Lubricant-impregnated Surfaces Research Articles

Physically consistent modelling of surface tension forces in the Volume-of-Fluid method for three or more phases

Multiphase numerical simulations have become a widely sought methodology for modelling capillary flows due to their scientific relevance and multiple industrial applications. Much progress has been achieved using different approaches, and the volume of fluid is one of the most popular methods widely used for modelling two or more phases due to its simplicity, accuracy and robustness. However, when prescribing the forces emerging from three or more fluid-fluid interfaces, the force balance is not guaranteed and can lead to spurious self-propulsion. Here, a new approach to account for the surface tension forces for multiphase flows with a correct force balance is proposed. The newly proposed method is successfully validated for a wide range of tests, including contact angles for the fluid-fluid and fluid-solid triple line. Additionally, complete spreading phenomena of fluid on fluid and fluid on solid have been found to emerge naturally from the newly proposed surface tension force model. Finally, simulation results are compared against experiments of lubricant-impregnated surfaces to demonstrate the practical applicability of the newly proposed method.

Biofilm formation of Pseudomonas aeruginosa in spaceflight is minimized on lubricant impregnated surfaces

The undesirable, yet inevitable, presence of bacterial biofilms in spacecraft poses a risk to the proper functioning of systems and to astronauts’ health. To mitigate the risks that arise from them, it is important to understand biofilms’ behavior in microgravity. As part of the Space Biofilms project, biofilms of Pseudomonas aeruginosa were grown in spaceflight over material surfaces. Stainless Steel 316 (SS316) and passivated SS316 were tested for their relevance as spaceflight hardware components, while a lubricant impregnated surface (LIS) was tested as potential biofilm control strategy. The morphology and gene expression of biofilms were characterized. Biofilms in microgravity are less robust than on Earth. LIS strongly inhibits biofilm formation compared to SS. Furthermore, this effect is even greater in spaceflight than on Earth, making LIS a promising option for spacecraft use. Transcriptomic profiles for the different conditions are presented, and potential mechanisms of biofilm reduction on LIS are discussed.

Bioinspired Scalable Lubricated Bicontinuous Porous Composites with Self-Recoverability and Exceptional Outdoor Durability.

Lubricant-impregnated surfaces (LIS) are promising as efficient liquid-repellent surfaces, which comprise a surface lubricant layer stabilized by base solid structures. However, the lubricant layer is susceptible to depletion upon exposure to degrading stimuli, leading to the loss of functionality. Lubricant depletion becomes even more pronounced in exposed outdoor conditions, restricting LIS to short-term lab-scale applications. Thus, the development of scalable and long-term stable LIS suitable for practical outdoor applications remains challenging. In this work, we designed "Lubricated Bicontinuous porous Composites" (LuBiCs) by infusing a silicone oil lubricant into a bicontinuous porous composite matrix of tetrapod-shaped zinc oxide microfillers and poly(dimethylsiloxane). LuBiCs are prepared in the meter scale by a facile drop-casting inspired wet process. The bicontinuous porous feature of the LuBiCs enables capillarity-driven spontaneous lubricant transport throughout the surface without any external driving force. Consequently, the LuBiCs can regain liquid-repellent function upon lubricant depletion via capillary replenishment from a small, connected lubricant reservoir, making them tolerant to lubricant-degrading stimuli (e.g., rain shower, surface wiping, and shearing). As a proof-of-concept, we show that the large-scale "LuBiC roof" retains slippery behavior even after more than 9 months of outdoor exposure.

Condensation heat transfer on slippery lubricant-impregnated nanohole-arrayed stainless-steel surface

The condensation heat transfer performance of lubricant-impregnated surfaces has a competitive advantage owing to the improved mobility of droplets and self-healing characteristics. In this study, we fabricated the lubricant-impregnated surfaces featured with Teflon-coated anodized nanoporous oxide layer on stainless steel infused with Krytox GPL103 oil. The characterization shows that the droplet state on the surface changes from the pinning to slippery states (sliding angle α of 1.8 ± 0.3°) enabled by the presence of the lubricant oil. The condensation heat transfer performances of the lubricant-impregnated Teflon-coated anodized stainless-steel surface (L-T-AS) are investigated from the aspects of the surface wettability and droplet mobility, compared to that of Teflon-coated stainless-steel surface (T-S) and Teflon-coated anodized stainless-steel surface (T-AS). Whereas coalescence-induced droplet shedding and droplet sweeping are found on the T-S and T-AS, the shedding of discrete droplets and their sweeping without further coalescence are noticed on the L-T-AS. The heat transfer coefficient of the L-T-AS is 398.4% higher than that of the T-AS at the low subcooling temperature of 0.2 K. The heat transfer performance of the dropwise condensation of the L-T-AS has further been elucidated by the theoretical model considering the heat transfer rate through a single droplet associated with the density distribution function of the droplet.

Droplet transportation on photosensitive lubricant-impregnated slippery surfaces in response to the light induced Marangoni effect and asymmetrical wetting ridges.

Flexible control of droplet transportation is crucial in various applications but is constrained by liquid-solid friction. The development of biomimetic lubricant-impregnated slippery surfaces provides a new solution for flexible manipulation of droplet transportation. Herein, a light strategy is reported for flexibly controlling droplet transportation on photosensitive lubricant-impregnated slippery surfaces. Owing to the localized heating effect of a focused laser beam via photothermal conversion, the resultant thermal Marangoni flow and horizontal component of the surface tension associated with the asymmetric wetting ridges are together responsible for actuating droplet transportation. It is found that the asymmetry of the wetting ridge is dominated by the thickness of the infused oil layer, which directly affects the droplet transportation. The feasibility of this light strategy is also demonstrated by uphill movement, droplet coalescence, and chemical reaction. This study provides a new design for potential applications in open droplet microfluidics, analytical chemistry, diagnosis, etc.

Laser-Enabled Surface Treatment of Disposable Endoscope Lens with Superior Antifouling and Optical Properties.

Endoscopes are ubiquitous in minimally invasive or keyhole surgeries globally. However, frequent removal of endoscopes from the patient’s body due to the lens contaminations results in undesirable consequences. Therefore, a cost-effective process chain to fabricate thermoplastic-based endoscope lenses with superior antifouling and optical properties is proposed in this research. Such multifunctional surface response was achieved by lubricant impregnation of nanostructures. Two types of topographies were produced by femtosecond laser processing of metallic molds, especially to produce single-tier laser-induced periodic surface structures (LIPSS) and two-tier multiscale structures (MS). Then, these two LIPSS and MS masters were used to replicate them onto two thermoplastic substrates, namely polycarbonate and cyclic olefin copolymer, by using hot embossing. Finally, the LIPSS and MS surfaces of the replicas were infiltrated by silicone oils to prepare lubricant-impregnated surfaces (LIS). Droplet sliding tests revealed that the durability of the as-prepared LIS improved with the increase of the lubricant viscosity. Moreover, the single-tier LIPSS replicas exhibited longer-lasting lubricant conservation properties than the MS ones. Also, LIPSS-LIS replicas demonstrated an excellent optical transparency, better than the MS-LIS ones, and almost match the performance of the reference polished ones. Furthermore, the LIPSS-LIS treatment led to superior antifouling characteristics, i.e., regarding fogging, blood adhesion, protein adsorption, and microalgae attachment, and thus demonstrated its high suitability for treating endoscopic lenses. Finally, a proof-of-concept LIPSS-LIS treatment of endoscope lenses was conducted that confirmed their superior multifunctional response.

Magnetic-Actuated Robot Enables High-Performance Underwater Bubble Maneuvering on Laser-Textured Biomimetic Slippery Surfaces.

Controllable underwater gas bubble (UGB) transport on a surface is realized by geography-/stimuli-induced wettability gradient force (Fwet-grad). Unfortunately, the high-speed maneuvering of UGBs along free routes on planar surfaces remains challenging. Herein, a regime of magnetism-actuated robot (MAR) mounting on biomimetic laser-ablated lubricant-impregnated slippery surfaces (LA-LISS) is reported. Leveraging on LA-LISS, MAR-entrained UGBs can move along arbitrary directions through the loading of a tracing magnetic trigger. The underlying hydrodynamics is that MAR-entrained UGBs would be actuated slipping upon a giant magnetic-induced towing force (FM//). Once the magnetism stimuli is discharged, FM// vanishes immediately to immobilize the UGBs on LA-LISS. Thanks to the MAR's robust bubble affinity, a typical UGB (20 μL) on the optimized LA-LISS can be accelerated at 500 mm/s2 and gain an ultrafast velocity of over 205 mm/s that far exceeds previously reported figures. Moreover, fundamental physics renders MAR antibuoyancy, steering locomotive UGBs on the inclined LA-LISS. Significantly, an MAR propelling UGBs to configure desirable patterns, realize on-demand coalescence, remedy the cutoff switch, as well as facilitate a programmable light-control-light optical shutter is successfully deployed. Compared with previous smart surfaces, the current multifunctional regime is more competent for harnessing UGBs featuring an unparalleled transport velocity independent of the feeble Fwet-grad.

Dropwise condensation of acetone and ethanol for a high-performance lubricant-impregnated thermosyphon

A gravity-assisted heat pipe called a two-phase closed thermosyphon (TPCT) is an efficient thermal device that transfers a large amount of heat at a relatively small temperature difference using the phase-change process. Promoting dropwise condensation on a condenser wall of a TPCT promises an increase in the effective thermal conductance of that device. Acetone and ethanol have been widely used for heat pipe applications as working fluids. Still, they pose a limitation in promoting dropwise condensation using typical hydrophobic coatings due to their lower surface energies. Recent works have shown that lubricant-impregnated surfaces (LIS) can promote dropwise condensation of low-surface-tension fluids, but no studies proved the effectiveness of the surfaces in TPCT applications. This study experimentally achieves dropwise condensation of acetone and ethanol within a TPCT using the LIS surface. We demonstrate heat transfer enhancement of a TPCT by examining temperature uniformity, condenser and evaporator heat transfer coefficients (HTCs), and overall and partial thermal resistances. We demonstrated that the LIS condenser increased the condenser HTC by 220% and decreased the overall thermal resistance of the TPCT by 44% compared to filmwise condensation. Our work also revealed a strong dependence of working fluid viscosity on dropwise condensation stability and heat transfer enhancement by comparing the two working fluids, acetone, and ethanol. This work presents the opportunity for improving thermal performance in heat pipes and applications that rely on the condensation of low-surface-tension fluids, such as HVAC, refrigeration, and organic Rankine cycle.

Lubricant-Mediated Strong Droplet Adhesion on Lubricant-Impregnated Surfaces.

Lubricant-impregnated surfaces have recently emerged as a new type of multifunctional coating with a promising capability in exhibiting low friction or contact angle hysteresis. However, lubricant-infused surfaces severely suffer from the tensile droplet-lubricant adhesion. In this study, we show that lubricant-infused surfaces allow for a strong tensile droplet adhesion, which results in the generation of an offspring residual droplet when a droplet detaches from the surface. Such tensile liquid-liquid adhesion and the corresponding liquid residue are solely mediated by the lubricant, independent of the underlying surface topography. We reveal how the lubricant film mediates droplet adhesion by measuring the maximum adhesion force and liquid residue and theoretically analyzing Laplace pressure force from the droplet shape and surface tension force depending on the contact line. Further, the presence of lubricant-induced adhesion considerably compromises the advantages of lubricant-infused surfaces in some applications. The lubricant-triggered tensile adhesion hampers the loss-free droplet transfer away from the surfaces in the photoelectrically and magnetically driven droplet manipulation. In addition, we demonstrate that the lubricant-triggered adhesion plays a dominant role in attenuating the efficiency of fog harvesting by impeding the shedding of the intercepted droplets by comparing the onset time, droplet radius, and collection efficiency. These findings advance our fundamental understanding of droplet adhesion on lubricant-infused surfaces and significantly benefit the design of lubricant-infused surfaces for various applications.

Droplet Mobility on Slippery Lubricant Impregnated and Superhydrophobic Surfaces under the Effect of Air Shear Flow.

The focus of this study is to investigate and compare the behavior of a droplet on superhydrophobic (SHS) and slippery lubricant impregnated (SLIPS) surfaces under the effect of air shear flow. In this regard, both experimental and numerical analyses have been conducted to compare their performance on droplet mobility under different air speeds. Two different lubricants have been utilized to scrutinize their effect on droplet movement. The numerical simulations have been performed based on the volume of fluid method coupled with the large eddy simulation turbulent model in conjunction with the dynamic contact angle method in addition to a model that can represent the effect of lubricants on slippery surfaces. The numerical simulations are compared with the experimental study in order to shed light on the underlying mechanisms. The results showed that under the same conditions, the critical velocity for droplet movement on the superhydrophobic surfaces is lower than that on the slippery lubricant impregnated surfaces due to the smaller droplet base diameter and the larger contact angle. The hydrodynamics of droplet mobility on superhydrophobic surfaces exhibits a rolling behavior while for the slippery lubricant impregnated surfaces a combination of rolling and sliding is observed. Beyond the critical airflow speed, a complete droplet shedding on all surfaces occurs. The wetting length and position of the droplet on superhydrophobic and slippery surfaces have been measured. On slippery surfaces, the speed of droplets is greatly affected by the lubricant properties while similar behavior in the wetting lengths is observed.

A thermal-driven self-replenishing slippery coating

The durability of the traditional lubricant impregnated surfaces was limited, due to the instability of the infused liquid lubricants and the mechanical weakness of the underlying textures. Herein, to address this challenge, we developed a carbon black @paraffin/PDMS organogel coating with the thermo-responsive self-replenishing property. The surface wetting behavior of the coating was reversible switching between the pinning state at room temperature to the sliding state for temperature above the melting point of paraffin. Programmable water motion was realized by melting the paraffin lubricant locally. The photothermal ability of the carbon black nanoparticles in the coating can accelerate the phase transition of paraffin to realize a quick self-replenishing upon the thermal trigger. In this way, the coating can heal itself easily under sunlight illumination when physical damage occurred. Exploiting its photothermal ability and water-repellency, the organogel coating was demonstrated as a de-icing surface and corrosion protection layer, respectively.

WO3-based slippery coatings with long-term stability for efficient fog harvesting

Inspired by Nepenthes pitcher plants, slippery liquid-infused porous surfaces (SLIPSs) have attracted wide attention and exhibited remarkable liquid repellency, droplet motion control and antifouling properties. However, lubricant-impregnated surfaces have poor durability, leading to loss of control of the movements of droplets during applications. Herein, WO3-based slippery coatings with high stability were prepared by the spray method and photocatalytic reaction. Notably, on the basis of the hierarchical structures, the strong intermolecular forces between the polydimethylsiloxane brush and silicone oil led to the formation of a stable lubricant layer on the WO3-based slippery coating, which can suppress lubricant loss during water collection. After a series of stability tests, such as high-speed centrifugation, long-term storage, acidic solution and multiple heating/cooling cycles, the biomimetic slippery surface still displays excellent surface-slippery stability. Furthermore, the slippery surface exhibits superior water mist capture, water droplet expansion and harvested water removal abilities, leading to good water collection performance. The silicone oil content in the collected water was 28 mg/L, demonstrating that the loss of oil was lower during the water collection process. Even under harsh environments, including multiple heating/cooling cycles, long-term storage and high shear force, lubricant-impregnated coatings can also maintain good water collection efficiency. Therefore, these slippery coatings are promising for widespread application.

A comparison of bioinspired slippery and superhydrophobic surfaces: Micro-droplet impact

Slippery lubricant impregnated surfaces (SLIPSs/LISs) exhibit remarkable features of repellency and droplet mobility to a broad range of complex fluids. Their performance in micro-droplet repellency has received less attention. In this study, the anti-wetting performance of SLIPSs in comparison to superhydrophobic surfaces (SHSs) is investigated for the micro-droplet impact on different textured surfaces. Different series of square-pillar arrays are modeled to consider the effect of surface morphology on droplet hydrodynamics. A multiphase numerical model in conjunction with an accurate contact angle method has been implemented to analyze details of three immiscible phases during the droplet impact on the SLIPS. Our findings revealed that on the SLIPS with a low-density micro-textured surface where the effect of lubricant is more significant, droplet repellency and mobility are improved compared to SHSs. It was illustrated that on the SLIPS, droplet pinning decreased significantly and in low Weber number cases where the effect of lubricant is more noticeable, partial bouncing occurred. It was also observed that slippery surfaces with a low-density of micro-pillars exhibit bouncing behavior, which indicated the repellency effect of lubricant in droplet hydrodynamics. Although micro-droplets failed to recoil at a higher Weber number (We≃160) on both the SHS and the SLIPS, droplet penetration within the micro-structured surface was considerably smaller on the SLIPS.

Contact-Angle Hysteresis and Contact-Line Friction on Slippery Liquid-like Surfaces.

Contact-line pinning and dynamic friction are fundamental forces that oppose the motion of droplets on solid surfaces. Everyday experience suggests that if a solid surface offers low contact-line pinning, it will also impart a relatively low dynamic friction to a moving droplet. Examples of such surfaces are superhydrophobic, slippery porous liquid-infused, and lubricant-impregnated surfaces. Here, however, we show that slippery omniphobic covalently attached liquid-like (SOCAL) surfaces have a remarkable combination of contact-angle hysteresis and contact-line friction properties, which lead to very low droplet pinning but high dynamic friction against the motion of droplets. We present experiments of the response of water droplets to changes in volume at controlled temperature and humidity conditions, which we separately compare to the predictions of a hydrodynamic model and a contact-line model based on molecular kinetic theory. Our results show that SOCAL surfaces offer very low contact-angle hysteresis, between 1 and 3°, but an unexpectedly high dynamic friction controlled by the contact line, where the typical relaxation time scale is on the order of seconds, 4 orders of magnitude larger than the prediction of the classical hydrodynamic model. Our results highlight the remarkable wettability of SOCAL surfaces and their potential application as low-pinning, slow droplet shedding surfaces.

Designing Lubricant-Impregnated Surfaces for Corrosion Protection

Corrosion is a detrimental process that can impact the performance and lifetime of many infrastructural systems. In this research, lubricant-impregnated surfaces (LIS) for corrosion protection are systematically developed and studied. Using microtextures with controlled geometry and spacing, this study shows that the corrosion resistance on LIS is greatly enhanced compared to bare iron as determined by a reduction in the corrosion current density by three orders of magnitude. Furthermore, it shows that the spreading characteristics of the lubricant are important toward ensuring effective corrosion protection. Krytox, a lubricant that covers both inside the textures as well as the top of the textures, provides two orders of magnitude greater corrosion protection as compared to silicone oil that does not cover texture tops. The practical applicability of LIS are highlighted to demonstrate corrosion protection on carbon steel in brine.

One‐Step Fabrication of Universal Slippery Lubricated Surfaces

AbstractAmong the various liquid‐repellent surfaces studied in recent times, lubricant‐impregnated surfaces (LISs) with a mobile lubricant top surface exhibit distinctive liquid repellency. For the fabrication of LISs with stable lubricant impregnation, there are two surface requirements: i) a porous/textured structure and ii) a hydrophobic surface. These limitations require complex processes to be overcome and impede the fabrication of a variety of potential substrates. Herein, a stable and homogeneous lubricant thin layer fabricated by a simple one‐step heating process is introduced on a nonhydrophobic surface with no porous/textured structures. Since the reaction minimizes the interfacial energy, a surface with a stable mobile lubricant thin layer, named “lubricated surface” (LuS), is fabricated. The LuS exhibits outstanding liquid repellency, anti‐biofouling properties against bacteria, and anticorrosivity for metals, along with excellent self‐healing characteristics. Additionally, the LuS can be applied on various materials and structures, including completely flat, nonporous surfaces. These superior virtues of the LuS can open new pathways for the production of universal slippery lubricant thin films for various potential applications.

Femtosecond laser-induced sub-micron and multi-scale topographies for durable lubricant impregnated surfaces for food packaging applications

Adhesion of viscous liquids on packaging surfaces could lead to wastage, an increase of recycling costs, and even customers' dissatisfaction in applications related to food, cosmetics and agrochemical industries. Lubricant-impregnated surfaces (LIS) gained much attention recently over other surface functionalisation technologies due to their non-sticking response to highly viscous liquids. This work reports an investigation into anti-adhesive properties of LIS, with an emphasis on their durability. It provides an insight into the rationale design of LIS topographies in order to maximise their lubricant retention in potential food packaging applications. Femtosecond laser processing and hot embossing were employed to produce two types of topographies for LIS on stainless steel, polypropylene and polystyrene surfaces. The first type was single-scale sub-micron laser induced periodic surface structures (LIPSS), while the second one was multi-scale (MS) structures with both micron and sub-micron features. Droplet shedding characteristics of such LIPSS-LIS and MS-LIS substrates with water, milk and honey were examined under vibration and shear. The critical sliding angles at which liquid droplets attained motion on LIS were observed to be less than 32° for all investigated liquids. However, the LIPSS-LIS substrates retained their functionality even after subjecting them to severe vibration, while the MS-LIS substrates partially lost their anti-adhesive characteristics. At the same time, the MS-LIS substrates exhibited premature pinning of droplets as compared to LIPSS-LIS substrates, under shear forces. Both vibration- and shear-induced loss of lubricant impacted the MS-LIS functionality.

Bubble-Induced Rupture of Droplets on Hydrophobic and Lubricant-Impregnated Surfaces.

Bursting of bubbles is ubiquitous with numerous applications ranging from spraying of pesticides, drug delivery, and inkjet printing to forming emulsions. Understanding the parameters that influence the dynamics of bubble rupture is crucial to design systems with improved performance. Here, we experimentally investigate the behavior of air-bubble-induced rupture of a sessile droplet placed on hydrophobic and lubricant-impregnated surfaces (LIS). We demonstrate that the bubble dynamics inside a sessile droplet and subsequent rupture of the thinning film is dependent on the nature of the underlying substrate and the thermodynamic state of the droplet on the substrate. The growth of the bubble is shown to be dependent on the contact angle hysteresis of water and air on the substrate and the presence of a cloaking oil film around the water droplet. On a plain and textured surface with a high contact angle hysteresis, bubble-induced rupture initiates at the apex of the droplet. In the case of LIS that offers negligible pinning forces, the bubble-induced rupture of liquid shell initiates near the triple contact line. The dynamics of rupture of droplets placed on LIS is shown to be dependent on the viscosity of the impregnating and cloaking lubricant.

Lubricant-Impregnated Surfaces for Mitigating Asphaltene Deposition.

Asphaltenes are heavy aromatic components of crude oil. Their complex chemical makeup-an aromatic core surrounded by aliphatic side chains-enables them to adhere to most surfaces. Their buildup in pipes can result in clogging and lead to interruption of production operations and expensive mechanical cleaning. We demonstrate the use of liquid-impregnated surfaces (LIS) to prevent asphaltene deposition and buildup on substrates. Indeed, these surfaces expose a liquid interface to the working fluid, which combines the benefits of a dynamic defect-free surface and tunable interfacial properties. In contrast to bulk additives that are typically mixed into the oil phase, the impregnating liquid also provides the great benefit of protecting the underlying solid surface with a stable and minimal layer of lubricant, thereby reducing costs and eliminating the need for subsequent downstream removal. We first select and confirm the thermodynamic stability of a suitable lubricant and its lack of interaction with asphaltenes. By using a carefully selected system composed of a textured and functionalized solid substrate in conjunction with a fluorinated lubricant, we show that asphaltene adsorption is prevented over long time scales. We further demonstrate the possibility of building such a system with representative industrial materials such as aluminum and expose the resulting substrate to an external shear flow to simulate pipe flow conditions.

Dynamic Wettability on the Lubricant-Impregnated Surface: From Nucleation to Growth and Coalescence.

The surface dynamic wettability during droplet nucleation and growth involved with phase change is different from the static wettability formed from a sessile drop. Revealing this dynamic wettability of the lubricant-impregnated surfaces (LISs) and identification of the consistency between the wettability during condensation and the static wettability are of significant importance. In this study, we investigated condensation of water droplets on LISs using molecular dynamics simulations. All possible morphologies on LISs were investigated considering the effects of interfacial tension and lubricant thickness. The exploration of droplet behaviors from nucleation to growth and coalescence revealed four nucleation mechanisms and six growth modes. The lubricant was observed to be beneficial for the formation of droplets and maintaining dropwise condensation mode. The present investigation also established that the consistency between the wettability during condensation and the static wettability was determined by the solid-water-oil interface and the lubricant thickness. A map was proposed which helps in deciding whether the wettability during condensation is the same as the static wettability on LIS.