Liquid-infused Surfaces Research Articles

Numerical validation of analytical formulas for channel flows over liquid-infused surfaces

This paper provides numerical validation of some new explicit, asymptotically exact, analytical formulas describing channel flows over liquid-infused surfaces, an important class of surfaces of current interest in surface engineering. The new asymptotic formulas, reproduced here, were derived in a recent companion paper by the authors. The numerical validation is done by presenting a novel computational method for calculating longitudinal flow in a periodic channel involving finite-length closed liquid-filled grooves with a flat two-fluid interface, a challenging problem given the two-fluid nature of the flow. The formulas are asymptotically exact for wide channels where the grooves on the lower wall of the channel are well separated; the numerical method devised here, however, is subject to no such restrictions. Significantly, it is shown here that the asymptotic formulas remain good global approximants for the flow over a wide range of flow geometries, including those well outside the asymptotic parameter range for which they were derived. It is found that the formulas are more reliable for liquid-infused surfaces than for superhydrophobic surfaces.

Modeling flow through tubes and annuli with liquid‐infused surfaces for enhanced stability of the fluid‐fluid interface

AbstractSuperhydrophobic (SHS) and liquid‐infused surfaces (LIS) have shown great potential in various engineering applications. Due to their heterogeneous surface properties, a mathematical description of the flow behavior along such surfaces is challenging. Circular textured surfaces are of particular importance. They are modeled as either axially traversed tubes or annuli consisting of no‐slip walls that are padded with rotationally symmetric finite‐shear regions. The latter represents a viscous interaction zone with a second fluid, assumed with layer thickness zero. Zimmermann and Schönecker provide analytical equations that describe the flow field and effective slip length for such geometries. They are applicable to Newtonian fluids of arbitrary viscosity ratio. This article emphasizes the development of principles and guidelines for the design of SHS and LIS to enhance sliding effects, based on these analytical models. The approach presented here facilitates an geometric evaluation of slippery circular surfaces, aiming to offer insights for the design. Through this research, the potential for significant energy savings and enhanced fluid transport performance can be realized, contributing to the development of more efficient fluid engineering systems.

Comparison of Superhydrophilic, Liquid-Like, Liquid-Infused, and Superhydrophobic Surfaces in Preventing Catheter-Associated Urinary Tract Infection and Encrustation.

Over the past decade, superhydrophilic zwitterionic surfaces, slippery liquid-infused porous surfaces, covalently attached liquid-like surfaces, and superhydrophobic surfaces have emerged as the most promising strategies to prevent biofouling on biomedical devices. Despite working through different mechanisms, they have demonstrated superior efficacy in preventing the adhesion of biomolecules (e.g., proteins and bacteria) compared with conventional material surfaces. However, their potential in combating catheter-associated urinary tract infection (CAUTI) remains uncertain. In this research, we present the fabrication of these four coatings for urinary catheters and conduct a comparative assessment of their antifouling properties through a stepwise approach. Notably, the superhydrophilic zwitterionic coating demonstrated the highest antifouling activity, reducing 72.3% of fibrinogen deposition and over 75% of bacterial adhesion (Escherichia coli and Staphylococcus aureus) when compared with an uncoated polyvinyl chloride (PVC) surface. The zwitterionic coating also exhibited robust repellence against blood and improved surface lubricity, decreasing the dynamic coefficient of friction from 0.63 to 0.35 as compared with the PVC surface. Despite the fact that the superhydrophilic zwitterionic and hydrophobic liquid-like surfaces showed great promise in retarding crystalline biofilm formation in the presence of Proteus mirabilis, it is worth noting that their long-term antifouling efficacy may be compromised by the proliferation and migration of colonized bacteria as they are unable to kill them or inhibit their swarming. These findings underscore both the potential and limitations of these ultralow fouling materials as urinary catheter coatings for preventing CAUTI.

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.

Hydrodynamic drag reduction in ribbed microchannel with infused non-Newtonian lubricants

Liquid-infused surfaces have recently gained prominence in engineering applications owing to their versatile characteristics such as self-cleaning, anti-fogging, drag reduction, and enhanced heat transfer. In this article, a numerical analysis of pressure-driven flow past a periodic array of rectangular transverse grooves infused with non-Newtonian immiscible lubricants is performed. The volume of fluid method is employed to capture the interface between primary and secondary fluids, and the power-law model is deployed to mimic the non-Newtonian lubricant. The drag reduction capability of the microchannel is examined for various parameters such as Reynolds number, liquid fraction, viscosity ratio, viscosity index, and contact angle. It is observed that the introduction of a non-Newtonian fluid (shear-thickening or shear-thinning) drastically modifies the interface velocity and hydrodynamic resistance. In particular, a shear-thinning lubricant enhances the slip length as the viscosity index (n) is reduced owing to the reduced viscosity at the interface. Note that, for a lubricant having n = 0.7, the percentage improvement in the slip length is 382% in comparison with a Newtonian counterpart having the same viscosity ratio, N = 0.1. Importantly, the introduction of a shear-thinning lubricant with a viscosity ratio N = 5, a liquid fraction of 0.8, and a behavior index n = 0.7 yielded a pressure drag reduction of 63.6% with respect to a classical no-slip channel and of 23% with reference to a microchannel with the Newtonian lubricant. Moreover, at high Reynolds numbers, Re→50, the drag mitigation is slightly lowered due to the primary vortex shift in the cavity. Furthermore, the effect of the interface contact angle (θc) is investigated, as θc drops from 90° (flat) to 45° (convex); the meniscus curvature is enhanced, and the effective slip length is reduced. These observations suggest that a shear-thinning lubricant-infused microchannel is a promising candidate for drag reduction in lab-on-chip applications.

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.

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.

Superior shear-stable slippery surface of porous carbon nanospheres (PCN)-oleogel

Frictional drag reduction (DR) is of great importance in marine environments as it has the potential to significantly reduce fuel consumption, leading to both economic and environmental benefits. While various conventional DR techniques exist, many of them are energy-inefficient and require external intervention to function effectively in real marine conditions. Unlike conventional methods, bioinspired surfaces offer cost-effective and sustainable DR. However, these surfaces suffer from (i) poor durability of superhydrophobic surfaces, (ii) limited scalability, (iii) substrate dependency of coatings, and (iv) fast depletion of impregnated lubricant oil of liquid infused surfaces (LIS). In this work, we proposed a double-layered LIS oleogel surface embedded with re-entrant surface (RES) morphology by adopting spin/spray-assisted deposition processes. The RES structure, achieved by depositing porous carbon nanospheres (PCN) on the oleogel surface, effectively stabilized and replenished a large amount of lubricated oil, ensuring long-lasting lubrication. The developed PCN-oleogel surface could retain its slippery property despite exposure to harsh physical abrasion, and various chemically contaminated aqueous solutions. The developed PCN-oleogel exhibited long-term lubrication performance and shear-stable durability under high-speed, high-pressure conditions up to 15 ms−1 — such shear-stable lubrication in extremely rare in literature. Conclusively, the proposed PCN-oleogel surface is envisioned to have a great potential in achieving sustainable underwater DR in harsh marine environments.

On the Rosensweig instability of ferrofluid-infused surfaces under a uniform magnetic field

This study numerically examines the Rosensweig instability of a ferrofluid-infused surface (FIS), which is an overlayer surface formed on the top of a porous substrate into which the ferrofluid is infused. FIS inherits the lubricating property of liquid-infused surfaces and the magnetic property of the ferrofluid, allowing an external control by a magnetic field. Currently, the evolution mechanisms of the Rosensweig instability of the FIS remain unclear. By using the recently developed coupled numerical scheme, which consists of the simplified multiphase lattice Boltzmann method and the self-correcting method for the Maxwell magnetic field equations, the intriguing mechanism and evolution dynamics of the ferrofluid spikes on the FIS are investigated systematically by examining the effects of overlayer thickness, substrate pore size, and magnetic field strength. The results are also analyzed by the Rosensweig instability theory. This study provides a theoretical and numerical basis for the magnetic field control of droplet or bubble motions on the FIS.

Droplet Self-Propulsion on Slippery Liquid-Infused Surfaces with Dual-Lubricant Wedge-Shaped Wettability Patterns.

Young's equation is fundamental to the concept of the wettability of a solid surface. It defines the contact angle for a droplet on a solid surface through a local equilibrium at the three-phase contact line. Recently, the concept of a liquid Young's law contact angle has been developed to describe the wettability of slippery liquid-infused porous surfaces (SLIPS) by droplets of an immiscible liquid. In this work, we present a new method to fabricate biphilic SLIP surfaces and show how the wettability of the composite SLIPS can be exploited with a macroscopic wedge-shaped pattern of two distinct lubricant liquids. In particular, we report the development of composite liquid surfaces on silicon substrates based on lithographically patterning a Teflon AF1600 coating and a superhydrophobic coating (Glaco Mirror Coat Zero), where the latter selectively dewets from the former. This creates a patterned base surface with preferential wetting to matched liquids: the fluoropolymer PTFE with a perfluorinated oil Krytox and the hydrophobic silica-based GLACO with olive oil (or other mineral oils or silicone oil). This allows us to successively imbibe our patterned solid substrates with two distinct oils and produce a composite liquid lubricant surface with the oils segregated as thin films into separate domains defined by the patterning. We illustrate that macroscopic wedge-shaped patterned SLIP surfaces enable low-friction droplet self-propulsion. Finally, we formulate an analytical model that captures the dependence of the droplet motion as a function of the wettability of the two liquid lubricant domains and the opening angle of the wedge. This allows us to derive scaling relationships between various physical and geometrical parameters. This work introduces a new approach to creating patterned liquid lubricant surfaces, demonstrates long-distance droplet self-propulsion on such surfaces, and sheds light on the interactions between liquid droplets and liquid surfaces.

Tunable encapsulation of sessile droplets with solid and liquid shells

Droplet encapsulations using liquid or solid shells are of significant interest in microreactors, drug delivery, crystallization, and cell growth applications. Despite progress in droplet-related technologies, tuning micron-scale shell thickness over a large range of droplet sizes is still a major challenge. In this work, we report capillary force assisted cloaking using hydrophobic colloidal particles and liquid-infused surfaces. The technique produces uniform solid and liquid shell encapsulations over a broad range (5–200 μm shell thickness for droplet volume spanning over four orders of magnitude). Tunable liquid encapsulation is shown to reduce the evaporation rate of droplets by up to 200 times with a wide tunability in lifetime (1.5 h to 12 days). Further, we propose using the technique for single crystals and cell/spheroid culture platforms. Stimuli-responsive solid shells show hermetic encapsulation with tunable strength and dissolution time. Moreover, scalability, and versatility of the technique is demonstrated for on-chip applications.

Unlocking the potential of 3D printed microfluidics for mass spectrometry analysis using liquid infused surfaces

Combining microfluidics with mass spectrometry (MS) analysis has great potential for enabling new analytical applications and simplifying existing MS workflows. The rapid development of 3D printing technology has enabled direct fabrication of microfluidic channels using consumer grade 3D printers, which holds great promise to facilitate the adoption of microfluidic devices by the MS community. However, photo polymerization-based 3D printed devices have an issue with chemical leeching, which can introduce contaminant molecules that may present as isobaric ions and/or severely suppress the ionization of target analytes when combined with MS analysis. Although extra cure and washing steps have alleviated the leeching issue, many such contaminant peaks can still show up in mass spectra. In this work, we report a simple surface modification strategy to isolate the chemical leachates from the channel solution thereby eliminating the contaminant peaks for MS analysis. The channel was prepared by fabricating a layer of polydimethylsiloxane graft followed by wetting the graft using silicone oil. The resulting liquid infused surface (LIS) showed significant reduction in contaminant peaks and improvement in the signal intensity of target analytes. The coating showed good stability after long-term usage (7 days) and long-term storage (∼6 months). Finally, the utility of the coating strategy was demonstrated by printing herringbone microfluidic mixers for studying fast reaction kinetics, which obtained comparable reaction rates to literature values. The effectiveness, simplicity, and stability of the present method will promote the adoption of 3D printed microdevices by the MS community.

Robust and durable self-lubricated surface by facile fabrication for anti-icing/deicing

Undesired ice accumulation can lead to serious negative impacts on daily life and equipment safety. Slippery liquid-infused surfaces (SLISs) have been widely studied for their low ice adhesion strength (τice). However, the lack of durability, weak robustness, and complex preparation process hinder the application of SLISs. In this work, robust oil-infused porous surface (RIPS) was obtained by a facile one-step method, which used epoxy resin (E44) as a substrate and contained fumed silica and silicone oil. The RIPS-6 displays outstanding anti-icing/deicing ability, superior liquid repellency, good self-cleaning performance, and excellent mechanical robustness. At −5 °C, the water droplet remained unfrozen after 7200 s. τice was only 6.9 kPa at −20 °C and remained below 15 kPa during 10 icing cycles. Furthermore, the mixture of E44 and fumed silica enhanced the mechanical properties including the hardness (4 H) and abrasion resistance of RIPS-6. The icephobicity can be remained (τice < 40 kPa) even after 150 cycles of sandpaper abrasion at 2.5 kPa. The anti-icing and deicing coating with excellent mechanical stability and durability significantly improves the practical application possibility of the coating in harsh environments.

Preparation of bio-inspired liquid-infused surface on epoxy resin coating for reliable hydrophobic and anti-corrosion properties

Epoxy resin (ER) is commonly used as an anti-corrosion coating, but the hydrophilicity hinders its effectiveness. Although liquid-infused surface (LIS) has shown promise in enhancing the hydrophobicity, its reliability is compromised because the vulnerability of modified layer. To address the above challenges encountered, a bio-inspired LIS (BI-LIS), which was proposed inspired by mussels and had improved modified layer, was prepared on ER. In preparation, by adding the modified mineral oil, consisting of the hydrolyzed modifier and mineral oil, dropwise onto the textured ER and placing it for 3 days, the BI-LIS specimen could be obtained. Energy dispersive spectroscopy results showed the gradual increase in the fluorine content on textured ER, indicating the formation of modified layer as the fluoroalkyl groups in the modifier molecules gradually adsorb. Not only did BI-LIS specimen exhibit improved hydrophobic and anti-corrosion properties than ER, but more importantly, it possessed the capability to restore the modified layer by the re-absorption of modifier molecules, which could response to the damage of modified layer and enhance the reliability of LIS. This study aims to enhance the hydrophobic and anti-corrosion properties of ER, and offers technical support to broaden the application scope of ER.

Studying Liquid-Infused Surfaces with Affordable Surface Features Infiltrated with Various Lubricants from Corrosion, Biofouling, and Fluid Drag Viewpoints

Studying Liquid-Infused Surfaces with Affordable Surface Features Infiltrated with Various Lubricants from Corrosion, Biofouling, and Fluid Drag Viewpoints

Slippery Graphene-Bridging Liquid Metal Layered Heterostructure Nanocomposite for Stable High-Performance Electromagnetic Interference Shielding.

Gallium-based liquid metal (LM) with intriguing high electrical conductivity and room-temperature fluidity has attracted substantial attention for its potential application in flexible electromagnetic interference (EMI) shielding. However, the EMI shielding performance of the existing LM-based composites is unsatisfying due to the irreconcilable contradiction between high EMI shielding efficiency (SE) and low thickness. In addition, the research on environmentally stable EMI shielding material has become an urgent need due to the increasingly sophisticated application scenarios. Herein, we prepared a reduced graphene oxide (rGO) bridging LM layered heterostructure nanocomposite with the liquid-infused slippery surface (S-rGO/LM), which exhibits an ultrahigh X-band EMI SE of 80 dB at a mere internal thickness of 33 μm, and an extremely high value of 100 dB at an internal thickness of 67 μm. More significantly, protected by the ultrathin (2 μm) yet effective slippery surface, the S-rGO/LM film exhibits exceptional EMI shielding stability (EMI SE stays above 70 dB) after enduring various harsh conditions (harsh chemical environments, extreme operating temperatures, and severe mechanical wearing). Moreover, the S-rGO/LM film also demonstrates satisfying photothermal behavior and excellent Joule heating performance (surface temperature of 179 °C at 1.75 V, thermal response <10 s), which endows it with the capability of anti-icing/de-icing. This work proposes a way to construct an LM-based nanocomposite with reliable high-performance EMI shielding capability, which shows great potential for applications in wearable devices, defense, and aeronautics and astronautics.

Bioinspired rapid self-replenishing liquid-infused surfaces for stable anti-adhesion of electrosurgical electrodes

Soft tissue adhesion on electrosurgical electrodes has been a significant concern in minimally invasive surgery. The liquid-infused surface (LIS) can effectively reduce the tissue adhesion on electrodes by constructing a lubricating interface. However, the tissue-cutting process involves a complex contact friction process and high temperature, resulting in lubricant depletion of LIS. Inspired by the rapid self-replenishment of liquid film on the peristome surface of Nepenthes, here, a novel self-replenishing liquid-infused surface (SR-LIS) is fabricated by the laser direct structuring method. SR-LIS consists of first-level microgrooves and second-level microcavities. The microcavities store lubricant for a long time and enhance the capillary force of microgrooves. The rapid self-replenishing lubricant of SR-LIS ensures that the cutting interface is composed of soft tissue-lubricant-steel, resulting in stable anti-adhesion. Soft tissue cutting experiments show that the SR-LIS could stably reduce tissue adhesion and tissue damage compared to LIS. The underlying mechanism is discussed, and further experiments conclude that the self-replenishing lubricant has the double action of lubrication and cooling. This novel structural design, preparation method, and the resulting surfaces have essential applications in electrosurgical electrodes and promote the development of slippery surfaces in various industries.

Durability of Slippery Liquid-Infused Surfaces: Challenges and Advances

Slippery liquid-infused porous surfaces (SLIPS) have emerged as a unique approach to creating surfaces that can resist fouling when placed in contact with aqueous media, organic fluids, or biological organisms. These surfaces are composed of essentially two components: a liquid lubricant that is locked within the protrusions of a textured solid due to capillarity. Drops, immiscible to the lubricant, exhibit high mobility and very-low-contact-angle hysteresis when placed on such surfaces. Moreover, these surfaces are shown to resist adhesion to a wide range of fluids, can withstand high pressure, and are able to self-clean. Due to these remarkable properties, SLIPS are considered a promising candidate for applications such as designing anti-fouling and anti-corrosion surfaces, drag reduction, and fluid manipulation. These collective properties, however, are only available as long as the lubricant remains infused within the surface protrusions. A number of mechanisms can drive the depletion of the lubricant from the interior of the texture, leading to the loss of functionality of SLIPS. Lubricant depletion is one challenge that is hindering the real-world application of these surfaces. This review mainly focuses on the studies conducted in the context of enhancing the lubricant retention abilities of SLIPS. In addition, a concise introduction of wetting transitions on structured as well as liquid-infused surfaces is given. We also discuss, briefly, the mechanisms that are responsible for lubricant depletion.

Droplet Manipulation on Bioinspired Slippery Surfaces: From Design Principle to Biomedical Applications.

Droplet manipulation with high efficiency, high flexibility, and programmability, is essential for various applications in biomedical sciences and engineering. Bioinspired liquid-infused slippery surfaces (LIS), with exceptional interfacial properties, have led to expanding research for droplet manipulation. In this review, an overview of actuation principles is presented to illustrate how materials or systems can be designed for droplet manipulation on LIS. Recent progress on new manipulation methods on LIS is also summarized and their prospective applications in anti-biofouling and pathogen control, biosensing, and the development of digital microfluidics are presented. Finally, an outlook is made on the key challenges and opportunities for droplet manipulation on LIS.