Lubricant-impregnated Surface Research Articles

A bubble energy generator featuring lubricant-impregnated surface with high durability and efficiency

Harvesting bubble energy emerges as a potential sustainable solution to harness tremendous marine energy sources. Yet, current bubble energy generators are susceptible to unstable performance owing to the degradation of the physiochemical properties of surface materials under the harsh underwater environment such as surface fouling and wetting transition. Here, we report the design of bubble energy generators featuring the cooperative integration of a slippery lubricant-impregnated porous surface and transistor-inspired architecture, referred to as SLIPS-TBG, that can generate electricity from moving bubbles with high stability in submerged conditions. In this design, the slippery and configurable lubricant layer serves as not only a smooth and mechanically stable surface for bubble transport but also a protective layer preventing the underlying solid materials from direct contact with the external harsh environment. Together with the transistor-inspired architecture, the SLIPS-TBG can deliver a stable electric output of 26.4 V, an instantaneous peak power density of 58.6 W m-3 from continuous bubble impingement in water media, even in extreme conditions involving mechanical abrasion and corrosion. We envision such a design represents a significant step toward real-life applications for bubble-based energy harvesting.

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.

Multifunctional Edible Oil-Impregnated Nanoporous Oxide Layer on AISI 304 Stainless Steel.

Slippery liquid-infused porous surface (SLIPS) realized on commercial materials provides various functionalities, such as corrosion resistance, condensation heat transfer, anti-fouling, de/anti-icing, and self-cleaning. In particular, perfluorinated lubricants infused in fluorocarbon-coated porous structures have showed exceptional performances with durability; however, they caused several issues in safety, due to their difficulty in degradation and bio-accumulation. Here, we introduce a new approach to create the multifunctional lubricant-impregnated surface with edible oils and fatty acid, which are also safe to human body and degradable in nature. The edible oil-impregnated anodized nanoporous stainless steel surface shows a significantly low contact angle hysteresis and sliding angle, which is similar with general surface of fluorocarbon lubricant-infused systems. The edible oil impregnated in the hydrophobic nanoporous oxide surface also inhibits the direct contact of external aqueous solution to a solid surface structure. Due to such de-wetting property caused by a lubricating effect of edible oils, the edible oil-impregnated stainless steel surface shows enhanced corrosion resistance, anti-biofouling and condensation heat transfer with reduced ice adhesion.

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.

Preparation for and performance of a Pseudomonas aeruginosa biofilm experiment on board the International Space Station

Biofilms are problematic on Earth due to their ability to both degrade the materials upon which they grow and promote infections. Remarkably, 65% of infections and 80% of chronic diseases on Earth are associated with biofilms. The impact of biofilms is even greater in space, as the crew's lives and mission success depend on nominal operation of mechanical systems which can be interrupted by material damage associated with biofilm growth. Furthermore, the isolated confined environment nature of spaceflight may increase the rates of disease transmission. In the case of the International Space Station (ISS), biofilms are an identified problem on the Environmental Control and Life Support System (ECLSS), namely on the water processor assembly (WPA). In late 2019, the bacterial component of the Space Biofilms experiment launched to ISS to (i) characterize the mass, thickness, morphology, and gene expression of biofilms formed in space compared to matched Earth controls, (ii) interrogate the expression of antimicrobial resistance genes, and (iii) test novel materials as potential biofilm control strategies for future ECLSS components. For this, 288 bacterial samples were prepared prior to the launch of the Northrop Grumman CRS-12 mission from NASA's Wallops Flight Facility. The samples were integrated into the spaceflight hardware, BioServe's Fluid Processing Apparatus (FPA), packed in sets of eight in Group Activation Packs (GAP). Half of these samples were activated and terminated on orbit by NASA astronauts Jessica Meir and Christina Koch, while the remaining half were processed equivalently on Earth. The spaceflight bacterial samples of Space Biofilms returned on board the SpaceX CRS-19 Dragon spacecraft in early 2020. We here describe the test campaign implemented to verify the experiment design and confirm it would enable us to achieve the project's scientific goals. This campaign ended with the Experiment Verification Test (EVT), from which we present example morphology and transcriptomic results. We describe in detail the sample preparation prior to flight, including cleaning and sterilization of the coupons of six materials (SS316, passivated-SS316, lubricant impregnated surface, catheter-grade silicone with and without a microtopography, and cellulose membrane), loading and integration of growth media, bacterial inoculum, and the fixative and preservative to enable experiment termination on orbit. Additionally, we describe the performance of the experiment on board the ISS, including crew activities, use of assets, temperature profile, and experiment timeline; all leading to a successful spaceflight experiment. Hence, this manuscript focuses on the steps implemented to ensure the experiment would be ready for spaceflight, as well as ISS and ground operations, with results presented elsewhere. The processes discussed here may serve as a guideline to teams planning their own gravitational microbiology experiments. This material is based upon work supported by the National Aeronautics and Space Administration under Grant No. 80NSSC17K0036.

Lubricant-enhanced self-transport of condensed nanodroplets trapped in Wenzel state

Condensates are extremely easy to be trapped in micro/nano-gaps of surfaces and stay in the sticky Wenzel wetting state due to the nucleation. Rather than avoiding the unexpected Wenzel state, an efficient detachment of Wenzel condensates is targeted in present work. A bionic lubricant-impregnated wedge-groove surface is proposed inspired by a combination of the conical cactus spines and the oil-immersion structure of pitcher plants. The dynamic behaviours of a single nanodroplet and condensates are investigated using molecular dynamics simulations. By taking advantage of the conical structure and the thoroughly wetted liquid–solid interface in the groove, droplets trapped in the Wenzel state are observed to move spontaneously on the proposed patterned surface, while droplets in the Cassie state are found immobile. Moreover, the nucleation sites can be controlled and the nucleation energy barriers are also reduced by the proposed surface. The present work can pave a new path to manipulate the droplets in the sticky Wenzel state with higher nucleation and drainage rate, which is desired to provide an energy-efficient method for efficient condensation.

Light-driven Locomotion of Underwater Bubbles on Ultrarobust Paraffin-impregnated Laser-ablated Fe3O4-doped Slippery Surfaces

Manipulating underwater bubbles (UGBs) is realized on morphology-tailored or stimuli-responsive slippery lubricant-impregnated porous surface (SLIPS). Unfortunately, the volatile lubricants (e. g., silicone oil, ferrofluid) greatly decrease their using longevity. Designed is light-responsive paraffin-infused Fe3O4-doped slippery surface (LR-PISS) by incorporation of hybrid lubricants and superhydrophobic micropillar-arrayed elastometric membranes resulted from one-step femtosecond laser vertically scanning. Upon LR-PISS, the dynamic motion control bwteen pinning and sliding along free routes over UGB could be realized by alternately loading/discharging NIR-trigger. The underlying principle is that when the NIR was applied, UGB would be actuated to slide along the NIR trace because the irradiated domain melts for a slippery surface within 1.0 s. Once the NIR is removed, the liquefied paraffin would be reconfigured to solid phase for pinning a moving UGB within 0.5 s. Newly explored hydrokinetics imparts us with capability of steering UGBs to arrange any desirable patterns and switch light-path behaving as the light-control-light optical shutter. In comparison with previously reported SLIPS, current LR-PISS unfolds unparalleled ultrarobust antidisturbance ability even in flowing liquid ambient. More significantly, even subjected to physical damage, underwater LR-PISS is capable of in situ self-healing within 13 s under the assistance of remote NIR. The results here could inspire the design of robust bubble manipulator and further boost their applications in optofluidics and all-optical modulators.

Failure and Recovery of Droplet Nucleation and Growth on Damaged Nanostructures: A Molecular Dynamics Study.

The condensate flooding during dropwise condensation causes serious deterioration in heat transfer performance. In this study, the three-dimensional large-scale molecular dynamics simulation is carried out to investigate the droplet state transition from local flooding mode to Wenzel or from Wenzel to Cassie due to the droplet coalescence under the effect of nanostructure size. In particular, the effect of nanostructure breakage on droplet nucleation and growth is discussed to reveal the mechanism of dropwise condensation heat transfer deterioration. As a potential solution, the lubricant-impregnated surface is proposed to recover the preferred Cassie state by regulating the dynamic wetting characteristics of droplets, and thus the detrimental effect of nanostructure breakage could be effectively avoided.

Interfacial Strategies for Smart Slippery Surfaces

The problem of contact line pinning on surfaces is pervasive and contributes to problems from ring stains to ice formation. Here we provide a single conceptual framework for interfacial strategies encompassing five strategies for modifying the solid-liquid interface to remove pinning and increase droplet mobility. Three biomimetic strategies are included, (i) reducing the liquid-solid interfacial area inspired by the Lotus effect, (ii) converting the liquid-solid contact to a solid-solid contact by the formation of a liquid marble inspired by how galling aphids remove honeydew, and (iii) converting the liquid-solid interface to a liquid-lubricant contact by the use of a lubricant impregnated surface inspired by the Nepenthes Pitcher plant. Two further strategies are, (iv) converting the liquid-solid contact to a liquid-vapor contact by using the Leidenfrost effect, and (v) converting the contact to a liquid-liquid-like contact using slippery omniphobic covalent attachment of a liquid-like coating (SOCAL). Using these approaches, we explain how surfaces can be designed to have smart functionality whilst retaining the mobility of contact lines and droplets. Furthermore, we show how droplets can evaporate at constant contact angle, be positioned using a Cheerios effect, transported by boundary reconfiguration in an energy invariant manner, and drive the rotation of solid components in a Leidenfrost heat engine. Our conceptual framework enables the rationale design of surfaces which are slippery to liquids and is relevant to a diverse range of applications.

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.

SLIPS-TENG: robust triboelectric nanogenerator with optical and charge transparency using a slippery interface.

Energy harvesting devices that prosper in harsh environments are highly demanded in a wide range of applications ranging from wearable and biomedical devices to self-powered and intelligent systems. Particularly, over the past several years, the innovation of triboelectric nanogenerators (TENGs) that efficiently convert ambient kinetic energy of water droplets or wave power to electricity has received growing attention. One of the main bottlenecks for the practical implications of such devices originates from the fast degradation of the physiochemical properties of interfacial materials under harsh environments. To overcome these challenges, here we report the design of a novel slippery lubricant-impregnated porous surface (SLIPS) based TENG, referred to as SLIPS-TENG, which exhibits many distinctive advantages over conventional design including optical transparency, configurability, self-cleaning, flexibility, and power generation stability, in a wide range of working environments. Unexpectedly, the slippery and configurable lubricant layer not only serves as a unique substrate for liquid/droplet transport and optical transmission, but also for efficient charge transfer. Moreover, we show that there exists a critical thickness in the liquid layer, below which the triboelectric effect is almost identical to that without the presence of such a liquid film. Such an intriguing charge transparency behavior is reminiscent of the wetting transparency and van der Waals potential transparency of graphene previously reported, though the fundamental mechanism remains to be elucidated. We envision that the marriage of these two seemingly totally different arenas (SLIPS and TENG) provides a paradigm shift in the design of robust and versatile energy devices that can be used as a clean and longer-lifetime alternative in various working environments.

Apparent Contact Angles on Lubricant-Impregnated Surfaces/SLIPS: From Superhydrophobicity to Electrowetting.

A fundamental limitation of liquids on many surfaces is their contact line pinning. This limitation can be overcome by infusing a nonvolatile and immiscible liquid or lubricant into the texture or roughness created in or applied onto the solid substrate so that the liquid of interest no longer directly contacts the underlying surface. Such slippery liquid-infused porous surfaces (SLIPS), also known as lubricant-impregnated surfaces, completely remove contact line pinning and contact angle hysteresis. However, although a sessile droplet may rest on such a surface, its contact angle can be only an apparent contact angle because its contact is now with a second liquid and not a solid. Close to the solid, the droplet has a wetting ridge with a force balance of the liquid-liquid and liquid-vapor interfacial tensions described by Neumann's triangle rather than Young's law. Here, we show how, provided the lubricant coating is thin and the wetting ridge is small, a surface free energy approach can be used to obtain an apparent contact angle equation analogous to Young's law using interfacial tensions for the lubricant-vapor and liquid-lubricant and an effective interfacial tension for the combined liquid-lubricant-vapor interfaces. This effective interfacial tension is the sum of the liquid-lubricant and the lubricant-vapor interfacial tensions or the liquid-vapor interfacial tension for a positive and negative spreading power of the lubricant on the liquid, respectively. Using this approach, we then show how Cassie-Baxter, Wenzel, hemiwicking, and other equations for rough, textured or complex geometry surfaces and for electrowetting and dielectrowetting can be used with the Young's law contact angle replaced by the apparent contact angle from the equivalent smooth lubricant-impregnated surface. The resulting equations are consistent with the literature data. These results enable equilibrium contact angle theory for sessile droplets on surfaces to be used widely for surfaces that retain a thin and conformal SLIPS coating.

Apparent slip and drag reduction for the flow over superhydrophobic and lubricant-impregnated surfaces

The flow within microscopic indentations filled with a lubricant fluid is studied by varying viscosity ratio, surface tension, and lubricant-to-working-fluid volume fraction. This yields protrusion heights that readily quantify the drag reduction of a lubricant-impregnated microstructured surface.

Design of a spaceflight biofilm experiment

Biofilm growth has been observed in Soviet/Russian (Salyuts and Mir), American (Skylab), and International (ISS) Space Stations, sometimes jeopardizing key equipment like spacesuits, water recycling units, radiators, and navigation windows. Biofilm formation also increases the risk of human illnesses and therefore needs to be well understood to enable safe, long-duration, human space missions. Here, the design of a NASA-supported biofilm in space project is reported. This new project aims to characterize biofilm inside the International Space Station in a controlled fashion, assessing changes in mass, thickness, and morphology. The space-based experiment also aims at elucidating the biomechanical and transcriptomic mechanisms involved in the formation of a “column-and-canopy” biofilm architecture that has previously been observed in space. To search for potential solutions, different materials and surface topologies will be used as the substrata for microbial growth. The adhesion of bacteria to surfaces and therefore the initial biofilm formation is strongly governed by topographical surface features of about the bacterial scale. Thus, using Direct Laser-Interference Patterning, some material coupons will have surface patterns with periodicities equal, above or below the size of bacteria. Additionally, a novel lubricant-impregnated surface will be assessed for potential Earth and spaceflight anti-biofilm applications. This paper describes the current experiment design including microbial strains and substrata materials and nanotopographies being considered, constraints and limitations that arise from performing experiments in space, and the next steps needed to mature the design to be spaceflight-ready.

Behavior of an evaporating water droplet on lubricant-impregnated nano-structured surface

The behavior of an evaporating water droplet on lubricant-impregnated nano-structured surface (LINS) was investigated experimentally. For comparison, the flat hydrophobic surface (FHS) and superhydrophobic nano-structured surface (SHNS) were also prepared and tested. The initial water contact angles (WCAs) of LINS, FHS, and SHNS showed 108°, 103°, and 153°, respectively, and their WCA hysteresis (WCAH) were 2°, 30°, and 23°, respectively. During the evaporation of water droplet, FHS showed a typical constant contact radius (CCR) mode. On the contrary, SHNS and LINS showed more complicated mixed mode. These differences of evaporation modes were closely related to the total evaporation time. Initial WCA of LINS was between FHS and SHNS, but the water droplet on LINS showed the longest total evaporation time in the test surfaces. This may be because LINS has the water droplet of the high aspect ratio during the evaporation and the low thermal effusivity. In such a case, the large height of water droplet plays a role to the longer thermal resistance path, which can result in extending the total evaporation time.

Viscous dewetting of metastable liquid films on substrates with microgrooves

We present a combined experimental and theoretical investigation of dewetting on substrates with parallel microgrooves. A thin, static liquid film has an equilibrium thickness so as to minimize the sum of the surface free energy and the gravitational potential energy. When the thickness of a liquid film is less than the equilibrium thickness, the film seeks the equilibrium through contraction of the wetted area, which is referred to as dewetting. We experimentally observed the dewetting of thin, metastable liquid films on substrates with parallel microgrooves. The experiments revealed that the films retract in the direction along the grooves and leaves liquid residues with various morphologies. We classify the residue morphologies into three modes and elucidate the dependence of the mode selection on the groove geometry and the equilibrium contact angle of the liquid. We also experimentally examined the dynamic motion of the receding contact lines of the dewetting films, and developed a mechanical model for the receding speed. Our results provide a basis for controlling liquid films using microstructures, which is useful for lubricant-impregnated surface production, painting, spray cooling, and surface cleaning.

3D Imaging of Water-Drop Condensation on Hydrophobic and Hydrophilic Lubricant-Impregnated Surfaces.

Condensation of water from the atmosphere on a solid surface is an ubiquitous phenomenon in nature and has diverse technological applications, e.g. in heat and mass transfer. We investigated the condensation kinetics of water drops on a lubricant-impregnated surface, i.e., a micropillar array impregnated with a non-volatile ionic liquid. Growing and coalescing drops were imaged in 3D using a laser scanning confocal microscope equipped with a temperature and humidity control. Different stages of condensation can be discriminated. On a lubricant-impregnated hydrophobic micropillar array these are: (1) Nucleation on the lubricant surface. (2) Regular alignment of water drops between micropillars and formation of a three-phase contact line on a bottom of the substrate. (3) Deformation and bridging by coalescence which eventually leads to a detachment of the drops from the bottom substrate. The drop-substrate contact does not result in breakdown of the slippery behaviour. Contrary, on a lubricant-impregnated hydrophilic micropillar array, the condensed water drops replace the lubricant. Consequently, the surface loses its slippery property. Our results demonstrate that a Wenzel-like to Cassie transition, required to maintain the facile removal of condensed water drops, can be induced by well-chosen surface hydrophobicity.

Ice Adhesion on Lubricant-Impregnated Textured Surfaces

Ice accretion is an important problem and passive approaches for reducing ice-adhesion are of great interest in various systems such as aircrafts, power lines, wind turbines, and oil platforms. Here, we study the ice-adhesion properties of lubricant-impregnated textured surfaces. Force measurements show ice adhesion strength on textured surfaces impregnated with thermodynamically stable lubricant films to be higher than that on surfaces with excess lubricant. Systematic ice-adhesion measurements indicate that the ice-adhesion strength is dependent on texture and decreases with increasing texture density. Direct cryogenic SEM imaging of the fractured ice surface and the interface between ice and lubricant-impregnated textured surface reveal stress concentrators and crack initiation sites that can increase with texture density and result in lowering adhesion strength. Thus, lubricant-impregnated surfaces have to be optimized to outperform state-of-the-art icephobic treatments.