We show that four or more LbL cycles introduce sufficient surface roughness to effectively immobilize the lubricant into the nanoporous coating and provide a stable liquid interface that repels water, low‐surface‐tension liquids and complex fluids. The absence of hierarchical structures and the small size of the silica nanoparticles enables complete transparency of the coating, with light transmittance exceeding that of normal glass. The coating is mechanically robust, maintains its repellency after exposure to continuous flow for several days and prevents adsorption of streptavidin as a model protein. The LbL process is conceptually simple, of low cost, environmentally benign, scalable, automatable and therefore may present an efficient synthetic route to non‐fouling materials.     

Fabrication of Frog-Skin-Inspired Slippery Antibiofouling Coatings Through Degradable Block Copolymer Wrinkling

Marine biofouling is a severe problem with a wide-reaching impact on ship maintenance, the economy, and ecosystem safety, among others. Inspired by complex multifunctional frogskins, wrinkled slippery coatings are created that exhibit remarkable antifouling, anti-icing, and self-cleaning properties through a combination of degradable di-block copolymer self-assembly [i.e., polystyrene-b-polylactide (PS-b-PLA)] and hydrolysis-driven dynamic release-induced surface wrinkling. Microwrinkled patterns can generate curved surfaces that are resistant to biofouling. Gyroid-forming PS-b-PLA can be used to produce nanoporous templates with cocontinuous nanochannels, which generate strong capillary forces for trapping and storing infiltrated lubricants. In this study, block-copolymer-derived hierarchically wrinkled slippery liquid-infused nanoporous surfaces (i.e., micro wrinkles with nanochannels infused with slippery fluids) are successfully fabricated after silicone oil infiltration. The antibiofouling performance of these surfaces is examined against different foulers under various conditions. The produced coatings exhibited flexible, stable, transparent, and easily tunable antibiofouling characteristics. In particular, the formation of an eco-friendly silicon-based lubricant layer without the use of fluorinated compounds and costly material precursors is an advantage in industrial practice that can be adopted in various applications, such as fuel transport, self-cleaning windows, anticorrosion protection, nontoxic coatings for medical devices, and optical instruments.     

Microstructured Nematic Liquid Crystalline Elastomer Surfaces with Switchable Wetting Properties

Inspired by the lotus leaf, scientists have developed many superhydrophobic surfaces, some of which show remarkable switching between hydrophobic and hydrophilic state under external stimuli. However, the switch usually relies on the change of chemical properties rather than on the modification of the topographic structure of the surface. In this paper, the roughness‐change‐related switchable wetting properties of microstructured responsive surfaces made of nematic liquid crystalline elastomers (LCEs) is reported. First, various carbonate LC monomers and side‐on LCEs are synthesized with low nematic‐to‐isotropic transition temperature, T_NI. Then, LCEs prepared from 3″‐vinylcarbonyloxypropyl 2,5‐di(4′‐octyloxybenzoyloxy)benzoate monomer, with T_NI of 76 °C and contraction of 34% are used to construct a surface covered with micropillar arrays by using a replica molding technique. The contraction of the micropillars induces a reversible roughness change of the microstructured surface. Water contact angle of this microstructured surface changed with temperature, indicating a successful approach at building a surface with switchable wetting properties.     

Stimuli‐Responsive Liquid Marbles: Controlling Structure,Shape, Stability,and Motion

“Liquid marbles” are liquid‐in‐gas dispersed systems stabilized by hydrophobic solid particles adsorbed at the gas‐liquid interface. The structure, stability and movement of these liquid marbles can be controlled by external stimuli such as pH, temperature, light, magnetic and electric fields, ultrasonic, mechanical stress and organic solvents. Stimuli‐responsive modes can be categorized into five classes: (i) liquid marbles whose stability can be controlled by adsorption/desorption of solid particles to/from liquid surfaces, (ii) liquid marbles that can open and close their particle‐coated surface by moving particles to and from the gas‐liquid surface, (iii) liquid marbles that can move, (iv) liquid marbles that can change their shape and (v) liquid marbles that can be split. As a result of these stimuli‐responsive characteristics, liquid marbles offer potential in the areas of controlled encapsulation, delivery and release.