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.     

Manipulating Bubbles in Aqueous Environment via a Lubricant‐Infused Slippery Surface

Designing functional interfaces to control solid/fluid interactions has emerged as an indispensable strategy for developing advanced materials and optimizing current technologies. Surfaces exhibiting special wettability offer many paradigms for regulating fluid behavior in practical applications including oil–water separation and fog harvesting. Nevertheless, the flexible manipulation of air bubbles under water still has room for further exploration. Here, it is reported that the lubricant‐infused slippery (LIS) surface with water repellency is applicable to manipulate bubbles in an aqueous environment. On the basis of the sufficient bubble adhesion, the shaped LIS tracks can be used in guiding the bubble delivery and facilitating continuous bubble distribution. Through the incorporation of an asymmetrical structure into the LIS surface, a triangle‐shaped bubble holder is capable of controlling a single bubble with ease. Moreover, the LIS surface is integrated with a H₂ microbubble evolving apparatus, demonstrating a potential method for in situ capture and delivery of microbubbles. The current finding reveals the meaningful interaction between underwater bubbles and the LIS surface, providing several examples for the applications of this bubble carrier, which should shed new light on the development of bubble‐controlling interfaces.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Silver‐Nanoparticle‐Colored Cotton Fabrics with Tunable Colors and Durable Antibacterial and Self‐Healing Superhydrophobic Properties

Colored cotton fabrics with satisfactory color fastness as well as durable antibacterial and self‐healing superhydrophobic properties are fabricated via a convenient solution‐dipping method that involves the sequential deposition of branched poly(ethylenimine) (PEI), silver nanoparticles (AgNPs), and fluorinated decyl polyhedral oligomeric silsesquioxane (F‐POSS) on cotton fabrics. The deposited AgNPs with tunable surface plasmon resonance endow the cotton fabrics with abundant color and and antibacterial ability. However, in general, water‐soluble AgNPs cannot be firmly deposited onto cotton fabrics to endure the laundering process. The integration of self‐healing superhydrophobicity into the cotton fabrics by depositing F‐POSS/AgNP/PEI films significantly enhances the color fastness of the AgNPs against laundry and mechanical abrasion, while retaining the antibacterial property of the AgNPs. The F‐POSS/AgNP/PEI‐coated cotton fabric accommodates an abundance of F‐POSS, which autonomically migrates to the cotton surface to repetitively restore its damaged superhydrophobicity. The self‐healing superhydrophobicity of the F‐POSS/AgNPs/PEI‐coated cotton fabric guarantees long‐term protection of the underlying AgNPs against laundry and abrasion and allows the cotton fabric to be cleaned by simple rinsing with water.     

Nature‐Inspired Boiling Enhancement by Novel Nanostructured Macroporous Surfaces

World energy crisis has triggered more attention to energy saving and energy conversion systems. Enhanced surfaces for boiling are among the applications of great interest since they can improve the energy efficiency of heat pumping equipment (i.e., air conditioners, heat pumps, refrigeration machines). Methods that are used to make the state‐of‐the‐art enhanced surfaces are often based on complicated mechanical machine tools, are quite material‐consuming and give limited enhancement of the boiling heat transfer. Here, we present a new approach to fabricate enhanced surfaces by using a simple electrodeposition method with in‐situ grown dynamic gas bubble templates. As a result, a well‐ordered 3D macro‐porous metallic surface layer with nanostructured porosity is obtained. Since the structure is built based on the dynamic bubbles, it is perfect for the bubble generation applications such as nucleate boiling. At heat flux of 1 W cm^‐2, the heat transfer coefficient is enhanced over 17 times compared to a plain reference surface. It's estimated that such an effective boiling surface would improve the energy efficiency of many heat pumping machines with 10–30%. The extraordinary boiling performance is explained based on the structure characteristics.     

Paper‐Based Surfaces with Extreme Wettabilities for Novel,Open‐Channel Microfluidic Devices

In this work, a facile methodology is discussed, involving fluoro‐silanization followed by oxygen plasma etching, for the fabrication of surfaces with extreme wettabilities, i.e., surfaces that display all four possible combinations of wettabilities with water and different oils: hydrophobic–oleophilic, hydrophilic–oleophobic, omniphobic, and omniphilic. Open‐channel, paper‐based microfluidic devices fabricated using these surfaces with extreme wettabilities allow for the localization, manipulation, and transport of virtually all high‐ and low‐surface tension liquids. This in turn expands the utility of paper‐based microfluidic devices to a range of applications never before considered. These include, as demonstrated here, continuous oil–water separation, liquid–liquid extraction, open‐channel microfluidic emulsification, microparticle fabrication, and precise measurement of mixtures' composition. Finally, the biocompatibility of the developed microfluidic devices and their utility for cell patterning are demonstrated.     

Droplets Crawling on Peristome‐Mimetic Surfaces

Controlling the mobility of liquids along surfaces is widely exploited in various technologies to achieve self‐lubrication, phase‐change heat transfer, and microfluidics. Despite commendable progress in directional liquid transport on peristome‐mimetic surfaces, liquid merely spreads directionally with a wetted trail remaining. It is a challenge to achieve directional contracting of spreading liquid at the rear side and ultimately unidirectional motion in bulk from one site to another. Here it is shown that liquids resting on the peristome‐mimetic surfaces can crawl directionally and rapidly in an inchworms‐like way under the action of sudden spontaneous bubbles levitation. Vacuuming or chemical reaction induces sudden nucleation, growth, coalescence (Ostwald ripening process), and rupture of bubbles in the asymmetric microcavities of the peristome‐mimetic surface with directional overpressure beneath the liquid, resulting in the guided contracting and spreading of the liquid. Bubbles regulate this new mode of liquid directional motion. The strategy offers opportunities for liquids directional motion for various applications, such as in microfluidic devices, oil–water separation, and water collection systems.     

Torrent Frog‐Inspired Adhesives: Attachment to Flooded Surfaces

Anatomic differences on the toe pad epithelial cells of torrent and tree frogs (elongated versus regular geometry) are believed to account for superior ability of torrent frogs to attach to surfaces in the presence of running water. Here, the friction properties of artificial hexagonal arrays of polydimethylsiloxane (PDMS) pillars (elongated and regular) in the presence of water are compared. Elongated pillar patterns show significantly higher friction in a direction perpendicular to the long axis. A low bending stiffness of the pillars and a high edge density of the pattern in the sliding direction are the key design criteria for the enhanced friction. The elongated patterns also favor orientation‐dependent friction. These findings have important implications for the development of new reversible adhesives for wet conditions.     

Tuning Specific Biomolecular Interactions Using Electro‐Switchable Oligopeptide Surfaces

The ability to regulate biomolecular interactions on surfaces driven by an external stimuli is of great theoretical interest and practical impact in the biomedical and biotechnology fields. Herein, a new class of responsive surfaces that rely on electro‐switchable peptides to control biomolecular interactions on gold surfaces is presented. This system is based upon the conformational switching of positively charged oligolysine peptides that are tethered to a gold surface, such that bioactive molecular moieties (biotin) incorporated on the oligolysines can be reversibly exposed (bio‐active state) or concealed (bio‐inactive state) on demand, as a function of surface potential. The dynamics of switching the biological properties is studied by observing the binding events between biotin and fluorescently labeled NeutrAvidin. Fluorescence microscope images and surface plasmon resonance spectral data clearly reveal opposite binding behaviors when +0.3 V or ?0.4 V vs. SCE are applied to the surface. High fluorescence intensities are observed for an applied positive potential, while minimal fluorescence is detected for an applied negative potential. Surface plasmon resonance spectroscopy (SPR) results provided further evidence that NeutrAvidin binding to the surface is controlled by the applied potential. A large SPR response is observed when a positive potential is applied on the surface, while a negative applied potential induces over 90% reduction in NeutrAvidin binding.     

Phase‐Separation‐Induced Micropatterned Polymer Surfaces and Their Applications

We report a route to fabricate micropatterned polymer films with micro‐ or nanometer‐scale surface concavities by spreading polymer solutions on a non‐solvent surface. The route is simple, versatile, highly efficient, low‐cost, and easily accessible. The concavity density of the patterned films is tuned from 10⁶ to 10⁹ features cm^–2, and the concavity size is controlled in the range from several micrometers to less than 100 nm, by changing the film‐forming parameters including the polymer concentration, the temperature of the non‐solvent and the interactions between polymer, solvent, and non‐solvent. We further demonstrate that these concavity‐patterned films have significantly enhanced hydrophobicity, owing to the existence of the surface concavities, and their hydrophobicity could be controlled by the concavity density. These films have been used as templates to successfully fabricate convex‐patterned polymer films, inorganic TiO₂ microparticles, and NaCl nanocrystals. Their other potential applications are also discussed.