Low Surface Tension Liquids Research Articles

Effects of Surface Modification by Means of Low-Temperature Plasma Nitriding on Wetting and Corrosion Behavior of Austenitic Stainless Steel

Low-temperature nitriding of austenitic stainless steels produces modified surface layers, consisting mainly of the S phase, which improve surface hardness and corrosion resistance. Because of the localized plastic deformations, owing to modified layer formation, and ion bombardment occurring during the process itself, this treatment produces also modifications of surface morphology and roughness, which can affect wettability and corrosion behavior. In this study the effects of plasma nitriding, performed using different treatment conditions, on the surface morphology and roughness, and thus on wettability and corrosion resistance, of AISI 202 specimens with different initial finishings (2D and polished finishing) were investigated. Different probe liquids, having both high (bi-distilled water and solution of 3.5% NaCl) and low (ethanol and rapeseed oil) surface tension, were employed for assessing the wetting behavior with the sessile drop method. The contact angle values for water increased markedly when nitriding was performed on polished samples, while this increase was smaller for 2D samples, and on selected specimens a hydrophobic behavior was observed. Very low contact angle values were registered using low surface tension liquids, suggesting an oleophilic behavior. Corrosion resistance in a 5% NaCl solution was assessed, and it depended on the characteristics of the nitrided specimens.

High Heat Flux Evaporation of Low Surface Tension Liquids from Nanoporous Membranes.

Water is often considered as the highest performance working fluid for liquid-vapor phase change due to its high thermal conductivity and large enthalpy of vaporization. However, a wide range of industrial systems require using low surface tension liquids where heat transfer enhancement has proved challenging for boiling and evaporation. Here, we enable a new paradigm of phase change heat transfer, which favors high volatility, low surface tension liquids rather than water. We utilized a nanoporous membrane of ≈600 nm thickness and <140 nm pore diameters supported on efficient liquid supply architectures, decoupling capillary pumping from viscous loss. Proof-of-concept devices were microfabricated and tested in a custom-built environmental chamber. We used R245fa, pentane, methanol, isopropyl alcohol, and water as working fluids with devices of total membrane area varying from 0.017 to 0.424 cm2. We realized a device-level pure evaporation heat flux of 144 ± 6 W/cm2 for water, and the highest evaporation heat flux was obtained with pentane at 550 ± 90 W/cm2. We developed a three-level model to understand vapor dynamics near the interface and thermal conduction within the device, which showed good agreement with experiments. We then compared pore-level heat transfer of different fluids, where R245fa showed approximately 10 times the performance of water under the same working conditions. Finally, we illustrate the usefulness of a figure of merit extracted from the kinetic theory for evaporation. The current work provides fundamental insights into the evaporation of low surface tension liquids, which can impact various applications such as refrigeration and air conditioning, petroleum and solvent distillation, and on-chip electronics cooling.

Superamphiphobic aluminum alloy with low sliding angles and acid-alkali liquids repellency

The achievement of superamphiphobic material with ultralow sliding angles is a highly worthwhile and challenging task because the current artificial superhydrophobicity can be easily contaminated and wetted by low surface tension liquids. Herein, we construct superamphiphobic 5083 aluminum alloy surface through a facile strategy including sono-assisted HCl etching, boiling H2O treatment and POTS grafting. Micro-nano dual scale hierarchical structure was achieved, attributing to air cushion formation and Cassie contacts. The as-prepared superamphiphobic HCl-H2O-POTS AA5083 surface has a wide range of liquids repellency to water, glycerol, ethylene glycol, peanut oil and even n-hexadecane (surface tension: 27.5 mN·m−1). The superamphiphobic surface also possesses outstanding self-cleaning ability for contaminants and chemical stability for liquid droplets with different pH values. All these advantages will endow the superamphiphobic HCl-H2O-POTS surface with broad applications prospects, including automobile, aerospace, military and marine industries.

Breakup dynamics of low-density gas and liquid interface during Taylor bubble formation in a microchannel flow-focusing device

This work aims to investigate the breakup dynamics of the low-density gas and liquid interface during bubble formation in a microchannel flow-focusing device. A high-contrast interface tracking method is developed. After the neck motion analysis in radial and axial directions, the time domain criterion between the liquid squeezing stage and the free pinch-off stage is proved to be two orders of magnitude less than the capillary time and is close to the viscous time of the liquid. Comparing to Nitrogen bubbles, the pinch-off point of Helium bubbles deflects downstream in viscous liquids and upstream in low surface tension liquids. Helium bubbles generate faster in viscous liquids and slower in low surface tension liquids. The power law exponents of thread diameter to the pinch-off remaining time in Helium experiments, which are larger than those in Nitrogen experiments, agree with previous studies in both ranges (1/3–1/2) and tendency.

Anisotropy-induced directional self-transportation of low surface tension liquids: a review.

Inspired by natural surfaces such as butterfly wings, cactus leaves, or the Nepenthes alata plant, synthetic materials may be engineered to directionally transport liquids on their surface without external energy input. This advantageous feature has been adopted for various mechanical and chemical processes, e.g. fog harvesting, lubrication, lossless chemical reactions, etc. Many studies have focused on the manipulation and transport of water or aqueous droplets, but significantly fewer have extended their work to low surface tension (LST) liquids, although these fluids are involved in numerous industrial and everyday processes. LST liquids completely wet most surfaces which makes spontaneous transportation an active challenge. This review focuses on recently developed strategies for passively and directionally transporting LST liquids.

Selective Liquid Sliding Surfaces with Springtail-Inspired Concave Mushroom-Like Micropillar Arrays.

Herein, a mushroom-like reentrant structure is proposed, inspired by springtails, to create a selective liquid sliding surface by implementing a simple yet sturdy silicon fabrication and lithography method. The fabricated arrays display high structural fidelity, presenting a novel geometry of a concave tip. The mushroom-like head shape of these structures is found to have superomniphobicity, which is independent of a variation of temperatures for even low surface tension liquids such as mineral oil. A design rule for the novel cap of the proposed structures, which results in a selective liquid sliding property with deionized (DI) water and mineral oil, is also investigated. It is demonstrated that oil starts to slide at a roll-off angle (ROA) 10° and then DI water rolls off at ROA 15° on the same fabricated transparent and flexible surface with repeatable durability.

Robust and transparent superoleophobic coatings from one-step spraying of SiO2@fluoroPOS

Transparent superoleophobic surfaces have attracted a great deal of attention due to their broad applications in daily use. But it has been proven that obtaining transparency and superoleophobicity simultaneously is challenging. Here, we report a one-step spraying method to prepare a robust transparent superoleophobic coating using SiO2@fluorinated polysiloxane, which derived through a sol–gel process of fumed SiO2 with tetraethylorthosilicate and perfluorooctyltrichlorosilane. The obtained coating exhibited high transmittance (above 82.3% for wavelength range 400–800 nm) and outstanding superoleophobicity to low surface tension liquids with the contact angles greater than 150° and sliding angles lower than 10°. The influence of SiO2 concentration and spray layer on the transmittance and superoleophobicity of coatings was also investigated. Results showed proper surface roughness and low surface energy are two main factors accounting for superoleophobicity. And to achieve highly transmittance, surface roughness of coating should be less than one-quarter of visible wavelength. More importantly, the superoleophobic coating demonstrated excellent mechanical and chemical stability, which confirmed through a series of experiments such as thermal treating at 300 °C, repelling corrosive liquid droplets, and high-speed sand impact. We envision our findings have a promising development space due to its simple preparation process and outstanding performance.

Evolution of Microdroplet Morphology Confined on Asymmetric Micropillar Structures.

The design of topological features to control the spreading of liquid has been widely investigated. Micropillar structures, for example, can retain stable droplets on the tip by inhibiting the contact line from advancing over a sharp solid edge. The pinning behavior of droplets on noncircular pillars, however, has received little attention. In this study, we analyze the retention of microdroplets with high and low surface tensions on axisymmetric and asymmetric porous micropillar structures. Circular, square, and triangular structures fabricated on silicon substrates are used to characterize the dynamic behavior of droplets before and after bursting. The critical pinning conditions are based on the visualization and pressure measurements of droplets. A theoretical model is developed based on a free energy analysis for predicting the change in pressure as the working fluid advances on the micropillar. For high surface tension liquids (e.g., water), the maximum pressure occurs when the contact line is pinned along the edge of the inner pore. For low surface tension liquids (e.g., Isopropanol and Novec 7500), the maximum pressure occurs when the contact line is pinned along the outer edge of the structure. The theoretical and experimental results demonstrate how a droplet pinned atop a triangular micropillar exhibits the smallest critical volume at the bursting moment. When using IPA solution (γ = 23 mN/m) and Novec 7500 (γ = 16 mN/m) as the working fluids, a change in the micropillar shape from circle to triangle, respectively, yields a 83% and 76% reduction in the critical burst volume. Meanwhile, the bursting pressure increases from 172 to 300 Pa and from 127 to 216 Pa for IPA and Novec 7500, respectively. These findings provide new insights to the rational design of surface micro/nanoengineered structures for tuning the surface wetting characteristics in scientific and engineering applications.

Self-Healable Superomniphobic Surfaces for Corrosion Protection.

Corrosion-protective surfaces are of the utmost relevance to ensure long-term stability and reliability of metals and alloys by limiting their interactions with corrosive species, such as water and ions. However, their practical applications are often limited either by the inability to repel low surface tension liquids such as oils and alcohols or by poor mechanical durability. Here, a superomniphobic surface is reported that can display very high contact angles for both high and low surface tension liquids as well as for concentrated acids and bases. Such extreme repellency allowed for approximately 20% of the corrosion rate compared to the conventional superhydrophobic corrosion protective coatings. Furthermore, the superomniphobic surface can autonomously repair mechanical damage at an elevated temperature (60 °C) within a short period of time (60 s), and the surface can restore its intrinsic corrosion protection performance. Such superomniphobic surfaces thus offer a wide range of potential applications, including pipelines, with sustainable corrosion protection and rust inhibitors for steel in reinforced concrete.

Facile fabrication of omniphobic PVDF composite membrane via a waterborne coating for anti-wetting and anti-fouling membrane distillation

Omniphobic membranes have been brought to great attention recently regarding the ability to resist both water and low surface tension liquids, thus can be utilized for desalination of saline wastewater containing low surface tension contaminants in membrane distillation (MD). In this study, a facile, environment-friendly, and one-step strategy is developed for fabricating an omniphobic membrane with re-entrant structure and low surface energy by spraying a waterborne coating solution comprising of fluorocarbon surfactant (FS, Zonyl@8867L), fluorinated alkyl silane (FAS), and silicon dioxide nano-particles (SiO2 NPs) onto amino functionalized polyvinylidene difluoride (PVDF) membrane via electrostatic adsorption. As a result, the prepared omniphobic FS/FAS/Si membrane possesses high liquid repellency to water, diiodomethane, mineral oil-in-water emulsion (0.10 % v/v), mineral oil, and SDS solution (0.40 mM) with the contact angle of 164.40 ± 0.95°, 159.12 ± 3.12°, 154.31 ± 3.71°, 153.31 ± 2.09°, and 157.14 ± 3.29°, respectively. Compared to the original PVDF membrane, the omniphobic FS/FAS/Si membrane displays higher rejection (about 99.99%) and superior anti-wetting and anti-fouling characteristics even to the saline feed solution with surfactants, oil pollutants, or hydrophilic organic pollutants in MD process. The results mentioned above verify that the obtained omniphobic FS/FAS/Si membrane has potential application for the desalination of the high saline wastewater containing low surface tension contaminants.

Dewetting of liquid film via vapour-mediated Marangoni effect

Liquid films on wettable solid surfaces can be disturbed to dewet when low surface tension liquids or surfactants are added because the surface tension difference gives rise to stresses on the film interface. Here we consider an alcohol drop placed above a thin aqueous film, which punctures a hole in the film starting from underneath the alcohol drop. Such film dewetting is attributed to the Marangoni effects caused by the spatial gradient of alcohol vapour concentration. We measure the liquid–gas interfacial tension of aqueous liquids rapidly responding to the surrounding isopropyl alcohol vapour concentration, and observe evolution of the film morphology consisting of central hole, fringe film, thinning region and bulk. We construct scaling laws to predict the dewetting rates of the film by considering the Marangoni stress, viscous shear stress and evaporation. It is shown that our experiments are consistent with our theory.

Superomniphobic Papers for On‐Paper pH Sensors

AbstractPaper‐based superomniphobic surfaces are of great interest because paper is flexible, inexpensive, lightweight, breathable, and recyclable. Prior reports on paper‐based superomniphobic surfaces have failed to demonstrate high mobility with low surface tension liquids. In order to overcome this issue, in this work, superomniphobic papers are developed through growth of nanofilaments on inherent microfibers of papers without noticeably altering their microscale features (i.e., diameter and distance of the microfibers). These superomniphobic papers display very low roll‐off angle, indicative of ultra‐high droplet mobility, even with low surface tension liquids. Here, a facile method is also developed to control the motion and adhesion of the droplets on the superomniphobic paper. Utilizing such liquid mobility in a controlled manner on these superomniphobic papers, a simple on‐paper pH sensor is fabricated. It is anticipated that this on‐paper, simple, and rapid detection methodology can also be extended to the colorimetric sensing of protein and chemical assays. Further, these superomniphobic papers have potential applications in water–oil separation and enhanced weight‐bearing capacity.

Spreading of low-viscous liquids on a stationary and a moving surface

This paper examines the time evolution for spreading of low surface tension liquids upon impact onto a surface, and highlights the differences with the same, for high surface tension liquids. Furthermore, it examines the role of the in-plane velocity (VP) on the time evolution of spreading phase of the impact phenomena; VP is seen when the surface is inclined, or when the surface is moving in the horizontal direction, for impact of a free-falling droplet. High-speed imaging was used to capture the spreading phenomenon from side and overhead views. It was observed that low and high surface tension liquids spread in a different manner on both stationary and moving surfaces with different outcomes regarding the time to the maximum spreading diameter, and the maximum spreading factor. Also, compared to high surface tension liquids, the stretching of lamella in the direction of the in-plane velocity vector, is more pronounced for low surface tension liquids. We observed that on a moving surface, the position of the maximum width shifts more to the center of the lamella for low surface tension liquids, compared to that of high surface tension liquids, and this shifting increases with an increase in in-plane velocity. We developed a method and related equations to describe the time evolution of the lamella as drop spreads on a hydrophilic surface. Using our method, one can predict the spreading of both low and high surface tension liquids over stationary and moving surfaces (i.e., when in-plane velocity exists). Top view of droplets impacting a moving surface: Same Impact Conditions, but different shapes for spreading, and time to max spreading seen, and explained.

Amphiphobic Nanostructured Coatings for Industrial Applications.

The search for surfaces with non-wetting behavior towards water and low-surface tension liquids affects a wide range of industries. Surface wetting is regulated by morphological and chemical features interacting with liquid phases under different ambient conditions. Most of the approaches to the fabrication of liquid-repellent surfaces are inspired by living organisms and require the fabrication of hierarchically organized structures, coupled with low surface energy chemical composition. This paper deals with the design of amphiphobic metals (AM) and alloys by deposition of nano-oxides suspensions in alcoholic or aqueous media, coupled with perfluorinated compounds and optional infused lubricant liquids resulting in, respectively, solid–liquid–air and solid–liquid–liquid working interfaces. Nanostructured organic/inorganic hybrid coatings with contact angles against water above 170°, contact angle with n-hexadecane (surface tension γ = 27 mN/m at 20 °C) in the 140–150° range and contact angle hysteresis lower than 5° have been produced. A full characterization of surface chemistry has been undertaken by X-ray photoelectron spectroscopy (XPS) analyses, while field-emission scanning electron microscope (FE-SEM) observations allowed the estimation of coatings thicknesses (300–400 nm) and their morphological features. The durability of fabricated amphiphobic surfaces was also assessed with a wide range of tests that showed their remarkable resistance to chemically aggressive environments, mechanical stresses and ultraviolet (UV) radiation. Moreover, this work analyzes the behavior of amphiphobic surfaces in terms of anti-soiling, snow-repellent and friction-reduction properties—all originated from their non-wetting behavior. The achieved results make AM materials viable solutions to be applied in different sectors answering several and pressing technical needs.

Surface Chemistry Enhancements for the Tunable Super-Liquid Repellency of Low-Surface-Tension Liquids.

Super-hydrophobic, super-oleo(amphi)phobic, and super-omniphobic materials are universally important in the fields of science and engineering. Despite rapid advancements, gaps of understanding still exist between each distinctive wetting state. The transition of super-hydrophobicity to super-(oleo-, amphi-, and omni-)phobicity typically requires the use of re-entrant features. Today, re-entrant geometry induced super-(amphi- and omni-)phobicity is well-supported by both experiments and theory. However, owing to geometrical complexities, the concept of re-entrant geometry forms a dogma that limits the industrial progress of these unique states of wettability. Moreover, a key fundamental question remains unanswered: are extreme surface chemistry enhancements able to influence super-liquid repellency? Here, this was rigorously tested via an alternative pathway that does not require explicit designer re-entrant features. Highly controllable and tunable vertical network polymerization and functionalization were used to achieve fluoroalkyl densification on nanoparticles. For the first time, relative fluoro-functionalization densities are quantitatively tuned and correlated to super-liquid repellency performance. Step-wise tunable super-amphiphobic nanoparticle films with a Cassie–Baxter state (contact angle of >150° and sliding angle of <10°) against various liquids is demonstrated. This was tested down to very low surface tension liquids to a minimum of ca. 23.8 mN/m. Such findings could eventually lead to the future development of super-(amphi)omniphobic materials that transcend the sole use of re-entrant geometry.

Superamphiphobic Cu/CuO Micropillar Arrays with High Repellency Towards Liquids of Extremely High Viscosity and Low Surface Tension

For almost all the research of super anti-wetting surfaces, pure liquids like water and n-hexadecane are used as the probes. However, liquids of diverse compositions are used in academic research, industrial production and our daily life. Obviously, the liquid repellency of super anti-wetting coatings is highly dependent on properties of the liquids. Here, we report the first superamphiphobic surface with high repellency towards liquids of extremely high viscosity and low surface tension. The surfaces were prepared by constructing a hierarchical micro-/nanostructure on the Cu micropillar arrays followed by modification with perfluorosilane. The surfaces are superamphiphobic towards the liquids with extremely high viscosity and low surface tension because of (i) the micro-/nanostructured surface composed of micropillars with proper pillar distance and CuO nano-flowers, and (ii) the abundant perfluorodecyl groups on the surface. The contact angles, sliding angles, apparent contact line at the solid-liquid interface and adhesion forces are the end products of micropillar distance, viscosity and surface tension. Smaller micropillar distance, higher viscosity and higher surface tension contribute to reducing the adhesion force. We in situ observed the process of microcapillary bridge rupture for the first time using highly viscous liquids. We also successfully reduced the adhesion forces and enhanced the average rolling velocity of liquids with extremely high viscosity and low surface tension by regulating the micropillar distance.

Self‐Cleaning Ultraviolet Photodetectors Based on Tree Crown‐Like Microtube Structure

AbstractPhotodetectors are extensively applied in a wide range of applications. In particular, hierarchical ZnO micro/nanostructure has achieved promising success in the fields of advanced photodetectors. By utilizing hierarchical structure, development of functional photodetectors with self‐cleaning properties would extend the applications of photodetectors in more severe environments. Recent reports have demonstrated the advantages of nanofabricated surfaces with re‐entrant geometry to obtain superoleophobicity that can repel low surface tension liquids such as biofluids or oils. Inspired by the excellent liquid repellency of re‐entrant geometry, here a self‐cleaning ultraviolet photodetector is reported based on tree crown‐like microtube (TCLMT) structure. The TCLMT consisted of a ZnO microtube‐trunk structure branched with numerous ZnO nanospikes on top, presenting a re‐entrant geometry that effectively repelled water and various biological fluids. The TCLMT is fabricated using a microporous membrane‐templated method, followed by multiple steps of material deposition, plasma etching, and hydrothermal growth. The fluorinated ZnO TCLMT‐based photodetector exhibits excellent photoelectric performance as well as resistance against biofouling solution such as bacterial suspension. This work provides a versatile approach to achieve controlled fabrication of re‐entrant structures with excellent liquid repellent properties as well as extending ZnO‐based photodetectors to be self‐cleaning and resistant to biofouling.

Formation of Liquid–Liquid Micropatterns through Guided Liquid Displacement on Liquid‐Infused Surfaces

AbstractHere a method is demonstrated to pattern liquids of varying surface tension and composition into droplets by utilizing slippery liquid‐infused surfaces prepared on chemically‐patterned substrates. The capability of different liquids to displace the lubricant from higher surface energy regions is studied and it is shown that both high and low surface tension liquids can imbibe the polymer, thereby forming droplets sharply following underlying surface energy patterns. For all liquids tested, droplet arrays of arbitrary shapes of each liquid are formed with precision down to 50 µm. By changing the chemical patterning from fluorinated to aliphatic groups, patterns of mineral and silicone oils are created. Finally, formation of 2D micropatterns of three‐phase liquid systems—fluorinated, organic, and aqueous phases—is demonstrated.

Nanostructure depositions on alumina hollow fiber membranes for enhanced wetting resistance during membrane distillation

Omniphobic alumina hollow fiber membranes were developed for direct contact membrane distillation (DCMD) with a low surface tension feed in this study. Alumina hollow fiber (HF) membranes were prepared as the membrane substrates, and chemical bath deposition methods were applied to deposit ZnO nanorods and nanoparticles on the HF membranes. The SEM, EDX, and AFM analyses showed that the ZnO nanostructures were effectively deposited on the membrane surfaces and able to increase the surface roughness. After surface fluorination by FAS17, the HF membranes deposited by ZnO nanorods and nanoparticles demonstrated contact angles of 128.7° and 138.1° for a 90% v/v ethanol/water mixture, both of which were higher than 114.8° for the pristine HF membrane without ZnO nanostructures. In the DCMD experiments with the sequential addition of SDS from 0.2 to 2.0 mM, the HF membranes with ZnO nanostructures exhibited superior wetting resistances with low surface tension feeds compared to that of the pristine HF membranes. Moreover, the HF membranes with the deposited ZnO nanoparticles had the best wetting resistance, and the permeate water flux was maintained for 24 h using a 2.0 mM SDS (70 °C 1 M NaCl) solution as an initial feed. The results not only suggested that the deposition of nanostructures enhanced the wetting resistance of the alumina hollow fiber membranes to low surface tension liquids but also showed the promise of utilizing these membranes for the desalination of low surface tension wastewaters.

3D re-entrant nanograss on microcones for durable superamphiphobic surfaces via laser-chemical hybrid method

The potential applications of superamphiphobic surfaces are widespread and attractive. The design of re-entrant structures is crucial to superamphiphobic surfaces. However, both the existing two strategies to design re-entrant structures face problems. Here a creative strategy to design 3D re-entrant nanostructures on ordered microstructures is proposed. A novel “Top-down” and “Bottom-up” hybrid method consisting of ultrafast laser ablation and chemical bath processing is used to fabricate hierarchical surfaces with 3D re-entrant CuO nanograss on Cu microcones. Thanks to the low surface energy from modification with perfluorodecyltrimethoxysilane and plenty of re-entrant geometries provided by the 3D distributed nanograss structures, the hierarchical surfaces show outstanding superamphiphobicity with water and low surface tension liquid like dodecane. The influence of the nanograss types and the microcone distribution on the superamphiphobicity are systematically studied. Besides, the microcones/nanograss hierarchical superamphiphobic surfaces show excellent long-term durability, high temperature durability and comprehensive mechanical durability, are promising for practically applications.