Fluid Slip and Drag Reduction on Liquid-Infused Surfaces under High Static Pressure.

Liquid-infused surfaces (LIS) have been shown to reduce the huge frictional drag affecting microfluidic flow and are expected to be more robust than superhydrophobic surfaces when exposed to external pressure as the lubricant in LIS is incompressible. Here, we investigate the effect of applying static pressure on the effective slip length measured on Teflon wrinkled surfaces infused with silicone oil through pressure measurements in microfluidic devices. The effect of static pressure on LIS was found to depend on air content in the flowing water: for degassed water, the average effective slip length was beff = 2.16 ± 0.90 µm, irrespective of applied pressure. In gassed water, the average effective slip length was beff = 4.32 ± 1.06 µm at zero applied pressure, decreased by 55% to 2.37 ± 0.90 µm when the pressure was increased to 50 kPa, and then remained constant up to 200 kPa. The result is due to nanobubbles present on LIS, which are compressed or partially dissolved under pressure, and the effect is more evident when the size and portion of surface nanobubbles are higher. In contrast, on superhydrophobic wrinkles, the decline in beff was more sensitive to applied pressure, with beff = 6.8 ± 1.4 µm at 0 kPa and, on average, beff = -1 ± 3 µm for pressures higher than 50 kPa for both gassed and degassed water. Large fluctuations in the experimental measurements were observed on superhydrophobic wrinkles, suggesting the nucleation of large bubbles on the surface. The same pressure increase did not affect the flow on smooth substrates, on which gas nanobubbles were not observed. Contrary to expectations, we observed that drag reduction in LIS is affected by applied pressure, which we conclude is because, in a similar manner to superhydrophobic surfaces, they lose the interfacial gas, which lubricates the flow.

Visualizing a Nanoscale Lubricant Layer under Blood Flow.

Tethered-liquid perfluorocarbons (TLPs) are a class of liquid-infused surfaces with the ability to reduce blood clot formation (thrombosis) on blood-contacting medical devices. TLP comprises a tethered perfluorocarbon (TP) infused with a liquid perfluorocarbon (LP); this LP must be retained to maintain the antithrombotic properties of the layer. However, the stability of the LP layer remains in question, particularly for medical devices under blood flow. In this study, the lubricant thickness is spatially mapped and quantified in situ through confocal dual-wavelength reflection interference contrast microscopy. TLP coatings prepared on glass substrates are exposed to the flow of 37% glycerol/water mixtures (v/v) or whole blood at a shear strain rate of around 2900 s-1 to mimic physiological conditions (similar to flow conditions found in coronary arteries). Excess lubricant (>2 µm film thickness) is removed upon commencement of flow. For untreated glass, the lubricant is completely depleted after 1 min of shear flow. However, on optimized TLP surfaces, nanoscale films of lubricants (thickness between 100 nm and 2 µm) are retained over many tens of minutes of flow. The nanoscale films conform to the underlying structure of the TP layer and are sufficient to prevent the adhesion of red blood cells and platelets.

Nanostructure Explains the Behavior of Slippery Covalently Attached Liquid Surfaces.

Slippery covalently-attached liquid surfaces (SCALS) with low contact angle hysteresis (CAH, <5°) and nanoscale thickness display impressive anti-adhesive properties, similar to lubricant-infused surfaces. Their efficacy is generally attributed to the liquid-like mobility of the constituent tethered chains. However, the precise physico-chemical properties that facilitate this mobility are unknown, hindering rational design. This work quantifies the chain length, grafting density, and microviscosity of a range of polydimethylsiloxane (PDMS) SCALS, elucidating the nanostructure responsible for their properties. Three prominent methods are used to produce SCALS, with characterization carried out via single-molecule force measurements, neutron reflectometry, and fluorescence correlation spectroscopy. CO2 snow-jet cleaning was also shown to reduce the CAH of SCALS via a modification of their grafting density. SCALS behavior can be predicted by reduced grafting density, Σ, with the lowest water CAH achieved at Σ≈2. This study provides the first direct examination of SCALS grafting density, chain length, and microviscosity and supports the hypothesis that SCALS properties stem from a balance of layer uniformity and mobility.

Cassie's Law Reformulated: Composite Surfaces from Superspreading to Superhydrophobic.

In 1948, Cassie provided an equation describing the wetting of a smooth, heterogeneous surface. He proposed that the cosine of the contact angle, θc, for a droplet on a composite surface could be predicted from a weighted average using the fractional surface areas, fi, of the cosines of contact angles of a droplet on the individual component surfaces, i.e., cos θc = f1 cos θ1 + f2 cos θ2. This was a generalization of an earlier equation for porous materials, which has recently proven fundamental to underpinning the theoretical description of wetting of superhydrophobic and superoleophobic surfaces. However, there has been little attention paid to what happens when a liquid exhibits complete wetting on one of the surface components. Here, we show that Cassie's equation can be reformulated using spreading coefficients. This reformulated equation is capable of describing composite surfaces where the individual surface components have negative (droplet state/partial wetting) or positive (film-forming/complete wetting) spreading coefficients. The original Cassie equation is then a special case when the combination of interfacial tensions results in a droplet state on the composite surface for which a contact angle can be defined. In the case of a composite surface created from a partial wetting (droplet state) surface and a complete wetting (film-forming) surface, there is a threshold surface area fraction at which a liquid on the composite surface transitions from a droplet to a film state. The applicability of this equation is demonstrated from literature data including data on mixed self-assembled monolayers on copper, silver, and gold surfaces that was regarded as definitive in establishing the validity of the Cassie equation. Finally, we discuss the implications of these ideas for super-liquid repellent surfaces.

Exploring the water capture efficiency of covalently attached liquid-like surfaces.

The capture of moisture from the atmosphere through condensation has the potential to provide a sustainable source of water. Here, we investigate the condensation of humid air at low subcooling condition (11 °C), similar to conditions for natural dew capture, and explore how water contact angle and contact angle hysteresis affect the rates of water capture. We compare water collection on three families of surfaces: (i) hydrophilic (polyethylene oxide, MPEO) and hydrophobic (polydimethylsiloxane, PDMS) molecularly thin coatings grafted on smooth silicon wafers, which produce slippery covalently attached liquid surfaces (SCALSs), with low contact angle hysteresis (CAH = 6°); (ii) the same coatings grafted on rougher glass, with high CAH (20°-25°); (iii) hydrophilic polymer surfaces [poly(N-vinylpyrrolidone), PNVP] with high CAH (30°). Upon exposure to water, the MPEO SCALS swell, which likely further increases their droplet shedding ability. MPEO and PDMS coatings collect similar volume of water (around 5 l m-2 day-1), both when they are SCALS and non-slippery. Both MPEO and PDMS layers collect about 20% more water than PNVP surfaces. We present a basic model showing that, under low heat flux conditions, on all MPEO and PDMS layers, the droplets are so small (600-2000 µm) that there is no/low heat conduction resistance across the droplets, irrespective of the exact value of contact angle and CAH. As the time to first droplet departure is much faster on MPEO SCALS (28 min) than on PDMS SCALS (90 min), slippery hydrophilic surfaces are preferable in dew collection applications where the collection time frame is limited.

Advances and challenges in slippery covalently-attached liquid surfaces.

Over the past decade, a new class of slippery, anti-adhesive surfaces known as slippery covalently-attached liquid surfaces (SCALS) has emerged, characterized by low values of contact angle hysteresis (CAH, less than 5°) with water and most solvents. Despite their nanoscale thickness (1 to 5 nm), SCALS exhibit behavior similar to lubricant-infused surfaces, including high droplet mobility and the ability to prevent icing, scaling, and fouling. To date, SCALS have primarily been obtained using grafted polydimethylsiloxane (PDMS), though there are also examples of polyethylene oxide (PEO), perfluorinated polyether (PFPE), and short-chain alkane SCALS. Importantly, the precise physico-chemical characteristics that enable ultra-low CAH are unknown, making rational design of these systems impossible. In this review, we conduct a quantitative and comparative analysis of reported values of CAH, molecular weight, grafting density, and layer thickness for a range of SCALS. We find that CAH does not scale monotonically with any reported parameter; instead, the CAH minimum is found at intermediate values. For PDMS, optimal behavior is observed at advancing contact angle of 106°, molecular weight between 2 and 10 kg mol-1, and grafting density of around 0.5 nm-2. CAH on SCALS is lowest for layers created from end-grafted chains and increases with the number of binding sites, and can generally be improved by increasing the chemical homogeneity of the surface through the capping of residual silanols. We review the existing literature on SCALS, including both synthetic and functional aspects of current preparative methods. The properties of reported SCALS are quantitatively analyzed, revealing trends in the existing data and highlighting areas for future experimental study.

Detection of Nanobubbles on Lubricant-Infused Surfaces Using AFM Meniscus Force Measurements.

So far, the presence of nanobubbles on lubricant-infused surfaces (LIS) has been overlooked, because of the difficulty in detecting them in such a complex system. We recently showed that anomalously large interfacial slip measured on LIS is explained by the presence of nanobubbles [Vega-Sánchez, Peppou-Chapman, Zhu and Neto, Nat. Commun., 2022 13, 351]. Crucial to drawing this conclusion was the use of atomic force microscopy (AFM) force-distance spectroscopy (meniscus force measurements) to directly image nanobubbles on LIS. This technique provided vital direct evidence of the spontaneous nucleation of nanobubbles on lubricant-infused hydrophobic surfaces. In this paper, we describe in detail the data collection and analysis of AFM meniscus force measurements on LIS and show how these powerful measurements can quantify both the thickness and distribution of multiple coexisting fluid layers (i.e., gas and oil) over a nanostructured surface. Using this technique, thousands of force curves were automatically analyzed. The results show that the interfacial tension of the nanobubbles is reduced from 52 ± 9 mN m-1 to 39 ± 4 mN m-1 by the presence of the silicone oil layer.

Mechanisms for Reduced Fibrin Clot Formation on Liquid-Infused Surfaces.

Biomedical devices are prone to blood clot formation (thrombosis), and liquid-infused surfaces (LIS) are effective in reducing the thrombotic response. However, the mechanisms that underpin this performance, and in particular the role of the lubricant, are not well understood. In this work, it is investigated whether the mechanism of LIS action is related to i) inhibition of factor XII (FXII) activation and the contact pathway; ii) reduced fibrin density of clots formed on surfaces; iii) increased mobility of proteins or cells on the surface due to the interfacial flow of the lubricant. The chosen LIS is covalently tethered, nanostructured layers of perfluorocarbons, infused with thin films of medical-grade perfluorodecalin (tethered-liquid perfluorocarbon), prepared with chemical vapor deposition previously optimized to retain lubricant under flow. Results show that in the absence of external flow, interfacial mobility is inherently higher at the liquid-blood interface, making it a key contributor to the low thrombogenicity of LIS, as FXII activity and fibrin density are equivalent at the interface. The findings of this study advance the understanding of the anti-thrombotic behavior of LIS-coated biomedical devices for future coating design. More broadly, enhanced interfacial mobility may be an important, underexplored mechanism for the anti-fouling behavior of surface coatings.

Slightly Depleted Lubricant-Infused Surfaces Are No Longer Slippery.

Textured surfaces infused with a lubricating fluid effectively reduce fouling and drag. These functions critically depend on the presence and distribution of the lubricant, which can be depleted by many mechanisms, including shear flow. We present a two-phase Couette flow computational dynamic simulation over lubricant-infused surfaces containing grooves oriented perpendicular to the flow direction, with the aim of revealing how interfacial slip, and therefore drag reduction, is impacted by lubricant depletion. We show that even a slight (20%) lubricant loss decreases slip to the point of making the lubricant superfluous, even for lubricants with lower viscosity than the flowing liquid and regardless of how well the lubricant wets the grooves. We explain that the drastic slip reduction is linked to a significant increase in the total viscous dissipation and to zero dissipation in the lubricant (similar to the one given by a no-slip boundary).

Nanobubbles explain the large slip observed on lubricant-infused surfaces.

Lubricant-infused surfaces hold promise to reduce the huge frictional drag that slows down the flow of fluids at microscales. We show that infused Teflon wrinkled surfaces induce an effective slip length 50 times larger than expected based on the presence of the lubricant alone. This effect is particularly striking as it occurs even when the infused lubricant's viscosity is several times higher than that of the flowing liquid. Crucially, the slip length increases with increasing air content in the water but is much higher than expected even in degassed and plain Milli-Q water. Imaging directly the immersed interface using a mapping technique based on atomic force microscopy meniscus force measurements reveals that the mechanism responsible for this huge slip is the nucleation of surface nanobubbles. Using a numerical model and the height and distribution of these surface nanobubbles, we can quantitatively explain the large fluid slip observed in these surfaces.

Convergent evolution of skin surface microarchitecture and increased skin hydrophobicity in semi-aquatic anole lizards.

Animals that habitually cross the boundary between water and land face specific challenges with respect to locomotion, respiration, insulation, fouling and waterproofing. Many semi-aquatic invertebrates and plants have developed complex surface microstructures with water-repellent properties to overcome these problems, but equivalent adaptations of the skin have not been reported for vertebrates that encounter similar environmental challenges. Here, we document the first evidence of evolutionary convergence of hydrophobic structured skin in a group of semi-aquatic tetrapods. We show that the skin surface of semi-aquatic species of Anolis lizards is characterized by a more elaborate microstructural architecture (i.e. longer spines and spinules) and a lower wettability relative to closely related terrestrial species. In addition, phylogenetic comparative models reveal repeated independent evolution of enhanced skin hydrophobicity associated with the transition to a semi-aquatic lifestyle, providing evidence of adaptation. Our findings invite a new and exciting line of inquiry into the ecological significance, evolutionary origin and developmental basis of hydrophobic skin surfaces in semi-aquatic lizards, which is essential for understanding why and how the observed skin adaptations evolved in some and not other semi-aquatic tetrapod lineages.

Depletion of the Lubricant from Lubricant-Infused Surfaces due to an Air/Water Interface.

Lubricant-infused surfaces (LIS) have emerged as an innovative way to combat several modern challenges such as biofouling, ice formation, and surface drag. The favorable properties of LIS are dependent on the presence and distribution of a lubricant layer coating the underlying substrate. Unfortunately, this layer is not indefinitely stable and depletes due to external forces. Here, we study how an air/water interface depletes the lubricant from LIS as a function of lubricant wettability on the substrate by varying the chemistry of both the lubricant and the substrate. The lubricants were chosen to represent some of those most commonly used in the literature (silicone oil, perfluoropolyethers, and mineral oil). We use an optical Wilhelmy plate tensiometer to measure the contact angle of the air/water interface on the LIS in situ as the sample is driven through the air/water interface and contact angle hysteresis as a qualitative measure of lubricant depletion. This data is augmented with ex situ quantitative mapping of lubricant thickness using atomic force microscopy (AFM) meniscus force measurements. We find that a thick layer of excess lubricant is always removed in just one dip, regardless of wettability, and that lubricants that do not spread fully on the substrate deplete faster due to their dewetting into droplets. We also find that lubricants that spread onto the air/water interface are more susceptible to depletion. Finally, we investigate the effect of repeated immersions on the properties of liquidlike poly(dimethylsiloxane) (PDMS) chains tethered to glass and find that dynamic contact angles on these surfaces remain constant over several dips and therefore their low hysteresis is unlikely due to unbound polymer.

Evaluating medical device and material thrombosis under flow: current and emerging technologies.

Although blood-contacting medical devices are used widely, blood clot formation (thrombosis) leads to device failure and potentially catastrophic adverse thrombotic events for patients, such as stroke or pulomonary embolism. Systemic anti-thrombotic drugs aimed at reducing these complications do not always prevent device thrombosis and can cause increased bleeding risks. Therefore, our understanding of material thrombosis mechanisms needs to be improved in order to develop next generation blood-contacting medical devices and materials. Medical device development requires material thrombogenicity evaluation according to the International Standards 10993-4 Biological evaluation of medical devices-Selection of tests for interactions with blood, which highlights that one of the key aspects for testing is a clinically relevant flow system. In this review, we first provide an overview of the current knowledge regarding material thrombosis and important physical and biological aspects of blood flow in relation to thrombus formation. We then examine commonly used in vitro flow systems to evaluate material and medical device thrombosis, focusing on their capabilities, advantages and disadvantages. Finally, we explore recent advances in technology that will aid in improving the design and fabrication of flow systems, mechanistic analysis and computational modelling.

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

Liquid-infused surfaces (or lubricant-infused surfaces) (LIS) are a new class of functional materials introduced in 2011. Their exceptional properties have earned them a place at the forefront of many fields including anti-biofouling, anti-icing, anti-corrosion, drag reduction, droplet manipulation and drop-wise condensation. Integral to their success is the infused lubricant layer which affords them their properties. In this review, we examine the current state of the literature relating to the lubricant layer. We consider the lubricant through all stages in the surface's lifecycle from design, to use, all the way through to depletion and eventual failure. First, we examine trends in lubricant choice and how to choose a lubricant, including environmental and medical considerations. We then look at the different methods used to infuse lubricant into surfaces and how lubricant depletes from the surface. We then report direct and indirect methods to characterise the thickness and distribution of the lubricant layer. Finally, we examine how droplets interact with LIS and the unique properties afforded by the lubricant before providing an outlook into where research centred on understanding the lubricant layer is heading in the new decade.

Large Effective Slip on Lubricated Surfaces Measured with Colloidal Probe AFM.

In this work, we study the interfacial boundary conditions at the interface between two immiscible liquids under a laminar flow. We measure the hydrodynamic drainage forces acting on a colloid probe as it approaches a flat and smooth Teflon film coated with silicone oil films, submerged in a sucrose solution using atomic force microscopy. On Teflon substrates, silicone oil films of thickness several hundred nanometers could be stabilized, and we found the effective slip length over these to be of the order of several hundred nanometers which increases with increasing silicone oil film thickness, as expected. The fitted slip length values weakly increased with increasing shear rates. The high values of effective slip length indicate that lubricant-infused surfaces are likely to reduce drag on length scales that approach the macroscopic scales.

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

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

Ultralow surface energy self-assembled monolayers of iodo-perfluorinated alkanes on silica driven by halogen bonding.

Compact self-assembled monolayers (SAMs) of perfluorododecyl iodide (I-PFC12) of reproducible thickness (1.2 nm) are shown to form on silicon wafers. The SAMs have a high fluorine content (95%) and convey an extremely low surface energy to the silicon wafers (4.3 mN m-1), lower than previously reported in the literature for perfluorinated monolayers, and stable for over eight weeks. Shorter chain iodo-perfluorinated (I-PFC8) or bromo-perfluorinated molecules (Br-PFC10) led to less dense layers. The monolayers are stable to heating up to 60 °C, with some loss up to 150 °C. The I-PFC12 monolayer increases the work function of silicon wafers from 3.6 V to 4.4 eV, a factor that could be gainfully used in photovoltaic applications. The I-PFC12 monolayers can be transferred into patterns onto silica substrates by micro-contact printing. The NMR data and the reproducible thickness point to an upright halogen bonding interaction between the iodine in I-PFC12 and the surface oxygen on the native silica layer.

Mapping Depletion of Lubricant Films on Antibiofouling Wrinkled Slippery Surfaces.

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

Influence of long-range forces and capillarity on the function of underwater superoleophobic wrinkled surfaces.

Underwater superoleophobic surfaces can be considered a particular type of lubricant-infused surface, that have anti-fouling properties by virtue of a trapped water layer that repels oils. However, as their function relies on a water layer being trapped in the surface roughness, it is crucial to understand the factors that determine the layer stability. In this work, the forces that are responsible for the stability of thin liquid films within structured surfaces were quantified, and the conclusions were tested against the performance of wrinkled surfaces as underwater superoleophobic coatings. Here, the system studied was a family of wrinkled surfaces made of hydrophilic poly(4-vinylpyridine) (P4VP), whereby the wrinkle width could be controllably tuned in the range 90 nm to 8000 nm. The van der Waals free energy was quantified and the capillary forces trapping water in the surface micro- and nano-wrinkle structure were estimated. P4VP surfaces with micro-scale wrinkles had underwater superoleophobic properties, and low adhesion to different oils with droplet roll-off angle below 6° ± 1°. Despite the van der Waals free energy of the system pointing to the dewetting of a water film under oil on top of a smooth P4VP film, the wrinkled structure is sufficient to induce a Cassie state with a trapped water layer. The micro-scale wrinkles (average width 4-12 µm) were found to be particularly effective in the trapping of the water in a Cassie non-adhesive state. The P4VP wrinkled surfaces are superamphiphobic, as when they were first infused with oil, and then exposed to a droplet of water under oil, they exhibited superhydrophobic behavior. The P4VP wrinkles have the additional useful feature of being transparent underwater, which makes them useful candidates for the protection of underwater cameras and sensors.

Marine Antifouling Behavior of Lubricant-Infused Nanowrinkled Polymeric Surfaces.

A new family of polymeric, lubricant-infused, nanostructured wrinkled surfaces was designed that effectively retains inert nontoxic silicone oil, after draining by spin-coating and vigorous shear for 2 weeks. The wrinkled surfaces were fabricated using three different polymers (Teflon AF, polystyrene, and poly(4-vinylpyridine)) and two shrinkable substrates (Polyshrink and shrinkwrap), and Teflon on Polyshrink was found to be the most effective system. The volume of trapped lubricant was quantified by adding Nile red to the silicone oil before infusion and then extracting the oil and Nile red from the surfaces in heptane and measuring by fluorimetry. Higher volumes of lubricant induced lower roll-off angles for water droplets, and in turn induced better antifouling performance. The infused surfaces displayed stability in seawater and inhibited growth of Pseudoalteromonas spp. bacteria up to 99%, with as little as 0.9 µL cm-2 of the silicone oil infused. Field tests in the waters of Sydney Harbor over 7 weeks showed that silicone oil infusion inhibited the attachment of algae, but the algal attachment increased as the silicone oil was slowly depleted over time. The infused wrinkled surfaces have high transparency and are moldable, making them suited to protect the windows of underwater sensors and cameras.