Direct observation of drops on slippery lubricant-infused surfaces

Antiadhesive Properties of Oil-Infused Gels against the Universal Adhesiveness of Polydopamine

Polydopamine (PDA) is well-known as the first material-independent adhesive, which firmly attaches to various substances, even hydrophobic materials, through strong coordinative interactions between the phenolic hydroxyl groups of PDA and the substances. In contrast, oil-infused materials such as self-lubricating gels (SLUGs) exhibit excellent antiadhesive properties against viscous liquids, ice/snow, (bio)fouling, and so on. In this study, we simply questioned: "What will happen when these two materials with contrary nature meet"? To answer this, we formed a PDA layer on a SLUG surface that exhibits thermoresponsive syneretic properties (release of liquid from the gel matrix to the outer surface) and investigated its interfacial behavior. The oil layer caused by syneresis from the SLUGs at -20 °C was found to show resistance to adhesion of universally adhesive PDA.

High Temperature Durability of Oleoplaned Slippery Copper Surfaces

Slippery surfaces, inspired by the functionality of trapping interfaces of specialized leaves of pitcher plants, have been widely used in self-cleaning, anti-icing, antifrost, and self-healing surfaces. They can be fabricated on metallic surfaces as well, presenting a more durable and low-maintenance anticorrosive surface on metals. However, the lack of studies on the durability of these slippery surfaces at high temperature prohibits their practical deployment in real industrial applications where thermal effects are critical and high temperature conditions are inevitable. We present here a unique fabrication technique of a copper-based oleoplaned slippery surface that has been tested for high temperature durability under repeated thermal cycles. Their slipperiness at high temperatures has also been tested in the absence of the Leidenfrost effect. Our findings suggest that these new substrates can be used for fabricating low maintenance surfaces for high temperature applications or even where the surface undergoes repeated thermal cycles like heat exchanger pipes, utensils, engine casings, and outdoor metallic structures.

Effect of Ageing on the Structure and Properties of Model Liquid-Infused Surfaces

Liquid-infused surfaces (LISs) exhibit unique properties that make them ideal candidates for a wide range of applications, from antifouling and anti-icing coatings to self-healing surfaces and controlled wetting. However, when exposed to realistic environmental conditions, LISs tend to age and progressively lose their desirable properties, potentially compromising their application. The associated ageing mechanisms are still poorly understood, and results reflecting real-life applications are scarce. Here, we track the ageing of a model LIS composed of glass surfaces functionalized with hydrophobic nanoparticles and infused with silicone oil. The LISs are fully submerged in aqueous solutions and exposed to acoustic pressure waves for set time intervals. The ageing is monitored by periodic measurements of the LIS's wetting properties. We also track the changes to the LIS's nanoscale structure. We find that the LISs rapidly lose their slippery properties because of a combination of oil loss, smoothing of the nanoporous functional layer, and substrate degradation when directly exposed to the solution. The oil loss is consistent with water microdroplets entering the oil layer and displacing oil away from the surface. These mechanisms are general and could play a role in the ageing of most LISs.

Printing on liquid elastomers

In recent years the research community has paid significant attention to geometrically engineered materials. These materials derive their unique properties from their structure rather than their chemistry alone. Despite their success in the laboratory, the assembly of such soft functional materials remains an outstanding challenge. Here, we propose a robust fluid-mediated route for the rapid fabrication of soft elastomers architected with liquid inclusions. Our approach consists of depositing water drops at the surface of an immiscible liquid elastomer bath. As the elastomer cures, the drops are encapsulated in the polymer and impart shape and function to the newly formed elastic matrix. Using the framework of fluid mechanics, we show how this type of composite material can be tailored.

Droplets on Lubricant-Infused Surfaces: Combination of Constant Mean Curvature Interfaces with Neumann Triangle Boundary Conditions

Superior mobility of droplets on lubricant-infused surfaces (LIS) has recently attracted significant attention for designing liquid-repellent surfaces. Unlike sessile droplets on flat surfaces wherein the contact line is easily visible in experiments, the contact line on LIS is masked by the lubricant meniscus, and special imaging techniques are required to visualize the hidden droplet-lubricant interface. Moreover, the overall shape deviates significantly from the spherical cap geometry even at very low droplet volumes. These difficulties necessitate the need to model interfaces in order to assess the effect of surface and fluid properties on LIS. In this work, we first numerically simulate the droplet shapes to show that at very small volumes, droplet-air and droplet-lubricant interfaces are constant mean curvature (CMC) interfaces. Moreover, we elucidate that these mean curvatures are related by the ratio of interfacial tensions of the droplet-air and the droplet-lubricant interfaces. These insights reduce the modeling of LIS interfacial profiles to a simplified geometric problem, which is solved using the parametric equations of CMC surfaces along with the angles of the Neumann triangle as the boundary conditions. Predicted profiles of the droplet-air interface as a spherical cap, the droplet-lubricant interface as a nodoid, and the lubricant-air interface as a catenoid/nodoid show good agreement with experimental results in the literature. Importantly, we for the first time provide a framework, which accurately predicts the true contact angle at the hidden solid contact line by just using the information of the top spherical cap portion visible in experiments.

Brushed lubricant-impregnated surfaces (BLIS) for long-lasting high condensation heat transfer

Recently, lubricant-impregnated surfaces (LIS) have emerged as a promising condenser surface by facilitating the removal of condensates from the surface. However, LIS has the critical limitation in that lubricant oil is depleted along with the removal of condensates. Such oil depletion is significantly aggravated under high condensation heat transfer. Here we propose a brushed LIS (BLIS) that can allow the application of LIS under high condensation heat transfer indefinitely by overcoming the previous oil depletion limit. In BLIS, a brush replenishes the depleted oil via physical contact with the rotational tube, while oil is continuously supplied to the brush by capillarity. In addition, BLIS helps enhance heat transfer performance with additional route to droplet removal by brush sweeping. By applying BLIS, we maintain the stable dropwise condensation mode for > 48 hours under high supersaturation levels along with up to 61% heat transfer enhancement compared to hydrophobic surfaces.

Life and death of liquid-infused surfaces: a review on the choice, analysis and fate of the infused liquid layer

We review the rational choice, the analysis, the depletion and the properties imparted by the liquid layer in liquid-infused surfaces – a new class of low-adhesion surface.

Dropwise condensation on solid hydrophilic surfaces

Droplet nucleation and condensation are ubiquitous phenomena in nature and industry. Over the past century, research has shown dropwise condensation heat transfer on nonwetting surfaces to be an order of magnitude higher than filmwise condensation heat transfer on wetting substrates. However, the necessity for nonwetting to achieve dropwise condensation is unclear. This article reports stable dropwise condensation on a smooth, solid, hydrophilic surface (θ_a = 38°) having low contact angle hysteresis (<3°). We show that the distribution of nano- to micro- to macroscale droplet sizes (about 100 nm to 1 mm) for coalescing droplets agrees well with the classical distribution on hydrophobic surfaces and elucidate that the wettability-governed dropwise-to-filmwise transition is mediated by the departing droplet Bond number. Our findings demonstrate that achieving stable dropwise condensation is not governed by surface intrinsic wettability, as assumed for the past eight decades, but rather, it is dictated by contact angle hysteresis.

Liquid infused surfaces with anti-icing properties

Ice accretion on solid surfaces, a ubiquitous phenomenon that occurs in winter, brings much inconvenience to daily life and can even cause serious catastrophes. Icephobic surfaces, a passive way of processing surfaces to prevent surface destruction from ice accumulation, have attracted much attention from scientists because of their special ice-repellent properties, and many efforts have been made to rationally design durable icephobic coatings. This review is aimed at providing a brief and crucial overview of ice formation processes and feasible de-icing strategies. Here, the excellent anti-icing performance of liquid infused surfaces (LIS) inspired from Nepenthes is emphatically introduced. After a short introduction, the recent progresses in ice nucleation theory and ice adhesion decrease mechanism are comprehensively reviewed to gain a general understanding of the long freeze process and low ice adhesion on LIS. Subsequently, the anti-icing performance of LIS is systematically evaluated from four aspects regarding water repellence, condensation-frosting, long freeze process, and low ice adhesion. Finally, this review focuses on discussing the advantages and disadvantages of LIS and the potential measures to eliminate and alleviate these drawbacks.

Dynamics of droplets on cones: self-propulsion due to curvature gradients

We study the dynamics of droplets driven by a gradient of curvature, as may be achieved by placing a drop on the surface of a cone. The curvature gradient induces a pressure gradient within the drop, which in turn leads to spontaneous propulsion of the droplet. To investigate the resulting driving force we perform a series of experiments in which we track a droplet's displacement, s, from the apex of a cone whose surface is treated to exhibit near-zero pinning effects. We find an s ∼ t1/4 scaling at sufficiently late times t. To shed light upon these dynamics, we perform an asymptotic calculation of the equilibrium shape of a droplet on a weakly curved cylinder, deriving the curvature-induced force responsible for its propulsion. By balancing this driving force with viscous dissipation, we recover a differential equation for the droplet displacement, whose predictions are found to be in good agreement with our experimental results.

Droplet Morphology and Mobility on Lubricant-Impregnated Surfaces: A Molecular Dynamics Study

Slippery liquid-infused porous surfaces (SLIPS) are gaining remarkable attention and have advanced performance in many fields. Although all SLIPS are related to lubricant-impregnation within nano/microstructures on a surface, they differ in many aspects, such as the morphology of droplets, the state of cloaking, the wetting edge, and the lubricant thickness. Requirements of the droplet morphology on SLIPS might change according to a specific application. A molecular-dynamics-based numerical model that can correctly simulate SLIPS is developed and is validated by comparing against the theoretical predictions for all possible stable states for a given droplet, lubricant, and solid surface. On the basis of this model, a detailed analysis of the equilibrium states is conducted. In particular, we discover that the four possible stable states on SLIPS predicted by theoretical studies can be extended to eight states by considering the effects of lubricant thickness and surface geometry in addition to the interfacial tension and surface wettability. These findings could be used to determine the conditions under which a thermodynamically stable state exists on SLIPS. The dynamic behavior of a nanodroplet on SLIPS is also studied, which provides insight into how a proper increase in the lubricant thickness might increase the sliding velocity. The above findings and developed model are expected to provide significant guidelines for designing SLIPS.

Slippery Liquid-Attached Surface for Robust Biofouling Resistance

Materials for biodevices and bioimplants commonly suffer from unwanted but unavoidable biofouling problems due to the nonspecific adhesion of proteins, cells, or bacteria. Chemical coating or physical strategies for reducing biofouling have been pursued, yet highly robust antibiofouling surfaces that can persistently resist contamination in biological environments are still lacking. In this study, we developed a facile method to fabricate a highly robust slippery and antibiofouling surface by conjugating a liquid-like polymer layer to a substrate. This slippery liquid-attached (SLA) surface was created via a one-step equilibration reaction by tethering methoxy-terminated polydimethylsiloxane (PDMS-OCH₃) polymer brushes onto a substrate to form a transparent "liquid-like" layer. The SLA surface exhibited excellent sliding behaviors toward a wide range of liquids and small particles and antibiofouling properties against the long-term adhesion of small biomolecules, proteins, cells, and bacteria. Moreover, in contrast to superomniphobic surfaces and liquid-infused porous surfaces (SLIPS) requiring micro/nanostructures, the SLA layer could be obtained on smooth surfaces and maintain its biofouling resistance under abrasion with persistent stability. Our study offers a simple method to functionalize surfaces with robust slippery and antibiofouling properties, which is promising for potential applications including medical implants and biodevices.

Control of Droplet Evaporation on Oil-Coated Surfaces for the Synthesis of Asymmetric Supraparticles

Controlling the droplet evaporation on surfaces is desired to get uniform depositions of materials in many applications, for example, in two- and three-dimensional printing and biosensors. To explore a new route to control droplet evaporation on surfaces and produce asymmetric particles, sessile droplets of aqueous dispersions were allowed to evaporate from surfaces coated with oil films. Here, we applied 1-50 μm thick films of different silicone oils. Two contact lines were observed during droplet evaporation: an apparent liquid-liquid-air contact line and liquid-liquid-solid contact line. Because of the oil meniscus covering part of the rim of the drop, evaporation at the periphery is suppressed. Consequently, the droplet evaporates mainly in the central region of the liquid-air interface rather than at the droplet's edge. Colloidal particles migrate with the generated upward flow inside the droplet and are captured by the receding liquid-air interface. A uniform deposition ultimately forms on the substrate. With this straightforward approach, asymmetric supraparticles have been successfully fabricated independent of particle species.

Formation of Hierarchical Porous Films with Breath-Figures Self-Assembly Performed on Oil-Lubricated Substrates

Hierarchical honeycomb patterns were manufactured with breath-figures self-assembly by drop-casting on the silicone oil-lubricated glass substrates. Silicone oil promoted spreading of the polymer solution. The process was carried out with industrial grade polystyrene and polystyrene with molecular mass M w = 35 , 000 g m o l . Both polymers gave rise to patterns, built of micro and nano-scaled pores. The typical diameter of the nanopores was established as 125 nm. The mechanism of the formation of hierarchical patterns was suggested. Ordering of the pores was quantified with the Voronoi tessellations and calculation of the Voronoi entropy. The Voronoi entropy for the large scale pattern was S v o r = 0.6 - 0.9 , evidencing the ordering of pores. Measurement of the apparent contact angles evidenced the Cassie-Baxter wetting regime of the porous films.

Responsive Ionogel Surface with Renewable Antibiofouling Properties

The synthesis of ionogels with a responsive, self-replenishing surface for combating biofouling is described. Ionogels are prepared by infiltrating poly(vinylidene fluoride-co-hexafluoropropylene) with binary mixtures of ionic liquids (IL): 1-octadecyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([C₁₈ C₁ im][NTf₂ ], melting point T_m = 55 °C) and 1-hexyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([C₆ C₁ im][NTf₂ ], T_m = -9 °C). The IL mixtures release spontaneously from the gel matrix and eventually crystallize on the surface. This leads to self-replenishment of the surface of ionogels even after mechanical damage. The incorporation of [C₆ C₁ im][NTf₂ ] provides the antimicrobial efficacy of ionogels while the crystals of [C₁₈ C₁ im][NTf₂ ] serve as a skeleton maintaining [C₆ C₁ im][NTf₂ ] on the surface. By heating, the ionogel surface transforms from solid to liquid-infused state-the removal of biofilms/bacteria developed under a long time of colonization is facilitated. The antimicrobial efficacy is maintained even after several cycles of biofilm formation and detachment. This work provides an opportunity to apply ionogels as functional coatings with renewable antibiofouling properties.

Guided droplet transport on synthetic slippery surfaces inspired by a pitcher plant

We show how anisotropic, grooved features facilitate the trapping and directed transport of droplets on lubricated, liquid-shedding surfaces. Capillary action pins droplets to topographic surface features, enabling transport along the feature while inhibiting motion across (or detachment from) the feature. We demonstrate the robustness of this capillary-based mechanism for directed droplet transport on slippery surfaces by combining experiments on synthetic, lubricant-infused surfaces with observations on the natural trapping surface of a carnivorous pitcher plant. Controlling liquid navigation on synthetic surfaces promises to unlock significant potential in droplet-based technologies. Our observations also offer novel insight into the evolution of the Nepenthes pitcher plant, indicating that the 'pitfall' trapping mechanism is enhanced by the lubricant-infused, macroscopic grooves on the slippery peristome surface, which guide prey into the trap in a way that is more tightly controlled than previously considered.

Liquid-Infused Surfaces: A Review of Theory, Design, and Applications

Due to inspiration from the Nepenthes pitcher plant, a frontier of devices has emerged with unmatched capabilities. Liquid-infused surfaces (LISs), particularly known for their liquid-repelling behavior under low tilting angles (<5°), have demonstrated a plethora of applications in medical, marine, energy, industrial, and environmental materials. This review presents recent developments of LIS technology and its prospective to define the future direction of this technology in solving tomorrow's real-life challenges. First, an introduction to the different models explaining the physical phenomena of these surfaces, their wettability, and viscous-dependent frictional forces is discussed. Then, an outline of different emerging strategies required to fabricate a stable liquid-infused interface is presented, including different substrates, lubricants, surface chemistries, and design parameters which can be tuned depending on the application. Furthermore, applications of LIS coatings in the areas of anticorrosion, antifouling, anti-icing, self-healing, droplet manipulation, and biomedical devices will be presented followed by the limitations and future direction of this technology.

Hydrophobic Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have diverse potential applications in catalysis, gas storage, separation, and drug delivery because of their nanoscale periodicity, permanent porosity, channel functionalization, and structural diversity. Despite these promising properties, the inherent structural features of even some of the best-performing MOFs make them moisture-sensitive and unstable in aqueous media, limiting their practical usefulness. This problem could be overcome by developing stable hydrophobic MOFs whose chemical composition is tuned to ensure that their metal-ligand bonds persist even in the presence of moisture and water. However, the design and fabrication of such hydrophobic MOFs pose a significant challenge. Reported syntheses of hydrophobic MOFs are critically summarized, highlighting issues relating to their design, characterization, and practical use. First, wetting of hydrophobic materials is introduced and the four main strategies for synthesizing hydrophobic MOFs are discussed. Afterward, critical challenges in quantifying the wettability of these hydrophobic porous surfaces and solutions to these challenges are discussed. Finally, the reported uses of hydrophobic MOFs in practical applications such as hydrocarbon storage/separation and their use in separating oil spills from water are summarized. Finally, the state of the art is summarized and promising future developments of hydrophobic MOFs are highlighted.

Self-Sustained Cascading Coalescence in Surface Condensation

Sustained dropwise condensation of water requires rapid shedding of condensed droplets from the surface. Here, we elucidate a microfluidic mechanism that spontaneously sweeps condensed microscale droplets without the need for the traditional droplet removal pathways such as use of superhydrophobicity for droplet rolling and jumping and utilization of wettability gradients for directional droplet transport among others. The mechanism involves self-generated, directional, cascading coalescence sequences of condensed microscale droplets along standard hydrophobic microgrooves. Each sequence appears like a spontaneous zipping process, can sweep droplets along the microgroove at speeds of up to ∼1 m/s, and can extend for lengths more than 100 times the microgroove width. We investigate this phenomenon through high-speed in situ microscale condensation observations and demonstrate that it is enabled by rapid oscillations of a condensate meniscus formed locally in a filled microgroove and pinned on its edges. Such oscillations are in turn spontaneously initiated by coalescence of an individual droplet growing on the ridge with the microgroove meniscus. We quantify the coalescence cascades by characterizing the size distribution of the swept droplets and propose a simple analytical model to explain the results. We also demonstrate that, as condensation proceeds on the hydrophobic microgrooved surface, the coalescence cascades recur spontaneously through repetitive dewetting of the microgrooves. Lastly, we identify surface design rules for consistent realization of the cascades. The hydrophobic microgrooved textures required for the activation of this mechanism can be realized through conventional, scalable surface fabrication methods on a broad range of materials (we demonstrate with aluminum and silicon), thus promising direct application in a host of phase-change processes.

Nature-Inspired Liquid Infused Systems for Superwettable Surface Energies

The development of an innovative interfacial wetting strategy known as liquid infused systems offers great promise for the advanced design of superwetting and superantiwetting substrates to overcome the drawbacks of textured surfaces classified under the heading of Cassie/Wenzel states. The potential value of nature-inspired surfaces has significant potential to address scientific and technological challenges within the field of interfacial chemistry. The objective of the current review is to provide insights into a fruitful and young field of research, highlight its historical developments, examine its nature-inspired design principles, gauge recent progress in emerging applications, and offer a fresh perspective for future research.

Microdroplet self-propulsion during dropwise condensation on lubricant-infused surfaces

Water vapor condensation is common in nature and widely used in industrial applications, including water harvesting, power generation, and desalination. As compared to traditional filmwise condensation, dropwise condensation on lubricant-infused surfaces (LIS) can lead to an order-of-magnitude increase in heat transfer rates. Small droplets (D ≤ 100 μm) account for nearly 85% of the total heat transfer and droplet sweeping plays a crucial role in clearing nucleation sites, allowing for frequent re-nucleation. Here, we focus on the dynamic interplay of microdroplets with the thin lubricant film during water vapor condensation on LIS. Coupling high-speed imaging, optical microscopy, and interferometry, we show that the initially uniform lubricant film re-distributes during condensation. Governed by lubricant height gradients, microdroplets as small as 2 μm in diameter undergo rigorous and gravity-independent self-propulsion, travelling distances multiples of their diameters at velocities up to 1100 μm s-1. Although macroscopically the movement appears to be random, we show that on a microscopic level capillary attraction due to asymmetrical lubricant menisci causes this gravity-independent droplet motion. Based on a lateral force balance analysis, we quantitatively find that the sliding velocity initially increases during movement, but decreases sharply at shorter inter-droplet spacing. The maximum sliding velocity is inversely proportional to the oil viscosity and is strongly dependent of the droplet size, which is in excellent agreement with the experimental observations. This novel and non-traditional droplet movement is expected to significantly enhance the sweeping efficiency during dropwise condensation, leading to higher nucleation and heat transfer rates.

Mobility of Aqueous and Binary Mixture Drops on Lubricating Fluid-Coated Slippery Surfaces

The mobility of liquid drops on lubricant-infused slippery surfaces depends strongly on various system parameters, for example, surface energy and roughness of the underlying solid surface and surface tension and viscosity of the test and the lubricating fluids. Here, we investigate lubricant-coated slippery surfaces fabricated on smooth hydrophobic solid surfaces and examine the influence of thickness and viscosity of the lubricating oil on the velocity of aqueous drops. We also investigate the effect of surface tension of the test liquid using a binary mixture of water and ethanol, on the apparent contact angle, which further affects their slip velocity. A theoretical model, based on various dissipative forces acting in different regions of the lubricating oil and a test drop, is also presented, which elucidates the dependence of drop velocity on lubricating oil viscosity and base radius of drops of test liquids.

Flow-Induced Long-Term Stable Slippery Surfaces

Slippery lubricant-infused surfaces allow easy removal of liquid droplets on surfaces. They consist of textured or porous substrates infiltrated with a chemically compatible lubricant. Capillary forces help to keep the lubricant in place. Slippery surfaces hold promising prospects in applications including drag reduction in pipes or food packages, anticorrosion, anti-biofouling, or anti-icing. However, a critical drawback is that shear forces induced by flow lead to depletion of the lubricant. In this work, a way to overcome the shear-induced lubricant depletion by replenishing the lubricant from the flow of emulsions is presented. The addition of small amounts of positively charged surfactant reduces the charge repulsion between the negatively charged oil droplets contained in the emulsion. Attachment and coalescence of oil droplets from the oil-in-water emulsion at the substrate surface fills the structure with the lubricant. Flow-induced lubrication of textured surfaces can be generalized to a broad range of lubricant-solid combinations using minimal amounts of oil.

Mobility of Yield Stress Fluids on Lubricant-Impregnated Surfaces

A common problem which we encounter on a daily basis is dispensing of yield stress fluids such as condiments, lotions, toothpaste, etc. from containers. Beyond consumer products, assuring the flow of yield stress fluids such as crude oil, mud, blood, paint, pharmaceutical products, and others, is essential for the respective industries. Elimination of wall-induced friction can lead to significant savings in the energy required for flow of yield stress fluids, as well as associated product loss and cleaning costs. Lubricant-impregnated surfaces (LIS) have been shown to change the dynamic behavior of yield stress fluids and enable them to flow without shearing. Despite the wide applicability of this technology and its general appeal, the fundamental physics governing the flow of yield stress fluids on LIS have not yet been fully explained. In this work, we study the mobility of yield stress fluids on LIS, and explain the relationship between their macroscale flow behavior and the microscale properties of LIS. We show that for yield stress fluids the thermodynamic state of an LIS can be the difference between mobility and immobility. We demonstrate that LIS can induce mobility in yield stress fluids even below their yield stress allowing them to move as a plug without shearing with an infinite slip length. We identify different mobility mechanisms and establish a regime map for drag reduction in terms of the shear stress to yield stress ratio and the microscopic properties of the LIS. We demonstrate these regimes in a practical application of pipe flow thereby providing key insights for the design of LIS to induce mobility of yield stress fluids in a broad range of practical applications.

Liquid-Repellent Metal Oxide Photocatalysts

Metal oxide photocatalysts (MOPCs) decompose organic molecules under illumination. However, the application of MOPCs in industry and research is currently limited by their intrinsic hydrophilicity because MOPCs can be wetted by most liquids. To achieve liquid repellency, the surface needs to possess a low surface energy, but most organic molecules with low surface energy are degraded by photocatalytic activity. Herein, current methods to achieve liquid repellency on MOPCs, while preventing degradation of hydrophobic coatings, are reviewed. Classically, composite materials containing MOPCs and hydrophobic organic compounds possess good liquid repellency. However, composites normally form irregular coatings and are hard to prepare on surfaces such as those that are mesoporous or nanostructured. In addition, the adhesion of composites to substrates is often weak, resulting in delamination. Recent studies have shown that the direct grafting reaction of polydimethylsiloxane (PDMS) from silicone oil (methyl-terminated PDMS) under illumination results in a stable polymer brush. This easy and simple grafting method allows us to create stable liquid-repellent surfaces on MOPCs of various types, structures, and sizes. In particular, super-liquid-repellent drops with an underlying air layer can be created on PDMS-grafted nano-/microstructured MOPCs. Potential applications of surfaces combining liquid repellency and photocatalytic activity are also discussed; thus offering new ways of using MOPCs in a wider range of applications.

Humidity-tolerant rate-dependent capillary viscous adhesion of bee-collected pollen fluids

We report a two-phase adhesive fluid recovered from pollen, which displays remarkable rate tunability and humidity stabilization at microscopic and macroscopic scales. These natural materials provide a previously-unknown model for bioinspired humidity-stable and dynamically-tunable adhesive materials. In particular, two immiscible liquid phases are identified in bioadhesive fluid extracted from dandelion pollen taken from honey bees: a sugary adhesive aqueous phase similar to bee nectar and an oily phase consistent with plant pollenkitt. Here we show that the aqueous phase exhibits a rate-dependent capillary adhesion attributed to hydrodynamic forces above a critical separation rate. However, the performance of this adhesive phase alone is very sensitive to humidity due to water loss or uptake. Interestingly, the oily phase contributes scarcely to the wet adhesion. Rather, it spreads over the aqueous phase and functions as a barrier to water vapor that tempers the effects of humidity changes and stabilizes the capillary adhesion.

How Slippery are SLIPS? Measuring Effective Slip on Lubricated Surfaces with Colloidal Probe Atmoc Force Microscopy

Lubricant-infused surfaces have attracted great attention recently and are described as slippery liquid-infused porous surfaces (SLIPS). Here, we measured the hydrodynamic drainage forces on SLIPS by colloid probe atomic force microscopy (AFM) and quantified the effective slip length over a nanothin silicone oil layer on hydrophobized [octadecyltrichlorosilane (OTS)-coated] silicon wafers. The thickness of a stable silicone oil film on OTS-Si under sucrose solution was determined to be 1.8 ± 1.3 nm and was found to induce an average effective slip length of 29 ± 3 nm, very close to that of an uninfused OTS substrate. These relatively low values of effective slip are confirmed by the relatively large macroscopic roll-off angle values of water droplets on the same substrates. Both nano- and macroscale results reflect the immobilized nature of a silicone oil layer of thickness around 2 nm within an underlying monolayer. These results have important implications in the design of drag-reducing coatings using lubricant infusion.

Wettability-Independent Droplet Transport by Bendotaxis

We demonstrate "bendotaxis," a novel mechanism for droplet self-transport at small scales. A combination of bending and capillarity in a thin channel causes a pressure gradient that, in turn, results in the spontaneous movement of a liquid droplet. Surprisingly, the direction of this motion is always the same, regardless of the wettability of the channel. We use a combination of experiments at a macroscopic scale and a simple mathematical model to study this motion, focusing in particular on the timescale associated with the motion. We suggest that bendotaxis may be a useful means of transporting droplets in technological applications, e.g., in developing self-cleaning surfaces, and discuss the implications of our results for such applications.

Motion of Viscous Droplets in Rough Confinement: Paradoxical Lubrication

We study the sedimentation of highly viscous droplets confined inside Hele-Shaw cells with textured walls of controlled topography. In contrast with common observations on superhydrophobic surfaces, roughness tends here to significantly increase viscous friction, thus substantially decreasing the droplets mobility. However, reducing confinement induces a jump in the velocity as droplets can slide on a lubricating layer of the surrounding fluid thicker than the roughness features. We demonstrate that increasing the viscosity of the surrounding liquid may counterintuitively enhance the mobility of a droplet sliding along a rough wall. Similarly, a sharp change of the droplet mobility is observed as the amplitude of the roughness is modified. These results illustrate the nontrivial friction processes at the scale of the roughness, and the coupling between viscous dissipation in the drop, in the front meniscus, and in the lubricating film. They could enable one to specifically control the speed of droplets or capsules in microchannels, based on their rheological properties.

Physics of pre-wetted, lubricated and impregnated surfaces: a review

Wetting phenomena occurring on pre-wetted flat and rough solid surfaces are reviewed. The wetting of lubricated flat surfaces is strongly correlated with the tribological properties of a solid/lubricant pair. The phenomena taking place on micro- and nano-rough oil-impregnated surfaces have attracted the attention of the scientific community due to their numerous promising applications as omniphobic, self-healing, anti-icing and anti-bacterial interfaces. On the other hand, these phenomena are rich in their physical content. The effects observed on natural and artificial, bioinspired oil-impregnated surfaces are discussed, including electrowetting of oil-impregnated surfaces, enabling low-voltage reversible control of the droplet shape. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology'.

Directional pumping of water and oil microdroplets on slippery surface

Transporting water and oil microdroplets is important for applications ranging from water harvesting to biomedical analysis but remains a great challenge. This is due to the amplified contact angle hysteresis and insufficient driving force in the micrometer scale, especially for low-surface energy oil droplets. Coalescence of neighboring droplets, which releases vast additional surface energy, was often required, but its relatively uncontrollable nature brings uncertainties to the droplet motion, and the methodology is not applicable to single droplets. Here we introduce a strategy based on slippery surface with immobilized lubricant menisci to directionally transport microdroplets. By simply mounting hydrogel dots on slippery surface, the raised menisci remotely pump microdroplets via capillary force with high efficiency, regardless of droplet size or surface energy. By proof-of-concept experiments, we demonstrate that our method allows for highly efficient water droplet collection and highly sensitive biomedical analyte detection.

Gibbsian Thermodynamic Study of Capillary Meniscus Depth

In the presence of gravity or other external fields, liquid surface curvature deviates from a spherical shape and the surface configuration can be found by numerical integration of the Young–Laplace equation and the typical initial point for integration is the apex of the interface. The meniscus shape in large Bond number systems, which have the central portion of the interface flattened, cannot be determined with the apex as the initial point for integration. Here we find the depth of capillary menisci by considering an initial point for integration to be at the three-phase-contact-line (TPCL) and evaluate the curvature at the TPCL by free energy analysis and inspect the effect of different parameters on the interface shape. A new parameter—which is the deviation of equilibrium curvature at the TPCL from the spherical shape (SR)—is introduced and inspected and it was found that at a Bond number of 13 the maximum deviation, approximately 0.8 of spherical curvature, takes place while for large enough Bond numbers the curvature at the three-phase contact line is near the spherical shape (0.95 < SR < 1). A potential application of this approach is to measure the capillary rise at the TPCL to find the surface tension in high Bond number systems such as those with low surface/interfacial tensions.

Bioinspired Ultra-Low Adhesive Energy Interface for Continuous 3D Printing: Reducing Curing Induced Adhesion

Additive manufacturing based on liquid resin curing is one of the most promising methods to construct delicate structures. However, precision and speed are limited by the vertical adhesion of in situ cured resin at the curing interface. To overcome the unavoidable adhesion and to develop a general curing interface, we propose a slippery surface taking inspiration of the peristome surface of the pitcher plant. Such surface shows ultra-low adhesive energy at the curing interface due to the inhibition of the direct contact between the cured resin and the solid surface, which also increases the refilling speed of liquid resin. This ultra-low adhesive energy interface is effective for continuous 3D printing and provides insights into the physical mechanisms in reducing vertical solid-solid interfacial adhesion.

Designing Liquid-Infused Surfaces for Medical Applications: A Review

The development of new technologies is key to the continued improvement of medicine, relying on comprehensive materials design strategies that can integrate advanced therapeutic and diagnostic functions with a variety of surface properties such as selective adhesion, dynamic responsiveness, and optical/mechanical tunability. Liquid-infused surfaces have recently come to the forefront as a unique approach to surface coatings that can resist adhesion of a wide range of contaminants on medical devices. Furthermore, these surfaces are proving highly versatile in enabling the integration of established medical surface treatments alongside the antifouling capabilities, such as drug release or biomolecule organization. Here, the range of research being conducted on liquid-infused surfaces for medical applications is presented, from an understanding of the basics behind the interactions of physiological fluids, microbes, and mammalian cells with liquid layers to current applications of these materials in point-of-care diagnostics, medical tubing, instruments, implants, and tissue engineering. Throughout this exploration, the design parameters of liquid-infused surfaces and how they can be adapted and tuned to particular applications are discussed, while identifying how the range of controllable factors offered by liquid-infused surfaces can be used to enable completely new and dynamic approaches to materials and devices for human health.

Droplets on Slippery Lubricant-Infused Porous Surfaces: A Macroscale to Nanoscale Perspective

A recent design approach in creating super-repellent surfaces through slippery surface lubrication offers tremendous liquid-shedding capabilities. Previous investigations have provided significant insights into droplet-lubricant interfacial behaviors that govern antiwetting properties but have often studied using macroscale droplets. Despite drastically different governing characteristics of ultrasmall droplets on slippery lubricated surfaces, little is known about the effects at the micro- and nanoscale. In this investigation, we impregnate a three-dimensionally, well-ordered porous metal architecture with a lubricant to confirm durable slippery surfaces. We then reduce the droplet size to a nanoliter range and experimentally compare the droplet behaviors at different length scales. By experimentally varying the lubricant thickness levels, we also reveal that the effect of lubricant wetting around ultrasmall droplets is intensely magnified, which significantly affects the transient droplet dynamics. Molecular dynamics computations further examine the ultrasmall droplets with varying lubricant levels or pore cut levels at the nanoscale. The combined experimental and computational work provides insights into droplet interfacial phenomena on slippery surfaces from a macroscale to nanoscale perspective.

Mapping Depletion of Lubricant Films on Antibiofouling Wrinkled Slippery Surfaces

Slippery liquid infused porous surfaces (SLIPS) have recently gained a lot of attention because of their wide range of applications. We recently showed that SLIPS with most of their surface depleted of lubricant, as little lubricant as 0.02 ± 0.01 μL cm^-1, were effective against marine biofouling. Characterization of the depletion and configuration of the immobilized liquid layer on SLIPS is crucial to optimizing their performance. Previous attempts at mapping lubricant thickness have been diffraction limited or indirectly measured thickness. Here, we use atomic force microscopy meniscus force measurements to directly map lubricant thickness with nanoscale resolution on wrinkled surfaces made from Teflon and poly(4-vinylpyridine) (P4VP). Using this method, we show that SLIPS are easily depleted and are effectively heterogeneous surfaces, where the majority of the surface is a thick lubricating layer stabilized by capillary forces and part nanothin layer stabilized long-range intermolecular forces. We found that the depleted silicone oil thickness on the tops of nonwettable (Teflon) wrinkles is approx. 5 nm, close to but greater than the minimum measurable thickness of approx. 3 nm. The silicone oil thickness on the tops of wettable (P4VP) wrinkles is approx. 15 nm. Surfaces in this state still show antibiofouling properties and thus show that a thick lubricating layer is not necessary for all favorable properties of SLIPS.

Omniphobic Metal Surfaces with Low Contact Angle Hysteresis and Tilt Angles

Various metal (Al, Ti, Fe, Ni, and Cu) surfaces with native oxide layers were rendered "omniphobic" by a simple thermal treatment of neat liquid trimethylsiloxy-terminated polymethylhydrosiloxanes (PMHSs) with a range of different molecular weights (MWs). Because of this treatment, the PMHS chains were covalently attached to the oxidized metal surfaces, giving 2-10 nm thick PMHS layers. The resulting surfaces were fairly smooth, liquid-like, and showed excellent dynamic omniphobicity with both low contact angle hysteresis (≲5°) and substrate tilt angles (≲8°) toward small-volume liquid drops (5 μL) with surface tensions ranging from 20.5 to 72.8 mN/m. Droplet mobility was improved overall as a result of heating the substrates to 70 °C. The reaction kinetics and final dynamic dewetting properties were found to be not dependent of the types of metals employed or MWs of PMHS, but mainly dominated by both reaction temperatures and reaction times.

In-situ ATR-FTIR for dynamic analysis of superhydrophobic breakdown on nanostructured silicon surfaces

Superhydrophobic surfaces are highly promising for self-cleaning, anti-fouling and anti-corrosion applications. However, accurate assessment of the lifetime and sustainability of super-hydrophobic materials is hindered by the lack of large area characterization of superhydrophobic breakdown. In this work, attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR) is explored for a dynamic study of wetting transitions on immersed superhydrophobic arrays of silicon nanopillars. Spontaneous breakdown of the superhydrophobic state is triggered by in-situ modulation of the liquid surface tension. The high surface sensitivity of ATR-FTIR allows for accurate detection of local liquid infiltration. Experimentally determined wetting transition criteria show significant deviations from predictions by classical wetting models. Breakdown kinetics is found to slow down dramatically when the liquid surface tension approaches the transition criterion, which clearly underlines the importance of more accurate wetting analysis on large-area surfaces. Precise actuation of the superhydrophobic breakdown process is demonstrated for the first time through careful modulation of the liquid surface tension around the transition criterion. The developed ATR-FTIR method can be a promising technique to study wetting transitions and associated dynamics on various types of superhydrophobic surfaces.

Dynamics of Ferrofluid Drops on Magnetically Patterned Surfaces

The motion of liquid drops on solid surfaces is attracting a lot of attention because of its fundamental implications and wide technological applications. In this article, we present a comprehensive experimental study of the interaction between gravity-driven ferrofluid drops on very slippery oil-impregnated surfaces and a patterned magnetic field. The drop speed can be accurately tuned by the magnetic interaction, and more interestingly, drops are found to undergo a stick-slip motion whose contrast and phase can be easily tuned by changing either the strength of the magnetic field or the ferrofluid concentration. This motion is the result of the periodic modulation of the external magnetic field and can be accurately analyzed because the intrinsic pinning due to chemical defects is negligible on oil-impregnated surfaces.

Drop Dynamics on Liquid-Infused Surfaces: The Role of the Lubricant Ridge

We employ a free-energy lattice-Boltzmann method to study the dynamics of a ternary fluid system consisting of a liquid drop driven by a body force across a regularly textured substrate, infused by a lubricating liquid. We focus on the case of partial wetting lubricants and observe a rich interplay between contact line pinning and viscous dissipation at the lubricant ridge, which become dominant at large and small apparent angles, respectively. Our numerical investigations further demonstrate that the relative importance of viscous dissipation at the lubricant ridge depends on the drop to lubricant viscosity ratio, as well as on the shape of the wetting ridge.

Multifunctional ferrofluid-infused surfaces with reconfigurable multiscale topography

Developing adaptive materials with geometries that change in response to external stimuli provides fundamental insights into the links between the physical forces involved and the resultant morphologies and creates a foundation for technologically relevant dynamic systems^1,2. In particular, reconfigurable surface topography as a means to control interfacial properties³ has recently been explored using responsive gels⁴, shape-memory polymers⁵, liquid crystals^6-8 and hybrid composites^9-14, including magnetically active slippery surfaces^12-14. However, these designs exhibit a limited range of topographical changes and thus a restricted scope of function. Here we introduce a hierarchical magneto-responsive composite surface, made by infiltrating a ferrofluid into a microstructured matrix (termed ferrofluid-containing liquid-infused porous surfaces, or FLIPS). We demonstrate various topographical reconfigurations at multiple length scales and a broad range of associated emergent behaviours. An applied magnetic-field gradient induces the movement of magnetic nanoparticles suspended in the ferrofluid, which leads to microscale flow of the ferrofluid first above and then within the microstructured surface. This redistribution changes the initially smooth surface of the ferrofluid (which is immobilized by the porous matrix through capillary forces) into various multiscale hierarchical topographies shaped by the size, arrangement and orientation of the confining microstructures in the magnetic field. We analyse the spatial and temporal dynamics of these reconfigurations theoretically and experimentally as a function of the balance between capillary and magnetic pressures^15-19 and of the geometric anisotropy of the FLIPS system. Several interesting functions at three different length scales are demonstrated: self-assembly of colloidal particles at the micrometre scale; regulated flow of liquid droplets at the millimetre scale; and switchable adhesion and friction, liquid pumping and removal of biofilms at the centimetre scale. We envision that FLIPS could be used as part of integrated control systems for the manipulation and transport of matter, thermal management, microfluidics and fouling-release materials.

Directional Passive Transport of Microdroplets in Oil-Infused Diverging Channels for Effective Condensate Removal

Condensation widely exists in nature and industry, and its performance heavily relies on the efficiency of condensate removal. Recent advances in micro-/nanoscale surface engineering enable condensing droplet removal from solid surfaces without extra energy cost, but it is still challenging to achieve passive transport of microdroplets over long distances along horizontal surfaces. The mobility of these condensate droplets can be enhanced by lubricant oil infusion on flat surfaces and frequent coalescence, which lead to fast growth but random motion of droplets. In this work, we propose a novel design of diverging microchannels with oil-infused surfaces to achieve controllable, long-distance, and directional transport of condensing droplets on horizontal surfaces. This idea is experimentally demonstrated with diverging copper and silicon microchannels with nanoengineered surfaces. Along these hierarchical surface structures, microdroplets condense on the top channel wall and submerge into microchannels owing to the capillary pressure gradient in infusing oil. Confined by the microchannel walls, the submerged droplets deform and maintain the back-front curvature difference, which enables the motion of droplets along the channel diverging direction. Subsequent droplet coalescences inside the channel further enhance this directional transport. Moreover, fast-moving deformed droplets transfer their momentum to downstream spherical droplets through the infusing oil. As a result, simultaneous passive transport of multiple droplets (20-400 μm) is achieved over long distances (beyond 7 mm). On these oil-infused surfaces, satellite microdroplets can further nucleate and grow on an oil-cloaked droplet, demonstrating an enlarged surface area for condensation. Our findings on passive condensate removal offer great opportunities in condensation enhancement, self-cleaning, and other applications requiring directional droplet transport along horizontal surfaces.

Origins of Extreme Liquid Repellency on Structured, Flat, and Lubricated Hydrophobic Surfaces

There are currently three main classes of liquid-repellent surfaces: micro- or nanostructured superhydrophobic surfaces, flat surfaces grafted with "liquidlike" polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measure the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities U=0.01-1  mm/s using a cantilever force sensor with submicronewton accuracy and correlate it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal (<0.1  s) resolutions. We find that the dissipative force-due to very different physical mechanisms at the contact line-is independent of velocity on superhydrophobic surfaces but depends nonlinearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.