Nanobubbles explain the large slip observed on lubricant-infused surfaces

Oil recovery enhancement by Nanobubbles: Insights from High-Pressure micromodel studies

HYPOTHESIS

Aqueous nanobubble solutions (NBs) have demonstrated a remarkable ability to displace hydrophobic fluids (e.g. oil) from porous media compared to blank water, although the underlying mechanisms remain unclear. Through detailed characterization of fluid behavior within porous spaces under controlled conditions, microfluidics can help uncover the fundamental origins of the NB-induced effects.

EXPERIMENTS

We systematically evaluate the impact of NBs on two-phase flow dynamics within porous media by applying glass micromodels that mimic both extreme wettability conditions: strongly hydrophilic (water-wet "WW") and strongly hydrophobic (oil-wet "OW"). An innovative system that combines membrane dispersion technique with microfluidic flow was used to generate NBs at elevated pressures for flooding tests.

FINDINGS

In OW scenarios, NBs demonstrated superior sweep efficiency compared to distilled water, achieving more uniform front propagation and reducing bypassed oil volumes. The improvement can be attributed to the interfacial activity of NBs along with their specific interactions with solid surfaces. In particular, NBs lowered the interfacial tension (IFT) between the oil and aqueous phases, leading to weaker capillary forces that aid in effective oil mobilization. At the pore walls, NBs induced a slippage effect that reduced the pressure drop across OW media, further facilitating displacement. Aside from these fundamental insights, our results demonstrate the utility of N2 NBs for oil recovery and related applications at elevated pressures, which are often encountered in practical settings.

Drop Friction and Failure on Superhydrophobic and Slippery Surfaces

The mobility of drops on a surface influences how much water and energy is required to clean the surface. By controlling drop mobility, it is possible to promote or reduce fogging, icing, and fouling. Superhydrophobic and slippery liquid-infused surfaces both display high drop mobility despite being 'lubricated' by fluids having very different viscosities. Superhydrophobic surfaces rely on micro- and/or nanoscale textures to trap air pockets beneath drops, minimizing solid-liquid contact. In contrast, on liquid-infused surfaces, these solid textures are filled with an immiscible liquid lubricant. Over the past few years, innovations in experimental and computational methods have provided detailed new insights into the static and dynamic wetting properties of drops on these surfaces. In this review, we describe the criteria needed to obtain stable wetting states with low drop friction and high mobility on both surfaces, and discuss the mechanisms that have been proposed to explain the origins of friction on each surface. Drops can collapse from the low-friction Cassie state to the high-friction Wenzel state on both surfaces, but the transition follows different pathways: on liquid-infused surfaces, the wetting ridge near the drop edge plays a central role in triggering collapse, a phenomenon not observed on superhydrophobic surfaces. This review emphasizes that a liquid-infused surface cannot be simply viewed as a superhydrophobic surface with the air pockets replaced by lubricant. The wetting ridge surrounding drops on liquid-infused surfaces significantly alters most of the drop's properties, including macroscopic shape, friction mechanisms, and the mechanism of collapse to a Wenzel state.

Material Design for Durable Lubricant-Infused Surfaces That Can Reduce Liquid and Solid Fouling

Liquid and solid fouling is a pervasive problem in numerous natural and industrial settings, significantly impacting energy efficiency, greenhouse emissions, operational costs, equipment lifespan, and human health. Inspired by pitcher plants, recently developed lubricant-infused surfaces (LISs) demonstrate resistance to both liquid and solid accretion under diverse environmental conditions, offering a potential solution to combat various foulants such as ice, bacteria, and mineral deposits. However, the commercial viability for most fouling-resistant LISs has thus far been compromised due to the challenges associated with maintaining a stable lubricant layer during operation. This review aims to address this important concern by providing systematic material design guidelines for fabricating durable LISs. We discuss fundamental design principles, methods for evaluating fouling resistance, and strategies to prevent lubricant loss. By presenting a comprehensive design methodology for this important class of materials, this review aims to aid future advancements in the field of antifouling surfaces, potentially impacting a variety of industries ranging from marine engineering to medical device manufacturing.

Unraveling the anti-biofouling mechanisms of slippery liquid-infused porous surface from molecular interaction perspective

Newly developed slippery liquid-infused porous surfaces (SLIPS) exhibit highly effective anti-fouling performance without harming organisms, making them a promising solution for both environmental and material protection. However, previous studies have primarily understood the anti-fouling effects of SLIPS from a mechanical perspective, neglecting the atomic interactions involved in the anti-fouling process. In this study, we combined microbiological experiments with multi-scale simulations to elucidate the microscopic mechanisms behind the unique anti-biofouling effects of SLIPS. After developing SLIPS with robust liquid-repellency, we characterized its physical and chemical properties and demonstrated its superior effectiveness in preventing Pseudomonas aeruginosa attachment. To probe the initial contact during bacterial attachment, all-atom molecular dynamics (MD) simulations were conducted, revealing that the liquid-liquid interface suppresses the effective pilin adhesion on SLIPS. Further analysis through steered MD, ab initio MD, and density functional theory calculations revealed that the flexible siloxane backbone and the non-polar nature of silicone oil molecules enhance the diffusivity of interfacial water and lead to the continuous nanoscale fluctuation of liquid-liquid interface, thus inhibiting the role of protein dynamics in promoting bio-adhesion. These novel insights into the characteristics of liquid-liquid and nano-bio interface during the anti-biofouling process of SLIPS may promote the future development of bio-inspired functional surfaces.

High drag reduction by spontaneous capture and transportation of bubbles

Gas lubrication is globally recognized as an effective technology for reducing drag. However, the injected gas bubbles are prone to escape, resulting in a low gas coverage area and insufficient drag reduction efficiency. Herein, we developed an underwater superaerophilic pattern composed of a serial structure and a brachistochrone profile to spontaneously capture and transport the gas bubbles, which effectively hindered bubble escape, expanded the gas coverage area, and increased the drag reduction rate. The maximum drag reduction rates at a horizontal sample surface and an inclined sample surface under a water velocity of 0.6 m s-1 were as high as 31.8% and 18.5%, respectively. In addition, two-phase flow simulation showed that the superaerophilic pattern could still work and achieve a drag reduction rate of 13.25% even under a high flow velocity of 10 m s-1. Moreover, the flowing gas layer on the superaerophilic pattern endowed an excellent corrosion resistance and marine antifouling ability. This high efficient and eco-friendly underwater drag reduction method will minimize the energy consumption required for ship navigation and greenhouse gas emission, which has a strong implication for sustainable human development.

Partially substrateless microchannels for direct monitoring of interfacial dynamics in hydrophobic surfaces

Superhydrophobic and liquid-infused surfaces are the most prominent techniques to achieve drag reduction in microchannels. However, they have specific drawbacks such as costly fabrication of complex and mechanically sensitive surfaces, surfaces susceptible to lubricant abrasion or involve hazardous chemicals. We present a partially substrateless microchannel whose upper wall features a large no-shear air/water meniscus at atmospheric pressure. On this wall, a self-assembled monolayer of hydrophobic alkyl silane was bonded covalently. Flow experiments reveal a drag reduction of up to 25% although only 4% of the wall fulfils the no-shear condition. These experiments demonstrated long-term stability and self-healing properties. Furthermore, White Light Interferometry (WLI) was used for direct monitoring of interfacial dynamics. By optical investigation of the full meniscus topography the contact-free evaluation of the spatially resolved static pressure distribution was possible. Conducted numerical simulations are in good agreement with the experimental findings and illustrate the drag reduction mechanism.

Slippery liquid-like surfaces as a promising solution for sustainable drag reduction

Drag reduction is crucial for many industries, ranging from aerospace to microfluidics, to enhance the energy efficiency and reduce costs. This work is the first to study drag reduction enabled by novel slippery liquid-like surfaces fabricated from flexible polymers. We experimentally characterized the drag reduction performance of slippery liquid-like surfaces in the laminar flow regime. Our results indicate that liquid-like surfaces can reduce fluid drag regardless of surface wettability and have achieved nearly 20% drag reduction. Furthermore, the durability tests show that these surfaces can maintain slipperiness over a month when exposed to air or water and the drag reduction capability for at least one week under a fluid flow. These findings highlight the potential of slippery liquid-like surfaces as a promising solution for sustainable drag reduction.

Enhanced Imbibition in Liquid-Infused Coated Microchannels

Spontaneous capillary imbibition has the potential to improve the performance of many micro and nanodevices since it does not require an external energy source to drive a fluid flow. Despite this advantage, controlling and reducing the friction exerted by the channel walls, which limits the speed of the liquid, remains a challenge. Here, we demonstrate experimentally that infusing the walls of a channel with a liquid lubricant substantially speeds up the imbibition process and reduces the overall viscous friction. By varying the viscosity of the lubricant, we observe a substantial reduction of the imbibition time of up to 50%. Our experimental results are in good agreement with previous theoretical predictions, providing a solid framework to study low-resistance spontaneous imbibition processes.

Ozone nanobubble technology as a novel AOPs for pollutants degradation under high salinity conditions

Conventional water treatment systems frequently exhibit diminished efficiency at high salinity - a significant issue especially for real industrial effluents - mostly due to the creation of intricate structures between pollutants and salts. One of the primary obstacles associated with high salinity conditions is the generation of by-products that pose additional hurdles for treatment. In this work, we have investigated the novel advanced oxidation process a so-called ozone nanobubble technology for degradation of the pollutants at high salinity conditions. The mass transfer is often the rate-limiting step in gas-liquid process and the poor rate of mass transfer diminishes the overall efficacy. One of the primary disadvantages associated with ozone is its restricted solubility and instability when dissolved in an aqueous solution. These characteristics impose limitations on its potential applications and need the use of specialized systems to facilitate gas-liquid interaction. In this work, we have also suggested enhancing the ozonation process through the utilization of ozone nanobubbles. The findings of our experiment and subsequent analysis indicate that the presence of nanobubbles enhances the process of ozonation through three key mechanisms: (i) an increased mass transfer coefficient, (ii) a higher rate of reactive oxygen species (ROS) generation attributed to the charged interface, and (iii) the nanobubble interface serving as an active surface for chemical reactions. The predicted mass transfer coefficients were found to range from 3 to 3.5 min-1, a value that is notably greater than that seen for microbubbles. The study showcased the degradation of methylene blue dye through the utilization of ozone nanobubbles, which exhibited a much higher rate of dye degradation compared to ozone microbubbles. The confirmation of the radical degradation mechanism was achieved by the utilization of electron spin resonance (ESR) measurements. The developed process has high potential for application in industrial scale textile wastewater treatment.

Multi-component liquid-infused systems: a new approach to functional coatings

Antifouling liquid-infused surfaces have generated interest in multiple fields due to their diverse applications in industry and medicine. In nearly all reports to date, the liquid component consists of only one chemical species. However, unlike traditional solid surfaces, the unique nature of liquid surfaces holds the potential for synergistic and even adaptive functionality simply by including additional elements in the liquid coating. In this work, we explore the concept of multi-component liquid-infused systems, in which the coating liquid consists of a primary liquid and a secondary component or components that provide additional functionality. For ease of understanding, we categorize recently reported multi-component liquid-infused surfaces according to the size of the secondary components: molecular scale, in which the secondary components are molecules; nanoscale, in which they are nanoparticles or their equivalent; and microscale, in which the additional components are micrometer size or above. We present examples at each scale, showing how introducing a secondary element into the liquid can result in synergistic effects, such as maintaining a pristine surface while actively modifying the surrounding environment, which are difficult to achieve in other surface treatments. The review highlights the diversity of fabrication methods and provides perspectives on future research directions. Introducing secondary components into the liquid matrix of liquid-infused surfaces is a promising strategy with significant potential to create a new class of multifunctional materials. Keywords: Active surfaces; Antimicrobial; Antifouling; Interfaces; Sensing surfaces.

Scalable Preparation of Liquid Infused Coatings for Lubrication of 103 m2 Dry Ski Slopes

To facilitate effective training for freestyle skiers on artificial dry ski slopes, it is crucial to reduce the friction coefficient of the slopes and closely match it with that of snow. Traditional lubrication methods, such as water or soapy water, come with multiple disadvantages, including water waste, which leads to environmental pollution, short-lived effectiveness, and high costs. In this study, we have successfully developed a method for the scalable preparation of a liquid-infused coating (LIC) by tandem spraying inexpensive and environmentally friendly SiO2 particles and silicone oil lubricants. Experimental results showed that the resulting LIC is capable of imparting slippery properties to various surfaces, regardless of the surface chemistry. Moreover, the presence of LIC could reduce the friction coefficient significantly. By carefully regulating the surface composition, we achieved a friction coefficient of 0.059 between a snowboard and the LIC-functionalized ski slope, closely matching that between the snowboard and snow in a typical skiing competition venue (∼0.06). We successfully applied LIC onto 103 m2 dry ski slopes, providing a training ground for professional freestyle skiers.

Bulk Nanobubbles through Gas Supersaturation Originated by Hot and Cold Solvent Mixing

The nucleation mechanism of bulk nanobubbles remains unclear despite the considerable attention they have received in recent years. We propose two hypotheses: (i) The gas supersaturation in the bulk liquid is the primary factor for nanobubble nucleation, and (ii) the mixing of the same solvent at varying gas solubilities should produce nanobubbles, provided that the first hypothesis is correct. To test this hypothesis, we performed extensive experiments on nanobubble nucleation in both water and organic solvents. The temperature difference between hot and cold samples ranged from 10 to 80 °C in pure solvents such as water, methanol, ethanol, propanol, and butanol prepared and mixed in equal proportions. To the best of our knowledge, we report bulk nanobubble nucleation by mixing hot and cold solvents for the first time. The refractive index value calculations using Mie scattering theory confirmed the existence of nanobubbles. When surface tension dominates over surface charge, the critical work for nanobubble formation is ΔFc ∝ 1/ξ2, and when surface charge dominates over surface tension, the critical work is ΔFc ∝ ξ1/4. Our experimental results verify such dependency by measuring nanobubbles nucleated with varying degrees of gas supersaturation.

Understanding the role of infusing lubricant composition in the interfacial interactions and properties of slippery surface

Liquid-infused surfaces (LISs) have attracted tremendous attention in recent years owing to their excellent surface properties, such as self-cleaning and anti-fouling. Understanding the effect of lubricant composition on LIS performance is of vital importance, which will help establish the criteria to choose suitable infusing lubricants for specific applications. In this work, the role of chemical composition of lubricant in the properties of LISs was investigated. The apparent water contact angle θapp was dependent on the temperature and beeswax/silicone oil ratio. Nevertheless, the trend of moving velocity of water drop on the tilted LISs did not follow that of θapp at 20 °C and 37 °C, which was attributed to the increased lubricant viscosity with beeswax/silicone oil ratio. At 60 °C, the drop velocity and θapp shared the similar variation trend with beeswax/silicone oil ratio, highlighting the significant role of chemistry of the components in beeswax. The alkanes and fatty acids promoted the drop movement, while the fatty acid esters impeded the movement. The interaction forces between water drop and lubricant surfaces were measured using atomic force microscopy. It was demonstrated that the interaction between water drop and lubricant was not the only factor to control the drop movement, while the interaction between lubricant and substrate as well as of lubricant itself also determined the movement. When the adhesions of water-lubricant and lubricant-substrate were similar for different lubricants, the influence of cohesion of lubricant became significant. This work provides useful insights into the fundamental understanding of the interfacial interactions of test drop, infusing lubricant and solid substrate of LISs, and the effect of infusing lubricant composition on the LIS performance.

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.

Energy harvesting from water streaming at charged surface

Water flowing at a charged surface may produce electricity, known as streaming current/potentials, which may be traced back to the 19th century. However, due to the low gained power and efficiencies, the energy conversion from streaming current was far from usable. The emergence of micro/nanofluidic technology and nanomaterials significantly increases the power (density) and energy conversion efficiency. In this review, we conclude the fundamentals and recent progress in electrical double layers at the charged surface. We estimate the generated power by hydrodynamic energy dissipation in multi-scaling flows considering the viscous systems with slipping boundary and inertia systems. Then, we review the coupling of volume flow and current flow by the Onsager relation, as well as the figure of merits and efficiency. We summarize the state-of-the-art of electrokinetic energy conversions, including critical performance metrics such as efficiencies, power densities, and generated voltages in various systems. We discuss the advantages and possible constraints by the figure of merits, including single-phase flow and flying droplets.

Multifunctional slippery photothermal coating

Slippery liquid-infused porous surface (SLIPS) has shown significant application values in various areas and has been commonly obtained by injecting the water-immiscible lubricant into a low-surface-energy modified micro/nano-structured surface. Constrained by the availability of desirable structured substrates or simple preparation schemes, the exploration of SLIPS with multifunctionality and universality that is facile to fabricate and robust in realistic applications remains challenging. Herein, we propose a one-step, fluoride-free and unconventional protocol based on a one-pot reaction of polysilazane (PSZ), silicone oils and multiwalled carbon nanotubes (MWCNT), which creates not only the favorable micro/nano-scale physical structures and surface chemistry for the excellent slippery property (sliding angle < 3°) and robust lubricant retention, but also the superior photothermal responsiveness for the potential multifunctional applications. It has been demonstrated that the proposed multifunctional slippery photothermal coating (MSPC) displayed outstanding potential in corrosion resistance, droplet manipulation and anti/de-icing. We envision that the proposed strategy could be realized in the real-life applications.

Enhanced Anticoagulation of Hierarchy Liquid Infused Surfaces in Blood Flow

Liquid infused surfaces (LIS) hold remarkable potential in anticoagulation. However, liquid loss of LIS in the bloodstream remains a challenge toward its clinical application. Here, micronano hierarchy structures are obtained on the titanium alloy substrate by regulating the microspheres' distribution. When the gap between the microspheres is smaller than the diameter of the red blood cell (RBC), the LIS is more stable under the blood wash and presents a better anticoagulation performance. The proper interval is found to prevent the RBCs from entering the gap and remove the liquid on the surface. The retained thickness of the liquid film is measured by the atomic force microscopy (AFM) technique. The LIS is applied on the front guide vane of an artificial heart pump and exhibits significant improvement on anticoagulation in the blood circulation in vitro for 25 h. The techniques and findings can be used to optimize the anticoagulation performance of LIS-related biomedical implant devices.

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.

On Characterization of Shear Viscosity and Wall Slip for Concentrated Suspension Flows in Abrasive Flow Machining

In the realm of abrasive flow machining (AFM), precise finishing and maintaining dimensional accuracy have remained challenging due to non-uniformities in the AFM process and complexities associated with the abrasive media's shear viscosity and wall slip behavior. By addressing these challenges, this study introduces a comprehensive framework, combining theoretical foundations, measurement techniques, and experimental setups. Utilizing capillary flow, a novel compensation strategy is incorporated within the Mooney method to counter entrance pressure drop effects. This enhanced capillary flow method emerges as a promising alternative to the conventional Cox-Merz empirical rule, enabling precise characterization of wall slip behavior and shear viscosity, particularly at elevated shear rates. The abrasive media exhibit a Navier nonlinear wall slip, as highlighted by the Mooney method. Rigorous verification of the proposed methodologies and models against supplemental experiments showcases a high degree of congruence between predicted and observed results, emphasizing their accuracy and broad application potential in AFM. This research illuminates the intricacies of the abrasive media's behavior, accentuating the need for meticulous characterization, and provides a robust foundation for genuine modeling and predictions in material removal within AFM.

Liquid-in-Aerogel Porous Composite Allows Efficient CO2 Capture and CO2 /N2 Separation

The pursuit of efficient CO2 capture materials remains an unmet challenge. Especially, meeting both high sorption capacity and fast uptake kinetics is an ongoing effort in the development of CO2 sorbents. Here, a strategy to exploit liquid-in-aerogel porous composites (LIAPCs) that allow for highly effective CO2 capture and selective CO2 /N2 separation, is reported. Interestingly, the functional liquid tetraethylenepentamine (TEPA) is partially filled into the air pockets of SiO2 aerogel with left permanent porosity. Notably, the confined liquid thickness is 10.9-19.5 nm, which can be vividly probed by the atomic force microscope and rationalized by tailoring the liquid composition and amount. LIAPCs achieve high affinity between the functional liquid and solid porous counterpart, good structure integrity, and robust thermal stability. LIAPCs exhibit superb CO2 uptake capacity (5.44 mmol g-1 , 75 °C, and 15 vol% CO2 ), fast sorption kinetics, and high amine efficiency. Furthermore, LIAPCs ensure long-term adsorption-desorption cycle stability and offer exceptional CO2 /N2 selectivity both in dry and humid conditions, with a separation factor up to 1182.68 at a humidity of 1%. This approach offers the prospect of efficient CO2 capture and gas separation, shedding light on new possibilities to make the next-generation sorption materials for CO2 utilization.

On Nanobubble Dynamics under an Oscillating Pressure Field during Salting-out Effects and Its DLVO Potential

We have investigated the origin, stability, and nanobubble dynamics under an oscillating pressure field followed by the salting-out effects. The higher solubility ratio (salting-out parameter) of the dissolved gases and pure solvent nucleates nanobubbles during the salting-out effect, and the oscillating pressure field enhances the nanobubble density further as solubility varies linearly with gas pressure by Henry's law. A novel method for refractive index estimation is developed to differentiate nanobubbles and nanoparticles based on the scattering intensity of light. The electromagnetic wave equations have been numerically solved and compared with the Mie scattering theory. The scattering cross-section of the nanobubbles was estimated to be smaller than the nanoparticles. The DLVO potentials of the nanobubbles predict the stable colloidal system. The zeta potential of nanobubbles varied by generating nanobubbles in different salt solutions, and it is characterized by particle tracking, dynamic light scattering, and cryo-TEM. The size of nanobubbles in salt solutions was reported to be higher than that in pure water. The novel mechanical stability model is proposed by considering both ionic cloud and electrostatic pressure at the charged interface. The ionic cloud pressure is derived by electric flux balance, and it is found to be twice the electrostatic pressure. The mechanical stability model for a single nanobubble predicts the existence of stable nanobubbles in the stability map.

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.

Augmenting the Leidenfrost Temperature of Droplets via Nanobubble Dispersion

Droplets may rebound/levitate when deposited over a hot substrate (beyond a critical temperature) due to the formation of a stable vapor microcushion between the droplet and the substrate. This is known as the Leidenfrost phenomenon. In this article, we experimentally allow droplets to impact the hot surface with a certain velocity, and the temperature at which droplets show the onset of rebound with minimal spraying is known as the dynamic Leidenfrost temperature (T_DL). Here we propose and validate a novel paradigm of augmenting the T_DL by employing droplets with stable nanobubbles dispersed in the fluid. In this first-of-its-kind report, we show that the T_DL can be delayed significantly by the aid of nanobubble-dispersed droplets. We explore the influence of the impact Weber number (We), the Ohnesorge number (Oh), and the role of nanobubble concentration on the T_DL. At a fixed impact velocity, the T_DL was noted to increase with the increase in nanobubble concentration and decrease with an increase in impact velocity for a particular nanobubble concentration. Finally, we elucidated the overall boiling behaviors of nanobubble-dispersed fluid droplets with the substrate temperature in the range of 150-400 °C against varied impact We through a detailed phase map. These findings may be useful for further exploration of the use of nanobubble-dispersed fluids in high heat flux and high-temperature-related problems and devices.

Observation of enhanced nanoscale creep flow of crystalline metals enabled by controlling surface wettability

Understanding and controlling interface friction are central to many science and engineering applications. However, frictional sliding is closely related to adhesion, surface roughness, surface chemistry, mechanical deformation of contact solids, which poses the major challenge to experimental studying and theoretical modeling of friction. Here, by exploiting the recent developed thermomechanical nanomolding technique, we present a simple strategy to decouple the interplay between surface chemistry, plastic deformation, and interface friction by monitoring the nanoscale creep flow of metals in nanochannels. We show that superhydrophobic nanochannels outperforming hydrophilic nanochannels can be up to orders of magnitude in terms of creep flow rate. The comparative experimental study on pressure and temperature dependent nanomolding efficiency uncovers that the enhanced creep flow rate originates from diffusion-based deformation mechanism as well as the superhydrophobic surface induced boundary slip. Moreover, our results reveal that there exists a temperature-dependent critical pressure below which the traditional lubrication methods to reduce friction will break down. Our findings not only provide insights into the understanding of mechanical deformation and nanotribology, but also show a general and practical technique for studying the fundamental processes of frictional motion. Finally, we anticipate that the increased molding efficiency could facilitate the application of nanoimprinting/nanomolding.

Light-Driven Liquid Conveyors: Manipulating Liquid Mobility and Transporting Solids on Demand

The intelligent transport of materials at interfaces is essential for a wide range of processes, including chemical microreactions, bioanalysis, and microfabrication. Both passive and active methods have been used to transport droplets, among which light-based techniques have attracted much attention because they are noncontact, safe, reversible, and controllable. However, conventional light-driven systems also involve challenges related to low transport ability and instability. Because of these shortcomings, technologies that can transport and manipulate droplets and microsolids on the same surface have yet to be realized. The present work demonstrates a light-driven system referred to as a liquid conveyor that enables the transport of both water droplets and microsolids. After the incorporation of an azobenzene-based molecular motor capable of undergoing photoisomerization into the surface liquid layer of this system, an isomerization gradient was induced by exposure to ultraviolet light emitting diodes that induced flow in this layer. Various parameters were optimized, including the concentration of the molecular motor compound, the light intensity, the viscosity of the liquid layer, and the droplet volume. This process eventually achieved the horizontal transport of droplets in any direction at varied rates. As a consequence of the limited heat buildup, the lack of droplet deformation, and extremely small contact angle hysteresis in this system, microsolids on droplets were also transported. This liquid conveyor is a promising platform for high-throughput omni-liquid/solid manipulation in the fields of biotechnology, chemistry, and mechanical engineering.

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).

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.

Liquid-Repellent Surfaces

Surfaces are vibrant sites for various activities with environments, especially as the transfer station for mass and energy exchange. In nature, natural creatures exhibit special wetting and interfacial properties such as water repellency and water affinity to adapt to various environmental challenges by taking advantage of air or liquid infusion media. Inspired by natural surfaces, various engineered liquid-repellent surfaces have been developed with a wide range of applications in both open and closed underwater environments. In particular, underwater conditions are characterized by high viscosity, high pressure, and complex compositions, which pose more challenges for the design of robust and functional repellent surfaces. In this Perspective, we take a parallel approach to introduce two classical liquid-repellent surfaces: an air-infused repellent surface and a lubricated liquid-repellent surface. Then we highlight fundamental challenges and design configurations of robust liquid-repellent surfaces both in air and underwater. We summarize the advantages and drawbacks of two kinds of repellent surfaces and list several applications of liquid-repellent surfaces for use in the ocean, medical care, and energy harvesting. Finally, we provide an outlook of research directions for robust liquid-repellent surfaces.

Growth Rates of Hydrogen Microbubbles in Reacting Femtoliter Droplets

Chemical reactions in small droplets are extensively explored to accelerate the discovery of new materials and increase the efficiency and specificity in catalytic biphasic conversion and high-throughput analytics. In this work, we investigate the local rate of the gas-evolution reaction within femtoliter droplets immobilized on a solid surface. The growth rate of hydrogen microbubbles (≥500 nm in radius) produced from the reaction was measured online with high-resolution confocal microscopic images. The growth rate of bubbles was faster in smaller droplets and near the droplet rim in the same droplet. The results were consistent for both pure and binary reacting droplets and on substrates of different wettability. Our theoretical analysis based on diffusion, chemical reaction, and bubble growth predicted that the concentration of the reactant depended on the droplet size and the bubble location inside the droplet, in good agreement with experimental results. Our results reveal that the reaction rate may be spatially nonuniform in the reacting microdroplets. The findings may have implications for formulating the chemical properties and uses of these droplets.