Slippery Lubricant-infused Porous Surfaces Research Articles

Room-temperature endogenous lubricant-infused slippery surfaces by evaporation induced phase separation.

We present a novel approach to fabricate endogenous slippery lubricant-infused porous surfaces (eSLIPS) at room temperature using an evaporation-induced phase separation process. The ternary coating system, comprising ethylene-propylene copolymer, caprylyl methicone, and n-hexane, forms a porous structure in situ infiltrated with lubricant, resulting in surfaces with remarkable anti-fouling and anti-icing properties.

Liquid-Liquid Triboelectric Nanogenerators Based on Bionic Slippery Lubricant-Infused Porous Surfaces

In the era of high energy dependence, self-powered smart devices have been widely used in the energy field, especially the continuous long-term operation in harsh environments is the biggest challenge at present. In the past few years, the innovation of Triboelectric nanogenerator (TENG) can effectively convert the kinetic energy of ocean waves and rain into electricity. However, this type of equipment encounters a bottleneck in practical applications, that is, the equipment is exposed to harsh environments for a long time, which will cause damage or aging due to inevitable physical and chemical effects, resulting in reduced power output. To overcome this difficulty, we propose an innovative liquid-liquid TENG (L-L TENG) integrated with slippery lubricant-infused porous surfaces (SLIPS), which is called SLIPS-TENG. Inspired by nepenthes, we use Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) imitates its unique structure as the base layer through electrospinning, and inject silicon oil (SO) to form a uniform, defect-free and slippery hydrophobic coating on the base layer to prevent liquids from adhering to the surface of the TENG device. The results show that the liquids on SLIPS-TENG show lower contact hysteresis than TENG fabricated with only hydrophobic surface, and the fast sliding enables efficient charge transfer during the drop process. In addition, combined with the versatility of SLIPS-TENG, its self-healing and self-cleaning capabilities enable it to continue operating stably in harsh environments or at extreme low temperatures.

Synergistic effect of helically-finned directional tracks and lubricant viscosity on droplet dynamics and condensation heat transfer of bioinspired slippery surfaces

Slippery lubricant-infused porous surfaces (SLIPSs) have been presented herein, developed on helically-fined grooves of copper tubes (HF-SLIPSs). The high viscosity lubricant (GPL 107) has been employed which is firmly locked by the nano-pores of oxidized layer fabricated on helically-finned grooves. The influences of synergistic cooperation of helically-finned grooves and infused lubricant on the condensed droplet dynamics and heat transfer are systematically investigated. Findings demonstrate that HF-SLIPSs not only improve the droplet mobility but also expand the condensing area and provide directional transport tracks where small-sized droplets quickly coalesce on fin surface and slide orderly along the helical grooves, resulting in short renewal time period of 2.99 s. Moreover, the condensation heat transfer on HF-SLIPSs(GPL 107) is better than other test tubes, and condensation heat transfer coefficients are 35.8% and 55.9% higher than that of smooth SLIPSs tubes impregnated with GPL 103 and GPL 107, respectively, and 98.1% and 204.4% higher comparing with smooth hydrophilic and helically-finned hydrophilic tubes, respectively. This indicates that the synergistic cooperation of high viscosity lubricant and helically-finned grooves plays a great role by providing certain directional and stable SLIPSs tracks where droplets are guided and transported faster which ensure the enhanced condensation heat transfer.

Fabrication of Hierarchical Patterned Surfaces Using a Functionalized CeO2-EPDM Composite for Crevice Corrosion Prevention on High-Voltage Insulators.

Crevice corrosion accounts for 62% of the recorded breakdown of insulators utilized in transmission lines, which may interfere with the reliability of power utilities. To address these challenges, sustainable and resilient slippery lubricant-infused porous surfaces (SLIPS) are developed on insulators to prevent electrochemically/biochemically induced crevice corrosion especially occurring in tropical and coastal environments. The conventional way of developing SLIPS by chemical and physical etching might interfere with the mechanical stability of insulators composed of pin (galvanized steel), cement, and shell (porcelain). The current study proposes a noble concept of developing hierarchical patterned textured surfaces on insulators to fabricate a resilient SLIPS coating without physical/chemical etching. The proposed coating exhibits 99% antiadhesion performance against a mixed culture of bacterial strains, superior hydrophobicity (contact angle: 160°, contact angle hysteresis: 4°), and crevice corrosion resistance performance at elevated temperatures (25-75 °C) and humidity. This study could facilitate a new route for the development of sustainable and highly reliable SLIPS coatings in the future.

Bioinspired superwetting materials for water manipulation

AbstractWater manipulation is crucial for a wide range of engineering applications, such as microfluidics, heat transfer, desalination, and water harvesting. Despite scientific achievements in the past few decades, it remains challenging to achieve efficient and well‐controllable droplet transport in practical applications. In nature, creatures have evolved ingenious wisdom to manipulate water to adapt to their environments. Learning from nature and combining bioinspirations with advanced manufacturing technology are promising to fabricate excellent materials for directional water transport. In this review, we summarize the mechanisms of water droplet manipulation on various biological surfaces and review the recent progress in fabricating hydrophilic surfaces, hybrid hydrophilic/hydrophobic surfaces, and slippery lubricant‐infused porous surfaces (SLIPSs) with bioinspiration for water manipulation. These novel bioinspired materials have great potential in many fields, especially for water harvesting. Finally, current challenges and future perspectives in this field are proposed.

Chlorotrimethylsilane treatment affords long-lasting contaminant-proof and transparent slippery covalently attached liquids surfaces

Slippery lubricant-infused porous surfaces (SLIPs) are effective in repelling low-surface tension fluids given their omniphobicity, anti-adhesive, and drag reduction properties. However, over time, SLIPs are subjected to lubricant losses and undergo dust fouling. In this research, we address these challenges by developing a new slippery covalently attached liquids (SCAL) surface. SCALs was composed of a molecularly-thick lubricant layer covalently bound to the solid support, generating long-lasting anti-fouling, hydrophobic and slippery effects. We show that our SCALs exhibit ultra-low contact angle hysteresis for a wide range of fluids and enable reduced colloidal forces in the vertical and lateral directions (adhesion and friction, respectively). In parallel, the SCALs exhibit a high resistance to contamination (from molecular adhesion to dust fouling), mechanical integrity, and light transmission. Overall, we introduce a facile, cost-effective, and scalable process to synthesize a SCALs system suitable to the demands of application in bioengineering, power generation and water harvesting.

Light-induced charged slippery surfaces.

Slippery lubricant-infused porous (SLIPS) and superhydrophobic surfaces have emerged as promising interfacial materials for various applications such as self-cleaning, anti-icing, and antifouling. Paradoxically, the coverage/screening of lubricant layer on underlying rough matrix endows functionalities impossible on superhydrophobic surfaces; however, the inherent flexibility in programming droplet manipulation through tailoring structure or surface charge gradient in underlying matrix is compromised. Here, we develop a class of slippery material that harnesses the dual advantages of both solid and lubricant. This is achieved by rationally constructing a photothermal-responsive composite matrix with real-time light-induced surface charge regeneration capability, enabling photocontrol of droplets in various working scenarios. We demonstrate that this light-induced charged slippery surface (LICS) exerts photocontrol of droplets with fast speed, long distance, antigravity motion, and directionally collective motion. We further extend the LICS to biomedical domains, ranging from specific morphological hydrogel bead formation in an open environment to biological diagnosis and analysis in closed-channel microfluidics.

Experimental and theoretical study of electrowetting dynamics on slippery lubricant-infused porous surfaces

The droplet electrowetting dynamic behaviors on slippery lubricant-infused porous surfaces (SLIPS) remain elusive because the soft liquid-liquid interface nature is much different from solid dielectric hydrophobic surfaces. To understand the dynamic process, the impact of the voltage applied, the oil thickness, and viscosity on dynamic electrowetting behavior was experimentally studied, respectively. Meanwhile, a numerical dynamic model was also developed for the quantitative interpretation of the droplet dynamic spreading process on SLIPS. It is found the droplet always spreads smoothly to the equilibrium state without overshooting on the SLIPS. The droplet is always over-damped with the increase of applied voltage, the settling time is proportional to the 0.9th power of the electrowetting number. Then, by changing the silicone oil viscosity, it is found the viscous dissipation of the oil-water interface becomes dominant, causing the droplet to spread slowly. By fitting the theoretical models to experimental results, it is found the friction coefficient is nearly proportional to 1/6th power of oil viscosity and rarely influenced by applied voltage and oil thickness. Finally, it is found both the initial oil thickness and the high wetting ridge have a minor influence on the electrowetting dynamic spreading. The relationship between the actual oil layer thickness and the initial oil layer thickness was estimated. This study will provide helpful information and theory support for electrowetting-on-dielectric device design, lab on a chip, and other potential applications on SLIPS.

Anti-icing propagation and icephobicity of slippery liquid-infused porous surface for condensation frosting

Anti-propagation and enhanced icephobicity both are critical for anti-icing surfaces. In this work, four types of functional surfaces were prepared to investigate condensation frosting mechanisms (hydrophilic surfaces, hydrophobic slippery lubricant-infused porous surfaces, and two types of superhydrophobic surfaces). Experimental results indicated that no directional icing was identified on both the hydrophilic and slippery surfaces, while it was observed on superhydrophobic surfaces. The mechanisms were explained based on microscopic photos, together with theoretical analysis and Molecular Dynamics simulations. On the hydrophilic surfaces, the initially frozen droplets can induce the icing of some of their neighbours. However, these inter-droplet icing spreading failed to develop into a directional spreading for the whole field of view. On the slippery surfaces, most of the droplets had to freeze locally because the distances among them were beyond the effects of icing bridge. Moreover, the lubricant effects on droplet wetting were investigated using Molecular Dynamics simulation, and the results can support the existence of a lubricant layer beneath droplets. The surface icephobicity was experimentally investigated by measuring the critical shear stress of a frozen droplet. As a whole, the icephobicity of the slippery surface was significantly enhanced. Especially, when the substrate was chilled to −5 °C, the high icephobicity of the slippery surface was highlighted by an order of magnitude lower than that on the hydrophilic surface. Also, the slippery surfaces were fabricated based on silica nanoparticles via a commercially coating spray. This method can be of low economic cost for scalable preparation. Overall, the present work can be helpful for developing anti-icing functional surfaces.

Liquid-solid triboelectric nanogenerators for a wide operation window based on slippery lubricant-infused surfaces (SLIPS)

Triboelectric nanogenerators (TENG) have emerged as suitable devices that are suitable to harvest widely abundant and renewable blue energy. However, a major drawback in related deployments include the reliability of the system when operating in variable atmospheric conditions. To address this limitation, we propose the integration of slippery lubricant-infused porous surfaces (SLIPS) as key TENG component. For instance, we combine SLIPS with transistor-inspired architectures to develop a robust single-electrode triboelectric nanogenerator (SLIPS-SE-TENG) that is shown to operate effectively under extreme temperature and humidity. For this purpose, we use the energy of water droplets (as in rain) impacting a surface to generate the electrical energy output. Our theoretical calculations and atomic force microscopy measurements show that a small volume of lubricant per surface area (4 μL/ per cm2 of porous PTFE) is sufficient to produce a low water contact angle hysteresis, leading to efficient energy conversion, provided the droplet’s moving velocity on the surface surpasses a threshold (0.3 mm/s). As such, we demonstrate SLIPS-SE-TENG to generate an instantaneous short-circuit current of 3 μA (charge density 8.8 nC/cm2). Importantly, we address the limitations of TENG fabricated with traditional superhydrophobic surfaces, affording a device that works normally and stably in harsh environments, under freezing temperatures or high humidity.

Bio-inspired Superwettable Surface for the Detection of Cancer Biomarker: A Mini Review.

Inspired by nature, superwettable material-based biosensors have aroused wide interests due to their potential in cancer biomarker detection. This mini review mainly summarized the superwettable materials as novel biosensing substrates for the development of evaporation-induced enrichment-based signal amplification and visual biosensing method. Biosensing applications based on the superhydrophobic surfaces, superwettable micropatterned surfaces, and slippery lubricant-infused porous surfaces for various cancer biomarker detections were described in detail. Finally, an insight of remaining challenges and perspectives of superwettable biosensor is proposed.

A Simple Approach for Flexible and Stretchable Anti-icing Lubricant-Infused Tape.

Unwanted icing has major safety and economic repercussions on human activities, affecting means of transportation, infrastructures, and consumer goods. Compared to the common deicing methods in use today, intrinsically icephobic surfaces can decrease ice accumulation and formation without any active intervention from humans or machines. However, such systems often require complex fabrication methods and can be costly, which limits their applicability. In this study, we report the preparation and characterization of several slippery lubricant-infused porous surfaces (SLIPSs) realized by impregnating with silicone oil a candle soot layer deposited on double-sided adhesive tape. Despite the use of common household items, these SLIPSs showed anti-icing performance comparable to other systems described in the literature (ice adhesion < 20 kPa) and a good resistance to mechanical and environmental damages in laboratory conditions. The use of a flexible and functional substrate as tape allowed these devices to be stretchable without suffering significant degradation and highlights how these systems can be easily prepared and applied anywhere needed. In addition, the possibility of deforming the substrate can “allow” the application of SLIPS technology in mechanical ice removal methodologies, drastically incrementing their performance.

When SLIPS meets TIPS: An endogenous lubricant-infused surface by taking the diluent as the lubricant

Slippery lubricant-infused porous surfaces (SLIPSs) hold great promise in fields requiring adhesion resistance such as self-cleaning, anti-fouling, anti-icing, and anti-scaling. However, their road to practical application is still blocked by the complicated fabrication process and the restricted coating area. To address these challenges, we propose a new concept of endogenous SLIPS in which the porous structure grows from the solution coating of polypropylene/caprylyl methicone undergoing a thermally induced phase separation (TIPS) process. Caprylyl methicone serves as the diluent during TIPS as well as the lubricant in the SLIPS so that the coating can be fabricated in one step and unrestrictedly. The dissolution and phase separation mechanisms have been revealed by molecular dynamic simulation and in situ microscopic observation. Such SLIPSs exhibit impressive sliding property even for sticky and viscous liquids and exhibit extremely low ice adhesion in anti-icing tests.

Preventing algae biofilm formation via designing long-term oil storage surfaces for excellent antifouling performance

Fabricating slippery lubricant-infused porous surfaces (SLIPS), typically composed of lubricant and microstructured surface, is an effective way to solve the problem of marine anti-fouling. In the present work, acetone acted as a volatile solvent, and epoxy resin acted as a linking agent were mixed with one-dimensional material with different length (300-nm carbon nanotubes, 300-μm carbon fiber, and 1-mm polyacrylonitrile fiber). After acetone volatilized, one-dimensional materials with different length could form various kind of internal or surface structures. 300-μm carbon fiber could form a hierarchical structure with both internal holes and vertical needle-shaped structures, which effectively ensured the storage stability of lubricant in the structure. And through 21 days of algae attachment experiment, compared with the bare epoxy resin, the lubricant-infused carbon fiber structure (SLIPS-EP/CF) displayed superior anti-fouling performance with the reduction rate of chlorella was 91.4% and the phaeodactylum tricornutum was 77.8%. Additionally, the TEM and SEM analysis showed the attachment of algae to the surface of lubricant, and it was interesting to observe that algae could be wrapped by silicone oil and could not produce algae biofilm, which was presented by the first time. Since the formation of algae biofilm was inhibited, a large number of algae attachment was avoided, thereby realizing effective antifouling performance.

Lubricant Depletion‐Resistant Slippery Liquid‐Infused Porous Surfaces via Capillary Rise Lubrication of Nanowire Array

AbstractDespite their promise in diverse applications, lubricant depletion over time presents an important challenge for slippery lubricant‐infused porous surfaces (SLIPS). The main source of lubricant depletion is the growth of a wetting ridge around a droplet of foreign liquid that removes the infused lubricant from the porous surface when the droplet slides off. This work describes the fabrication of liquid‐infused ZnO nanowire array by capillary rise lubrication and the effect of array morphology on suppressing wetting ridge formation. The orientation of nanowires strongly affects lubricant infiltration and the stability of SLIPS properties. The infiltrated lubricant in random ZnO nanowire arrays does not form a visible wetting ridge around a water droplet because of the strong capillarity that retains the lubricant confined in the nanoporous structure. The lack of wetting ridge gives rise to a stable SLIPS that exhibits self‐recovery of the infused lubricant. Such a structure retains lubricant under a strong centrifugal force, showing high stability. This study provides a straightforward, robust, and potentially scalable approach for making stable SLIPS that suppresses lubricant depletion by taking advantage of the unique morphology of the random nanowire arrays and capillary rise lubrication.

Smart Control for Water Droplets on Temperature and Force Dual-Responsive Slippery Surfaces.

Responsive slippery lubricant-infused porous surfaces (SLIPSs), featuring excellent liquid repelling/sliding capabilities in response to external stimuli, have attracted great attention in smart droplet manipulations. However, most of the reported responsive SLIPSs function under a single stimulus. Here, we report a kind of smart slippery surface capable of on-demand control between sliding and pinning for water droplets via alternately freezing/thawing the stretchable polydimethylsiloxane sheet in different strains. Diverse parameters are quantified to investigate the critical sliding volume of the droplet, including lubricant infusion amount, laser-scanning power, and pillar spacing. By virtue of the cooperation of temperature and force fields acting on the SLIPS, we demonstrate the intriguing applications including controllable chemical reaction and on-demand electrical circuit control. We envision that this dual-responsive surface should provide more possibilities in smart control of microscale droplets, especially in active vaccine-involved biochemical microreactions where a lower temperature is highly favored.

Fabrication of a Porous Slippery Icephobic Surface and Effect of Lubricant Viscosity on Anti-Icing Properties and Durability

A breakdown caused by the icing of power generation infrastructure is one of the serious disasters occurring in the power system. Slippery lubricant-infused porous surfaces (SLIPSs), whose ice adhesion strength is extremely low, have a promising application in the anti-icing field. In the present study, we fabricated SLIPSs with low ice adhesion strength by infusing silicone oil into an anodic aluminum oxide (AAO) substrate. In addition, the effects of the viscosity of silicone oil on the anti-icing properties and durability of the SLIPSs were investigated. The results show that a lower viscosity silicone oil brings about more slippery surfaces and lower ice adhesion strength. The ice adhesion strength was reduced by 99.3% in comparison with the bare Al alloy. However, low-viscosity silicone oil has worse de-icing resistance and heat resistance. Additionally, the porous films filled with low-viscosity silicone oil possess a better self-healing property after icing/de-icing cycles and followed by exposure to the atmosphere. When the viscosity of silicone oil is 50 mPa·s, the SLIPSs exhibit the best durability for anti-icing. Even after 21 de-icing tests or 168 h of heating at 90 °C, the ice adhesion strength still remains below 10% compared with that of bare Al. This work provides some useful advice for the design and fabrication of anti-icing SLIPSs.

Superhydrophobic surfaces with flake-like structures and lubricant-infused composite surfaces to enhance anti-icing ability

We prepared superhydrophobic surfaces (SHS) with various morphologies on 7075 aluminum, demonstrating that the SHS with small nanoscale asperities of ~80 nm yielded the lowest ice adhesion forces of ~80 kPa, due to the lubricating effect of the overcooled water, and the ice menisci caused by the high surface roughness deteriorated the surface’s icephobicity. To overcome this, slippery lubricant-infused porous surfaces (SLIPS) were prepared, only to find that the removal ability of water on SLIPS was not sufficient to represent its anti-icing ability, and the ice-adhesion force on SLIPS exhibited a negative correlation with the surface roughness.

Precursor-Film-Mediated Thermocapillary Motion of Low-Surface-Tension Microdroplets.

In contrast to microdroplet condensation with high contact angles, the one with low contact angles remains unclear. In this study, we investigated dynamics of microdroplet condensation of low-surface-tension liquids on two flat substrate surfaces by using reflection interference confocal microscopy. Spontaneous migration toward relatively larger droplets was first observed for the microdroplets nucleated on the hydrophilic quartz surface. The moving microdroplets showed a contact angle hysteresis of ∼0.5°, which is much lower than the values observed on typical flat substrates and is within the range observed on slippery lubricant-infused porous surfaces. Because the microdroplets on the hydrophobic polydimethylsiloxane surface did not move, we concluded that the ultrathin precursor film is formed only on the hydrophilic surface, which reduces a resistive force to migration. Also, reduced size of droplets promotes the thermocapillary motion, which is induced by a gradient in local temperature inside a small microdroplet arising due to the difference in size of adjacent droplets.

Condensate droplet size distribution and heat transfer on hierarchical slippery lubricant infused porous surfaces

In recent years, slippery lubricant infused porous surfaces (SLIPSs) have received important attention due to their excellent performance in applications such as condensation, low friction, self-cleaning and anti-icing, which is owed to the presence of an infused lubricant or oil effectively decreasing the liquid-solid interactions and enhancing droplet mobility when compared to hydrophobic and/or to superhydrophobic surfaces. In this work, we fabricate and make use of hierarchical micro-/nano-structured and nano-structured SLIPSs for condensation phase-change. Optical microscopy and macroscopic experimental observations are coupled to extract the droplet size distribution at different condensation times. Heat transfer resistance model through individual condensing droplets is further extended here to account for the presence of both micro- and nano-structures. Then, heat transfer through individual droplets is coupled to the droplet number density to estimate the heat transfer at different condensation times and their overall performance. A 100% greater heat transfer performance is reported on nano-structured SLIPSs when compared to hierarchical micro-/nano-structure SLIPSs due to the greater thermal resistance imposed by the micro-structures and the lubricant present within the structures. We conclude that although the presence of micro-structures shifts the droplet number density towards greater population of smaller sized droplets, this effect is not enough to overcome the greater heat transfer predicted on solely nano-structured SLIPSs. Findings presented here complement current research on SLIPSs and condensation phase-change heat transfer and are of great importance for the effective design of SLIPSs with enhanced condensation heat transfer performance.