Ultralow Contact Angle Hysteresis Research Articles

Patterned Quasi‐Liquid Surfaces for Condensation of Low Surface Tension Fluids

AbstractExtensive research concerns dropwise condensation of low surface tension fluids to promote energy efficiency and decarbonization in thermal energy systems. However, it is challenging as these fluids typically result in filmwise condensation. Drawing inspiration from the Namib desert beetle that enhances condensation through patterned wettability, conventional beetle‐inspired surfaces excel in water condensation but flood when condensing low surface tension fluids. In this work, a patterned quasi‐liquid surface is reported that achieves exceptional dropwise condensation of low surface tension fluids. The surface consists of alternating stripes with low surface energy, that is, a perfluoropolyether (PFPE) and fluorinated quasi‐liquid surface (FQLS), that shows ultralow contact angle hysteresis for ethanol and hexane. The PFPE stripes are slightly more slippery, acting as slippery bridges that accelerate droplet coalescence and removal. It is experimentally demonstrated that the striped PFPE‐FQLS pattern exhibits a heat transfer coefficient 85%, 330%, and 550% higher than that of PFPE, fluorinated silane, and filmwise condensation, respectively. This study reveals that a high contact angle is desired to sustain dropwise condensation, irrespective of contact angle hysteresis. These findings provide a new paradigm for promoting the dropwise condensation of low surface tension fluids and offer valuable insights into surface design for energy sustainability.

Non-fluorine oil repellency: To what extent can it substitute perfluoroalkyl substances?

Oil-repellency of fabric is essentially based on perfluoroalkyl substances (PFAS, or called perfluorochemicals (PFCs)). However, plans to phase out the use of these chemicals, even those short-chain ones (C4-C6), have been announced because they accumulate in the human body and are extremely persistent in the environment. It is urgent to develop non-fluorine surface chemistry for oil-repellent fabric finishing. Here, we report the non-fluorine oil repellency of brush-like polydimethylsiloxane (PDMS) coating (g-PDMS), and the comparison of its oil repellency with that of a common PFAS crosslink polysiloxane coating (C4-PFAS). A catalyst-free, solvent-free chemical vapor deposition (CVD) process was used to prepare the g-PDMS coating. For smooth glass, g-PDMS coating exhibits ultralow contact angle hysteresis (∆θ) for various oils (1.3–7.3°), while the ∆θ of C4-PFAS coating is much higher (30.7–49.7°). For fabrics, g-PDMS and C4-PFAS achieved almost the same contact angles to castor oil (~134°). According to GB/T 19977-2014 standard, the oil repellency of coated fabrics was rated Grade 5 for both g-PDMS and C4-PFAS. Therefore, the non-fluorine oil repellency of g-PDMS is able to meet the basic requirement of oil-repellent fabric finishing, and it is expected that the oil repellency can be further improved by the rational design on surface topography.

Nature-Inspired High Temperature Scale-Resistant Slippery Lubricant-Induced Porous Surfaces (HTS-SLIPS).

Scale formation is a longstanding and unresolved problem in a number of fields, including power production, petroleum exploration, thermal desalination, and construction. Herein, a high-temperature scale-resistant slippery lubricant-induced surface (HTS-SLIPS) is developed by one-step electrodeposition and lubricant infusion. The fractal cauliflower-like morphology with lubricant oil is conducive to forming an ultralow contact angle hysteresis of ≈1°. The 10-d real-world boiling trial indicates that by replacing the uncoated surface with HTS-SLIPS, the reduction in scale mass is greater than 200% because of the low surface free energy (4.3 mJ m-2 ) and outstanding smoothness (Ra = 41 ± 8nm) of HTS-SLIPS. Thanks to the scale retardation, the bubble departure frequency of HTS-SLIPS is eightfold higher than that of uncoated surfaces, signifying superior heat transfer efficiency. In these demonstrations, HTS-SLIPS coated spiral tube exhibits better flowability and lower pressure drop than the uncoated one. In addition, favorable compatibility between HTS-SLIPS and mechanical vibration is experimentally verified to strengthen the descaling of SLIPS synergistically. It is anticipated that the simple and scalable coating fabrication approach will be applicable in numerous industrial high-temperature processes where scale formation is encountered.

Chlorotrimethylsilane treatment affords long-lasting contaminant-proof and transparent slippery covalently attached liquids surfaces

Slippery lubricant-infused porous surfaces (SLIPs) are effective in repelling low-surface tension fluids given their omniphobicity, anti-adhesive, and drag reduction properties. However, over time, SLIPs are subjected to lubricant losses and undergo dust fouling. In this research, we address these challenges by developing a new slippery covalently attached liquids (SCAL) surface. SCALs was composed of a molecularly-thick lubricant layer covalently bound to the solid support, generating long-lasting anti-fouling, hydrophobic and slippery effects. We show that our SCALs exhibit ultra-low contact angle hysteresis for a wide range of fluids and enable reduced colloidal forces in the vertical and lateral directions (adhesion and friction, respectively). In parallel, the SCALs exhibit a high resistance to contamination (from molecular adhesion to dust fouling), mechanical integrity, and light transmission. Overall, we introduce a facile, cost-effective, and scalable process to synthesize a SCALs system suitable to the demands of application in bioengineering, power generation and water harvesting.

Droplet Spreading Characteristics on Ultra-Slippery Solid Hydrophilic Surfaces with Ultra-Low Contact Angle Hysteresis

Dynamic interactions of the droplet impact on a solid surface are essential to many emerging applications, such as electronics cooling, ink-jet printing, water harvesting/collection, anti-frosting/icing, and microfluidic and biomedical device applications. Despite extensive studies on the kinematic features of the droplet impact on a surface over the last two decades, the spreading characteristics of the droplet impact on a solid hydrophilic surface with ultra-low contact angle hysteresis are unclear. This paper clarifies the specific role of the contact angle and contact angle hysteresis at each stage of the droplet impact and spreading process. The spreading characteristics of the droplet impact on an ultra-slippery hydrophilic solid surface are systematically compared with those on plain hydrophilic, hydroxylated hydrophilic, and plain hydrophobic surfaces. The results reveal that the maximum spreading factor (βmax) of impacting droplets is mainly dependent on the contact angle and We. βmax increases with the increase in We and the decrease in the contact angle. Low contact angle hysteresis can decrease the time required to reach the maximum spreading diameter and the time interval during which the maximum spreading diameter is maintained when the contact angles are similar. Moreover, the effect of the surface inclination angle on the spreading and slipping dynamics of impacting droplets is investigated. With the increase in the inclination angle and We, the gliding distance of the impacting droplet becomes longer. Ultra-low contact angle hysteresis enables an impacting droplet to slip continuously on the ultra-slippery hydrophilic surface without being pinned to the surface. The findings of this work not only show the important role of the surface wettability in droplet spreading characteristics but also present a pathway to controlling the dynamic interactions of impacting droplets with ultra-slippery hydrophilic surfaces.

Quasi-Liquid Surfaces for Sustainable High-Performance Steam Condensation.

Sustainable high-performance steam condensation is critical to reducing the size, weight, and cost of water and energy systems. It is well-known that dropwise condensation can provide a significantly higher heat-transfer coefficient than filmwise condensation. Tremendous efforts have been spent to promote dropwise condensation by achieving a nonwetting state on superhydrophobic surfaces and a slippery state on liquid-infused surfaces, but these surfaces suffer from severe durability challenges. Here, we report sustainable high-performance dropwise condensation of steam on newly developed durable quasi-liquid surfaces, which are easily made by chemically bonding quasi-liquid polymer molecules on solid substrates. As a result, the solid/water interface is changed to a quasi-liquid/water interface with minimal adhesion and extraordinary durability. The quasi-liquid surface with ultralow contact angle hysteresis down to 1° showed a heat-transfer coefficient up to 70 and 380% higher than those on conventional hydrophobic and hydrophilic surfaces, respectively. Furthermore, we demonstrated that the quasi-liquid coating exhibited a sustainable heat-transfer coefficient of 71 kW/(m2 K) at a heat flux of 420 kW/m2 under a prolonged period of 39 h in continuous steam condensation. Such a quasi-liquid surface has the potential to sustain high-performance dropwise condensation of steam and address the long-standing durability challenge in the field.

Gradient Quasi‐Liquid Surface Enabled Self‐Propulsion of Highly Wetting Liquids

AbstractSelf‐propulsion of highly wetting liquids is important in heat exchanger, air conditioning, and refrigeration systems. However, it is challenging to achieve such a spontaneous motion as these liquids tend to wet all the surfaces due to their ultralow surface tensions. Despite that extensive asymmetric surface structures and gradient chemical coatings are developed for directional droplet transport, they will be flooded and covered by these liquids. Here, this challenge is addressed by creating a gradient quasi‐liquid surface to achieve the self‐propulsion of droplets with surface tensions down to 10.0 mN m−1. Such a surface engineered by tethering flexible polymers with gradient grafting density shows ultralow contact angle hysteresis (<1o) to highly wetting liquids. Thus, the surface can simultaneously provide sufficient driving forces through the gradient wettability and negligible retention forces through the slippery boundary lubrication for spontaneous droplet movement. Moreover, continual self‐propulsion of tiny droplets is achieved by spraying highly wetting liquids in simulated condensation conditions and demonstrates that adding temperature gradient can further accelerate the self‐propulsion. The study provides a new paradigm to promote passive removal of highly wetting droplets, leading to potential impacts in enhancing condensation heat transfer regardless of surface orientations.

Passive Removal of Highly Wetting Liquids and Ice on Quasi-Liquid Surfaces.

Surfaces with ultralow adhesion to liquids and solids have attracted broad interests in both fundamental studies and engineering applications from passive removal of highly wetting liquids and water harvesting to anti-/de-icing. The current state-of-the-art superomniphobic surfaces (rely on air lubricant) and liquid-infused surfaces (rely on liquid lubricant) suffer from severe issues for liquid repellency and ice removal: air/liquid lubricant loss or topography damage. Here, we create a durable quasi-liquid surface by tethering flexible polymer on various solid substrates. The untethered end of the polymer has mobile chains that behave like a liquid layer and greatly reduce the interfacial adhesion between the surface and foreign liquids/solids. Such a quasi-liquid surface with a 30.1 nm flexible polymer layer shows ultralow contact angle hysteresis (≤1.0°) to liquids regardless of their surface tensions. The highly wetting perfluorinated liquids like FC72 and Krytox101, as well as complex fluids like urine and crude oil, can be repelled from the surface. Moreover, wind can remove accreted ice from the surface in harsh conditions due to the negligible ice adhesion. We have demonstrated that the quasi-liquid surface shows robust performances in repelling highly wetting liquids, harvesting water, and removing ice, respectively.

Non-Fluorinated Omniphobic Paper with Ultralow Contact Angle Hysteresis.

Existing methods for fabricating oil-repellent paper rely on highly fluorinated and therefore toxic chemicals. Non-fluorinated omniphobic paper with low contact angle hysteresis (CAH) has not been demonstrated. We report a facile method to prepare omniphobic paper through the vapor-phase deposition of chlorosilane molecules to create "liquid-like" polymer brushes on commercially available release liners. Compared to polymer brushes grafted from solution, this solvent-free method avoided physical deformation of the paper, such as curling or wrinkling. The obtained paper displayed low CAH (<6°) and roll-off angles for liquids exhibiting a broad range of surface tension, from 72.8 to 22.4 mN m-1. A hexadecane droplet (15 μL, 27.5 mN m-1) slid off the paper at a tilt angle less than 4°. The effects of surface roughness, composition, and the presence of particle additives on the wetting properties were investigated. The utility of the omniphobic paper was demonstrated in microfluidic, oil funnel, microtiter plate, and food packaging container applications.

Extreme Antiscaling Performance of Slippery Omniphobic Covalently Attached Liquids.

Scale formation presents an enormous cost to the global economy. Classical nucleation theory dictates that to reduce the heterogeneous nucleation of scale, the surface should have low surface energy and be as smooth as possible. Past approaches have focused on lowering surface energy via the use of hydrophobic coatings and have created atomically smooth interfaces to eliminate nucleation sites, or both, via the infusion of low-surface-energy lubricants into rough superhydrophobic substrates. Although lubricant-based surfaces are promising candidates for antiscaling, lubricant drainage inhibits their utilization. Here, we develop methodologies to deposit slippery omniphobic covalently attached liquids (SOCAL) on arbitrary substrates. Similar to lubricant-based surfaces, SOCAL has ultralow roughness and surface energy, enabling low nucleation rates and eliminating the need to replenish the lubricant. To enable SOCAL coating on metals, we investigated the surface chemistry required to ensure high-quality functionalization as measured by ultralow contact angle hysteresis (<3°). Using a multilayer deposition approach, we first electrophoretically deposit (EPD) silicon dioxide (SiO2) as an intermediate layer between the metallic substrate and SOCAL. The necessity of EPD SiO2 is to smooth (<10 nm roughness) as well as to enable the proper surface chemistry for SOCAL bonding. To characterize antiscaling performance, we utilized calcium sulfate (CaSO4) scale tests, showing a 20× reduction in scale deposition rate than untreated metallic substrates. Descaling tests revealed that SOCAL dramatically decreases scale adhesion, resulting in rapid removal of scale buildup. Our work not only demonstrates a robust methodology for depositing antiscaling SOCAL coatings on metals but also develops design guidelines for the creation of antifouling coatings for alternate applications such as biofouling and high-temperature coking.

Self-cleaning poly(L-dopa)-based coatings with exceptional underwater oil repellency for crude oil/water separation

As a new derivative material of polydopamine, poly(L-dopa) (PDopa) possesses abundant catechol, carboxyl and amino groups, endowing a new platform for construction of multifunctional coatings. In the present work, negatively charged PDopa was synthesized on a large scale by self-polymerization of L-3,4-dihydroxyphenylalanine (L-Dopa) in aqueous solutions. The PDopa-based coatings were fabricated via layer-by-layer assembly of positively charged polyelectrolyte and negatively charged PDopa. The as-obtained coatings demonstrate excellent underwater oil repellency with oil contact angles of 165 ± 2° and ultra-low contact angle hysteresis arising from electrostatically-induced and hydrogen bonding-induced hydration layers formed between functional groups of PDopa and water. Unlike traditional underwater superoleophobic coatings realized by introducing hydrophilic nano/microstructures, the smooth PDopa-based coatings can clean up oil fouling simply by water action. The coatings applied onto steel meshes facilitate high-performance separation of both phase-separated and emulsified oil/water mixtures with high flux and separation efficiency solely driven by gravity. By employing the separation apparatus with a sloping plate structure, crude oil/water mixture collected from oilfield with 75–80% of water can be effectively separated under mixture temperature of 35–50 °C, with oil content in water of ~20 to ~70 ppm, and water content in oil of 1.9–4.3%.

Stable Dropwise Condensation of Ethanol and Hexane on Rationally Designed Ultrascalable Nanostructured Lubricant-Infused Surfaces.

Vapor condensation is a widely used industrial process for transferring heat and separating fluids. Despite progress in developing low surface energy hydrophobic and micro/nanostructured superhydrophobic coatings to enhance water vapor condensation, demonstration of stable dropwise condensation of low-surface-tension fluids has not been achieved. Here, we develop rationally designed nanoengineered lubricant-infused surfaces (LISs) having ultralow contact angle hysteresis (<3°) for stable dropwise condensation of ethanol (γ ≈ 23 mN/m) and hexane (γ ≈ 19 mN/m). Using a combination of optical imaging and rigorous heat transfer measurements in a controlled environmental chamber free from noncondensable gases (<4 Pa), we characterize the condensation behavior of ethanol and hexane on ultrascalable nanostructured CuO surfaces impregnated with fluorinated lubricants having varying viscosities (0.496 < μ < 5.216 Pa·s) and chemical structures (branched versus linear, Krytox and Fomblin). We demonstrate stable dropwise condensation of ethanol and hexane on LISs impregnated with Krytox 1525, attaining about 200% enhancement in condensation heat transfer coefficient for both fluids compared to filmwise condensation on hydrophobic surfaces. In contrast to previous studies, we use 7 h of steady dropwise condensation experiments to demonstrate the importance of rational lubricant selection to minimize lubricant drainage and maximize LIS durability. This work not only demonstrates an avenue to achieving stable dropwise condensation of ethanol and hexane, it develops the fundamental design principles for creating durable LISs for enhanced condensation heat transfer of low-surface-tension fluids.

Rigorous testing to assess the self-cleaning properties of an ultra-water-repellent silicone rubber surface

Ultra-water-repellent silicone-based surfaces were produced to study their self-cleaning properties. First, we investigated the consistency of the micro-nano air pockets that are present between the surface asperities responsible for the formation of the Cassie-Baxter regime. We then performed a comprehensive series of self-cleaning experiments involving both suspended and non-suspended contaminants using various materials (e.g., kaolin, carbon black, silica, etc.) and contaminant-applying methods (e.g., dropwise, spraying, wet or dry contaminants). In this paper, the self-cleaning tests were arranged from the less severe, i.e., non-suspended contamination tests, to more severe, i.e., wet suspended contamination test, and ending with the most severe, i.e., dry suspended contamination test. Due to the ultra-low contact angle hysteresis, the produced surfaces showed favorable self-cleaning properties against the various types of contaminants and the different means of contaminant application. The produced surfaces retained their water repellency properties following application of the contaminants and after the cleaning of the surfaces, thus verifying the self-cleaning performance and resistance of the fabricated superhydrophobic silicone surfaces.

Self-healing atypical liquid-infused surfaces: Superhydrophobicity and superoleophobicity in submerged conditions

Abstract Self-healing atypical liquid-infused surfaces (ALIS) which demonstrate very high bubble/drop angles (>160°) and ultra-low contact angle hysteresis (CAH

Flexible superhydrophobic SERS substrates fabricated by in situ reduction of Ag on femtosecond laser-written hierarchical surfaces

The ability to fingerprint a few molecules via surface enhanced Raman scattering (SERS) continues to be of considerable utility in diverse fields encompassing physics, chemistry, materials sciences, nanotechnology, biomedicine, and environmental engineering. However, the development of facile and low cost approaches towards the fabrication of flexible substrates with very high SERS signal enhancement remains a challenge. Compared to conventional plasmonic-based sensors, a superhydrophobic plasmonic surface provides the combined advantage of concentration enrichment of solute molecules, the fourth power dependent localized electric field enhancement as well as the enhanced light scattering on rough surface. We demonstrate here a method to fabricate flexible SERS substrates by replicating laser-written patterns created on polymethylmethacrylate (PMMA) onto a soft-elastomer, namely polydimethylsiloxane (PDMS), followed by in situ reduction of silver nanoparticles on the surface. Laser writing of structures is fluence dependent and leads to substantial enhancement of Raman signals compared to a conventional plasmonic surface. Our fabricated surfaces provide a limit of detection of eight femtomolar for Rhodamine 6G upon 532 nm resonance excitation; an enhancement factor of ∼1010 is achieved for methyl orange. Studies of how water droplets spread on our laser-written surfaces indicate a fluence dependent enhancement in apparent contact angle with a concomitant increase in the contact angle hysteresis (CAH). Most significantly, our replicated patterns exhibit lotus effect (superhydrophobic surface with ultra-low contact angle hysteresis), that upon reduction of silver nanoparticles, exhibit superhydrophobicity with high contact angle hysteresis (rose petal effect). Our findings offer fresh opportunities for expanding the applicability of the SERS technique via superhydrophobic flexible plasmonic nanostructures.

Droplet Cloaking Imaging and Characterization

Lubricant infused surfaces (LIS or SLIPS) have potential to enhance a variety of applications due to their non-wetting characteristics. Recently, LIS and SLIPS have been shown to greatly improve dropwise condensation heat transfer performance by enabling ultra-low contact angle hysteresis (&lt; 5°) and droplet-surface adhesion. The low adhesion characteristics arise from the ultra-smooth liquid-liquid interface that depends on the lubricating fluid remaining infused into the surface by capillary forces. A key limiting phenomenon of LIS and SLIPS is cloaking, whereby the infused lubricant spreads and forms a very thin layer (∼100 nm) over liquid droplet residing on the substrate. This can lead to degradation due to drainage of the lubricant layer. Cloaking involves several fluid interfaces between the lubricant, droplet and air, and the resolution of the lubricant-droplet interface is essential to cloaking imaging and characterization. Our work involves goniometric measurements of advancing and receding contact angles, imaging of cloaked water droplets on LIS and SLIPS, and the characterization of cloaking during shear induced flow. The LIS and SLIPS substrates were created by spin coating lubricants of various viscosities (5 – 2712 cSt) onto superhydrophobic nanostructured boehmite (AlO(OH)). Cloaking effects were observed by inhibition of water droplet evaporation (Fig. a). Furthermore, the lubricantdroplet interface for non-cloaking droplets was resolved using lubricant-miscible dyes (Fig. a). Cloaking was also characterized by studying shear induced flow of both condensate and artificially injected droplets, resulting in lubricant drainage as shown in Figures (b) and (c). The results of this work not only provide a basis for understanding the lubricant-droplet interactions on LIS and SLIPS, but also material design guidelines for future LIS and SLIPS coatings.

Fast Transport of Water Droplets over a Thermo-Switchable Surface Using Rewritable Wettability Gradient.

In spite of the reported temperature dependent tunability in wettability of poly(N-isopropylacrylamide) (PNIPAAm) surfaces for below and above lower critical solution temperature (32 °C), the transport of water droplets is inhibited by the large contact angle hysteresis. Herein, for the first time, we report on-demand, fast, and reconfigurable droplet manipulation over a PNIPAAm grafted structured polymer surface using temperature-induced wettability gradient. Our study reveals that the PNIPAAm grafted on intrinsically superhydrophobic surfaces exhibit hydrophilic nature with high contact angle hysteresis below 30 °C and superhydrophobic nature with ultralow contact angle hysteresis above 36 °C. The transition region between 30 and 36 °C is characterized by a large change in water contact angle (∼100°) with a concomitant change in contact angle hysteresis. By utilizing this "transport zone" wherein driving forces overcome the frictional forces, we demonstrate macroscopic transport of water drops with a maximum transport velocity of approximately 40 cm/s. The theoretical calculations on the force measurements concur with dominating behavior of driving forces across the transport zone. The tunability in transport velocity by varying the temperature gradient along the surface or the inclination angle of the surface (maximum angle of 15° with a reduced velocity 0.4 mm/s) is also elucidated. In addition, as a practical application, coalescence of water droplets is demonstrated by using the temperature controlled wettability gradient. The presented results are expected to provide new insights on the design and fabrication of smart multifunctional surfaces for applications such as biochemical analysis, self-cleaning, and microfluidics.

Extraordinarily Rapid Rise of Tiny Bubbles Sliding beneath Superhydrophobic Surfaces.

Tiny bubbles readily stick onto substrates owing to contact angle hysteresis (CAH). Nevertheless, they can slide slowly on a tilted surface with ultralow CAH because capillarity is overcome by buoyancy. It is surprising to observe experimentally that bubbles of 3-15 μL (diameter 1.79-3.06 mm) slide beneath a tilted superhydrophobic surface at a vertical ascent rate faster than that of freely rising ones of high Reynold numbers ≈O(102). As the tilting angle increases, the drag coefficient remains essentially the same as that of a freely rising bubble, but the frontal area of the flat bubble rises monotonically. Nonetheless, the frontal area of the sliding bubble always stays much smaller than that of a freely rising bubble. Consequently, the small drag force associated with the sliding bubbles is attributed to their substantially small frontal areas on superhydrophobic surfaces.

Ultralow contact angle hysteresis and no-aging effects in superhydrophobic tangled nanofiber structures generated by controlling the pore size of a 99.5% aluminum foil

Superhydrophobic surfaces designed to improve hydrophobicity have high advancing contact angles corresponding to the Cassie state, but these surfaces also exhibit high contact angle hysteresis. We report here a simple and inexpensive method for fabricating superhydrophobic tangled nanofiber structures with ultralow contact angle hysteresis and no-aging degradation, based on a widening process. The resulting nanostructures are suitable for diverse applications including microfluidic devices for biological studies and industrial self-cleaning products for automobiles, ships and houses.