Slippery Liquid-infused Surfaces Research Articles

Self-regulated secretory materials for long-term icephobicity

Passive icephobic coatings attract increasing attention due to their harmless strategy for preventing undesirable ice accumulation. Slippery liquid-infused surfaces display extremely low ice adhesion (τice) but are argued for their poor stabilities and longevities due to inevitable liquid consumption. Herein we reported a class of lubricated polysiloxane coatings that can maintain low τice (∼2.2 kPa) for a long time (>800 icing/deicing cycle). The coatings have slippery lubricated surfaces and switchable porous matrices loading a large amount of liquid in isolated porevoids. Such droplet-embedded structure allows the surfaces to continuously maintain highly swelling states in a self-adaptive manner, i.e., only in the conditions icing or oil consumption occur dose oil is released, and thus show excellent long-term icephobicity. Besides, these materials exhibit good mechanical properties, antifatigue, and substrate adhesion. Because the coatings can be prepared via facile and green method from cheap starting materials, we foresee their broad application prospect in many fields.

Silicon gels with sustainable self-replenishment: Fluid drag reduction through viscosity dependency

Slippery liquid infused surfaces (SLISs) provide excellent dynamic lubrication properties by storing the lubricant internally to form a homogeneous liquid layer to insulate external ambient fluids. However, the kinetic instability caused by lubricant depletion gradually degrades the surface slippery performance. Herein, inspired by skin gland secretion system, silicon gels with sustainable self-replenishment were prepared using polydimethylsiloxane (PDMS) and commercially available dimethyl silicone oils. By carefully tuning the kinematic viscosity and content of infused oil, the surfaces of corresponding silicon gels demonstrated significant syneresis behavior, and the lubricating oils released from the surface were still detected after continuous wiping for more than 120 days. More importantly, when the ratio of working fluid to infused oil was 6.8, the maximum drag reduction of silicon gel in Couette flow could reach up to about 16%, while the surfaces remained long-lasting water repellency under the continuous shear action of encapsulated water. Self-replenishable silicon gels exhibiting strong kinetic stability were expected to provide valuable design insights for robust slippery interfaces.

Droplet Self-Propulsion on Slippery Liquid-Infused Surfaces with Dual-Lubricant Wedge-Shaped Wettability Patterns.

Young's equation is fundamental to the concept of the wettability of a solid surface. It defines the contact angle for a droplet on a solid surface through a local equilibrium at the three-phase contact line. Recently, the concept of a liquid Young's law contact angle has been developed to describe the wettability of slippery liquid-infused porous surfaces (SLIPS) by droplets of an immiscible liquid. In this work, we present a new method to fabricate biphilic SLIP surfaces and show how the wettability of the composite SLIPS can be exploited with a macroscopic wedge-shaped pattern of two distinct lubricant liquids. In particular, we report the development of composite liquid surfaces on silicon substrates based on lithographically patterning a Teflon AF1600 coating and a superhydrophobic coating (Glaco Mirror Coat Zero), where the latter selectively dewets from the former. This creates a patterned base surface with preferential wetting to matched liquids: the fluoropolymer PTFE with a perfluorinated oil Krytox and the hydrophobic silica-based GLACO with olive oil (or other mineral oils or silicone oil). This allows us to successively imbibe our patterned solid substrates with two distinct oils and produce a composite liquid lubricant surface with the oils segregated as thin films into separate domains defined by the patterning. We illustrate that macroscopic wedge-shaped patterned SLIP surfaces enable low-friction droplet self-propulsion. Finally, we formulate an analytical model that captures the dependence of the droplet motion as a function of the wettability of the two liquid lubricant domains and the opening angle of the wedge. This allows us to derive scaling relationships between various physical and geometrical parameters. This work introduces a new approach to creating patterned liquid lubricant surfaces, demonstrates long-distance droplet self-propulsion on such surfaces, and sheds light on the interactions between liquid droplets and liquid surfaces.

Highly stable fluorine-free slippery liquid infused surfaces

A promising Slippery Liquid Infused Surface (SLIS) has been developed, that is both easy-to-produce and environmentally friendly. Using a double replication method, a 200 µm-thick texturized cross-linked PDMS film was obtained, leading to a grid of micrometric hills and deep wells. A comparison between perfluoropolyether (PFPE) lubricant and silicone oil of different viscosities highlights the advantage of silicone oil, which penetrates the cross-linked PDMS matrix even at high viscosity. Chemical modification with OTS (octadecyltrichlorosilane) and introduction of secondary structures through plasma etching, increased the surface's shear resistance. As a result, stable sliding properties were maintained after hundreds of cycles of immersion in water and, more originally still after tests under wind blowing exposure up to 78 m/s. Furthermore, addition of water sprayed during wind blowing exposure allows partial fouling release of the surface.

Robust and durable self-lubricated surface by facile fabrication for anti-icing/deicing

Undesired ice accumulation can lead to serious negative impacts on daily life and equipment safety. Slippery liquid-infused surfaces (SLISs) have been widely studied for their low ice adhesion strength (τice). However, the lack of durability, weak robustness, and complex preparation process hinder the application of SLISs. In this work, robust oil-infused porous surface (RIPS) was obtained by a facile one-step method, which used epoxy resin (E44) as a substrate and contained fumed silica and silicone oil. The RIPS-6 displays outstanding anti-icing/deicing ability, superior liquid repellency, good self-cleaning performance, and excellent mechanical robustness. At −5 °C, the water droplet remained unfrozen after 7200 s. τice was only 6.9 kPa at −20 °C and remained below 15 kPa during 10 icing cycles. Furthermore, the mixture of E44 and fumed silica enhanced the mechanical properties including the hardness (4 H) and abrasion resistance of RIPS-6. The icephobicity can be remained (τice < 40 kPa) even after 150 cycles of sandpaper abrasion at 2.5 kPa. The anti-icing and deicing coating with excellent mechanical stability and durability significantly improves the practical application possibility of the coating in harsh environments.

Durability of Slippery Liquid-Infused Surfaces: Challenges and Advances

Slippery liquid-infused porous surfaces (SLIPS) have emerged as a unique approach to creating surfaces that can resist fouling when placed in contact with aqueous media, organic fluids, or biological organisms. These surfaces are composed of essentially two components: a liquid lubricant that is locked within the protrusions of a textured solid due to capillarity. Drops, immiscible to the lubricant, exhibit high mobility and very-low-contact-angle hysteresis when placed on such surfaces. Moreover, these surfaces are shown to resist adhesion to a wide range of fluids, can withstand high pressure, and are able to self-clean. Due to these remarkable properties, SLIPS are considered a promising candidate for applications such as designing anti-fouling and anti-corrosion surfaces, drag reduction, and fluid manipulation. These collective properties, however, are only available as long as the lubricant remains infused within the surface protrusions. A number of mechanisms can drive the depletion of the lubricant from the interior of the texture, leading to the loss of functionality of SLIPS. Lubricant depletion is one challenge that is hindering the real-world application of these surfaces. This review mainly focuses on the studies conducted in the context of enhancing the lubricant retention abilities of SLIPS. In addition, a concise introduction of wetting transitions on structured as well as liquid-infused surfaces is given. We also discuss, briefly, the mechanisms that are responsible for lubricant depletion.

Hybrid electrodes effective for both electrowetting‐ and dielectrowetting‐driven digital microfluidics

AbstractElectrowetting on dielectric (EWOD) and dielectrowetting (DEW) are two major principles to drive droplets in digital microfluidics. EWOD is effective to manipulate (create, transport, split, and merge) conductive droplets being currently used for many biological, chemical, and optical applications. DEW can also manipulate droplets but more efficiently with dielectric (nonconductive) fluids. A digital microfluidic platform efficiently operable by both EWOD and DEW would offer higher versatility in handling a wide range of fluids, regardless of their conductivities. In this regard, this article presents a new hybrid electrode design enabling EWOD and DEW to drive various kinds of droplet fluids on a single platform. In addition, a slippery liquid‐infused surface (SLIPS) is integrated with the hybrid electrodes. The SLIPS is well known to resist biofouling and repel sticky fluids, which endows the hybrid electrodes with much wider application spectra. As a result, the present SLIPS‐integrated hybrid electrodes facilitate actuating various kinds of fluids which would not be driven by conventional EWOD and/or DEW electrodes. This paper presents the successful transportation of not only conductive fluids including water, protein solution, glycerol, and honey but also nonconductive fluids including dodecane, silicone oil, and light and heavy crude oil, all driven by the SLIPS‐integrated hybrid electrodes. The performance comparisons among solid, interdigitating, and hybrid electrodes are made by testing both conductive and nonconductive droplets.

The preparation of slippery liquid-infused surface and the droplets bounce behavior

The preparation of slippery liquid-infused surface and the droplets bounce behavior

Durability and Degradation Mechanisms of Antifrosting Surfaces.

Rapid implementation of renewable energy technologies has exacerbated the potential for economic loss and safety concerns caused by ice and frost accretion, which occurs on the surfaces of wind turbine blades, photovoltaic panels, and residential and electric vehicle air-source heat pumps. The past decade has seen advances in surface chemistry and micro- and nanostructures that can promote passive antifrosting and enhance defrosting. However, the durability of these surfaces remains the major obstacle preventing real-life applications, with degradation mechanisms remaining poorly understood. Here, we conducted durability tests on antifrosting surfaces, including superhydrophobic, hydrophobic, superhydrophilic, and slippery liquid-infused surfaces. For superhydrophobic surfaces, we demonstrate durability with progressive degradation for up to 1000 cycles of atmospheric frosting-defrosting and month-long outdoor exposure tests. We show that progressive degradation, as reflected by increased condensate retention and reduced droplet shedding, results from molecular-level degradation of the low-surface-energy self-assembled monolayer (SAM). The degradation of the SAM leads to local high-surface-energy defects, which further deteriorate the surface by promoting accumulation of atmospheric particulate matter during cyclic condensation, frosting, and melt drying. Furthermore, cyclic frosting and defrost tests demonstrate the durability and degradation mechanisms of other surfaces to show, for example, the loss of water affinity of superhydrophilic surfaces after 22 days due to atmospheric volatile organic compound (VOC) adsorption and significant lubricant drainage for lubricant-infused surfaces after 100 cycles. Our work reveals the degradation mechanism of functional surfaces during exposure to long-term frost-defrost cycling and elucidates guidelines for the development of future surfaces for real-life antifrosting/icing applications.

A facile method to fabricate the durable and self-protective coating for marine applications

Endowing coatings with effective protective function for metal substrates is of great interest. An excellent example is slippery liquid infused surface (SLIS) which possesses potential applications owing to its unique liquid repellency and self-cleaning properties. However, their widespread applications are facing obstacles of complex preparation, poor protection and low durability. Herein, a facile and efficient liquid infusion method was proposed to construct SLIS by altering the heterogeneous microstructure on the SiOx layer of the coatings with direct UV irradiation. The coatings were fabricated by conducting an appropriate UV irradiation treatment on polydimethylsiloxane (PDMS) film infused with methyl oleate (MO). The as-prepared transparent anti-fouling coatings showed outstanding anti-fouling properties against various contaminants such as algal (Chlorella), sand, rust (Fe2O3), paints (both water-based and oil-based) and daily life liquid pollutants (e.g., water, coffee, cola, soy sauce and milk). In addition, results revealed that the coatings exhibited a prominent anti-corrosion performance. Moreover, the coatings had excellent mechanical, chemical and thermal durability regarding the great repellency of the pollutants. We proposed this synergistic strategy between lubricant infusion and coating heterogeneous microstructure to widen new possibilities for fabricating coatings with promissing anti-corrosive and anti-fouling functionality.

Laser-Induced Slippery Liquid-Infused Surfaces with Anticorrosion and Wear Resistance Properties on Aluminum Alloy Substrates.

Slippery liquid-infused surfaces (SLISs) are developed as a potential alternative to superhydrophobic surfaces (SHSs) to resolve the issues of poor durability in corrosion protection and wear resistance. In this work, we used a simple laser processing technology to prepare a SLIS on the aluminum alloy (7075) surface. The superhydrophobicities of the modified surface and the oil film formed by liquid injection make the corrosive medium difficult to directly contact the surface and thus have a significant effect on corrosion resistance. The water and oil repellent SLIS exhibits durable corrosion resistance and excellent tribological properties compared with the SHS. The anticorrosion and wear resistance performances provided by the composite film have been assessed by multiple methods including the electrochemical test, immersion test, and friction wear test. The results indicate that compared to the bare surface, laser-ablated surface (LAS), and fluoroalkyl silane-modified SHS, the SLIS composite coating has better corrosion resistance and wear resistance, which is of great significance to expand the potential applications of 7075 aluminum alloys. The work provides a research basis for expanding the practical application of SLISs in complex environments.

Patterned Liquid-Infused Nanocoating Integrating a Sensitive Bacterial Sensing Ability to an Antibacterial Surface.

The slippery liquid-infused surfaces show a great antibacterial property. However, most liquid-infused surfaces cannot detect whether or not the unknown aqueous samples contain microorganisms. Therefore, it is highly necessary but a challenge to integrate bacterial sensing capability into antibacterial surface. In this work, we prepared a slippery patterned liquid-infused nanocoating on the glass substrate for integrating bacterial sensing capability into the bacterial repellence surface. Dendritic mesoporous silica nanoparticles (DMSNs) with a suitable particle size of ca. 128 nm were employed as a building block to fabricate the multifunctional nanocoating with a superhydrophilic microwell and hydrophobic periphery by a dip-coating strategy, hydrophobic treatment, photomask-mediated plasma etching, and liquid infusion. Dendritic porous silica nanoparticles (DPSNs) with a larger particle size of ca. 260 nm were uniformly loaded with Au nanoparticles (NPs), providing large surface area for the modification of Raman reporter (4-mercaptobenzoic acid (4-MBA)) and aptamer. Thus, as a Raman tag, the formed DPSNs-Au-MBA-aptamer could achieve sensitive surface-enhanced Raman spectroscopy (SERS) detection of target bacteria. Combined with the Raman tag, the patterned liquid-infused nanocoating not only completely repelled bacteria on the hydrophobic area but also enabled sensitive SERS detection of Staphylococcus aureus in a very low sample volume (1 μL) with a low detection limit of 2.6 colony formation units (CFU)/mL on the antibody-modified superhydrophilic microwell. This research provided a novel and reliable strategy to construct a multifunctional nanocoating with microbial repellence and sensing capabilities.

The investigation of droplet directional self-transport ability on the slippery liquid-infused surface with anisotropic structure

Liquid directional transport on functional surfaces has promising application in society. Inspired by cactus spine and Nepenthes pitcher plant, a series of slippery liquid-infused microstructure surfaces (SLIMS) with uniform texture have been fabricated on T2 copper substrates by laser texturing technique. Such substrates show geometry-gradient shape with uniformly rough structure, which can successfully realize droplet directional self-transport behaviour. Combined with experimental findings, the mechanism of droplet self-transport is introduced based on the analyses of driven force induced by Laplace pressure and the adhesive force between droplet and surface. The experiment results indicate that the droplet displacement is well consistent with theoretical results on the SLIMS with uniform surface texture. Different from previous reports, we have particularly investigated the droplet self-transport property on SLIMS with rice-leaf-like anisotropic grooves. The results demonstrate significant difference between the parallel groove-patterned SLIMS (SLIMS//) and the perpendicular groove-patterned SLIMS (SLIMS⊥). By considering both the adhesive force and droplet spreading ability, the variation of droplet transport distance is determined by synergistic effect of adhesive force in different directions (Fre// and Fre⊥). We believe this work will provide novel insights to improve the droplet transport distance, which can widen liquid directional transport applications in both industrial and academic fields.

Enhancing osseointegration and mitigating bacterial biofilms on medical-grade titanium with chitosan-conjugated liquid-infused coatings

Titanium alloys, in particular, medical-grade Ti-6Al-4 V, are heavily used in orthopaedic applications due to their high moduli, strength, and biocompatibility. Implant infection can result in biofilm formation and failure of prosthesis. The formation of a biofilm on implants protects bacteria from antibiotics and the immune response, resulting in the propagation of the infection and ultimately resulting in device failure. Recently, slippery liquid-infused surfaces (LIS) have been investigated for their stable liquid interface, which provides excellent repellent properties to suppress biofilm formation. One of the current limitations of LIS coatings lies in the indistinctive repellency of bone cells in orthopaedic applications, resulting in poor tissue integration and bone ingrowth with the implant. Here, we report a chitosan impregnated LIS coating that facilitates cell adhesion while preventing biofilm formation. The fabricated coating displayed high contact angles (108.2 ± 5.2°) and low sliding angles (3.56 ± 4.3°). Elemental analysis obtained using X-ray photoelectron spectroscopy (XPS) confirmed the availability of fluorine and nitrogen, indicating the presence of fluorosilane and chitosan in the final coating. Furthermore, our results suggest that chitosan-conjugated LIS increased cell adhesion of osteoblast-like SaOS-2 cells and significantly promoted proliferation (a fourfold increase at 7-day incubation) compared to conventional titanium liquid-infused surfaces. Furthermore, the chitosan conjugated LIS significantly reduced biofilm formation of methicillin-resistant Staphylococcus aureus (MRSA) by up to 50% and 75% when compared to untreated titanium and chitosan-coated titanium, respectively. The engineered coating can be easily modified with other biopolymers or capture molecules to be applied to other biomaterials where tissue integration and biofilm prevention are needed.

Robust and durable liquid-repellent surfaces.

Liquid-repellent surfaces, such as superhydrophobic surfaces, superoleophobic surfaces, and slippery liquid-infused surfaces, have drawn keen research interest from the communities engaged in chemical synthesis, interfacial chemistry, surface engineering, bionic manufacturing and micro-nano machining. This is due to their great potential applications in liquid-proofing, self-cleaning, chemical resistance, anti-icing, water/oil remediation, biomedicine, etc. However, poor robustness and durability that notably hinders the real-world applications of such surfaces remains their Achilles heel. The past few years have witnessed rapidly increasing publications that address the robustness and durability of liquid-repellent surfaces, and many breakthroughs have been achieved. This review provides an overview of the recent progress made towards robust and durable liquid-repellent surfaces. First, we discuss the wetting of solid surface and its generally-adopted characterisation methods, and introduce typical liquid-repellent surfaces. Second, we focus on various evaluation methods of the robustness and durability of liquid-repellent surfaces. Third, the recent advances in design and fabrication of robust and durable liquid-repellent surfaces are reviewed in detail. Fourth, we present the applications where these surfaces have been employed in fields like chemistry, engineering, biology and in daily life. Finally, we discuss the possible research perspectives in robust and durable liquid-repellent surfaces. By presenting such state-of-the-art of this significant and fast-developing area, we believe that this review will inspire multidisciplinary scientific communities and industrial circles to develop novel liquid-repellent surfaces that can meet the requirements of various real-world applications.

Slippery magnetic track inducing droplet and bubble manipulation.

It is difficult for traditional droplet manipulation to combine transportation and rapid capture of droplets on an inclined surface. In this work, a slippery magnetic track (SMT) is presented to manipulate droplets and bubbles in a magnetic field. By changing the direction of the magnetic field, the transitions from non-pinning to pinning states on the SMT can be achieved. Through the SMT surface, it is possible to capture and release droplets and bubbles in the vertical direction. The detailed theoretical and experimental studies of droplet and bubble manipulation are discussed. This work demonstrates the versatility of magnetic manipulation, including the transition of droplet trajectory and bubble removal, which will facilitate the research of intelligent interfaces in energy transmission, drug transport and micro engineering.

Condensation Heat Transfer Performance of Vertical Condenser Tube with Slippery Liquid Infused Surface Coatings

Condensation Heat Transfer Performance of Vertical Condenser Tube with Slippery Liquid Infused Surface Coatings

Oil Retention Properties of Elastomer Based Slippery Liquid Infused Surfaces Under Extreme Conditions

Oil Retention Properties of Elastomer Based Slippery Liquid Infused Surfaces Under Extreme Conditions

Easily fabricated icephobic surface with external and self-replenishing properties

Ice accretion has adverse effects on outdoor equipment, and slippery liquid-infused surface has attracted increasing attention due to its low ice adhesion strength (τice) and great potential in practical application. However, the surface lubricant loss leads to the failure of icephobicity in this system. In this paper, the icephobic surface with external and self-replenishing properties (RDLS) is prepared to enhance anti-icing and deicing properties by the facile method with infusing the silicone oil and fumed silica into the epoxy resin adhesive. As an outcome, the prepared RDLS exhibit anti-icing and deicing properties, and it also shows mechanical durability. Meanwhile, the use of fumed silica is proposed to act as a reservoir for silicone oil to extend the lifetime of oil-releasing properties. The icephobicity of the coating can also be improved by the proper amount of fumed silica. Furthermore, obtained RDLS showed superior sustainability, and the silicone oil in the coating can be replenished easily. The problem of icephobic failure caused by lubricant loss is solved effectively. This work provides a new and facile way to fabricate an icephobic coating with external and self-replenishing properties.

Self-healing dual biomimetic liquid-infused slippery surface in a partition matrix: Fabrication and anti-corrosion capability for magnesium alloy

Biomimetics affords versatile ways to develop new routes for protecting metals from corrosion. In this report, focusing on the inhibition of corrosion for magnesium alloy, different biomimetic strategies were studied. Using a facile immersion method followed by surface modification, a lotus leaf-inspired superhydrophobic surface (SHS) is achieved. However, in the water phase, the SHS is found to be vulnerable, and a hydrophilic defect spot can irreversibly deteriorate the corrosion inhibition capability. Based on the infusion of dimethyl silicone oil into a SHS matrix, a dual biomimetic slippery liquid-infused porous surface (SLIPS) in a partition matrix inspired by both Nepenthes pitcher and pomegranate fruit was prepared. In a 3.5 wt% NaCl solution, the corrosion resistance of the SLIPS is found to be four orders of magnitude larger than that of the untreated and superhydrophobic magnesium alloys. In comparison with the vulnerable SHS, the SLIPS can tolerate repeat mechanical damage, exhibiting prominent self-healing properties. A facile route leads to dual biomimetic slippery liquid-infused surface in partition matrix onto magnesium alloy. Slippery liquid-infused porous surface has long-term corrosion inhibition and good self-healing capability. • Immersion and surface modification leads to superhydrophobic AZ31B. • Durability of superhydrophobicity is short for corrosion inhibition. • Slippery liquid-infused porous surface protects AZ31B from corrosion.