Ambient-mediated wetting on smooth surfaces

A consensus was built in the first half of the 20th century, which was further debated more than 3 decades ago, that the wettability and condensation mechanisms on smooth solid surfaces are modified by the adsorption of organic contaminants present in the environment. Recently, disagreement has formed about this topic once again, as many researchers have overlooked contamination due to its difficulty to eliminate. For example, the intrinsic wettability of rare earth oxides has been reported to be hydrophobic and non-wetting to water. These materials were subsequently shown to display dropwise condensation with steam. Nonetheless, follow on research demonstrated that the intrinsic wettability of rare earth oxides is hydrophilic and wetting to water, and that a transition to hydrophobicity occurs in a matter of hours-to-days as a consequence of the adsorption of volatile organic compounds from the ambient environment. The adsorption mechanisms, kinetics, and selectivity of these volatile organic compounds are empirically known to be functions of the substrate material and structure. However, these mechanisms, which govern the surface wettability, remain poorly understood. In this contribution, we introduce current research demonstrating the different intrinsic wettability of metals, rare earth oxides, and other smooth materials, showing that they are intrinsically hydrophilic. Then we provide details on research focusing on the wetting to non-wetting transition to hydrophobicity due to adsorption of volatile organic compounds. A state-of-the-art figure of merit mapping the wettability of different smooth solid surfaces to ambient exposure and surface carbon content is developed. In addition, we analyse recent works that address the wetting transitions so to shed light on how such processes affect droplet pinning and lateral adhesion. We then conclude with objective perspectives about research on wetting to non-wetting transitions on smooth solid surfaces in an attempt to raise awareness regarding surface contamination within the engineering, interfacial science, and physical chemistry domains

The Apparent Surface Free Energy of Rare Earth Oxides is Governed by Hydrocarbon Adsorption

The surface free energy of rare earth oxides (REOs) has been debated during the last decade, with some reporting REOs to be intrinsically hydrophilic and others reporting hydrophobic. Here, we investigate the wettability and surface chemistry of pristine and smooth REO surfaces, conclusively showing that hydrophobicity stems from wettability transition due to volatile organic compound adsorption. We show that, for indoor ambient atmospheres and well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water increase with exposure time. We examined the surfaces comprehensively with multiple surface analysis techniques to confirm hydrocarbon adsorption and correlate it to wettability transition mechanisms. We demonstrate that both physisorption and chemisorption occur on the surface, with chemisorbed hydrocarbons promoting further physisorption due to their high affinity with similar hydrocarbon molecules. This study offers a better understanding of the intrinsic wettability of REOs and provides design guidelines for REO-based durable hydrophobic coatings

Atmosphere-Mediated Superhydrophobicity of Rationally Designed Micro/Nanostructured Surfaces

Superhydrophobicity has received significant attention over the past three decades owing to its significant potential in self-cleaning, anti-icing and drag reduction surfaces, energy-harvesting devices, antibacterial coatings, and enhanced heat transfer applications. Superhydrophobicity can be obtained via the roughening of an intrinsically hydrophobic surface, the creation of a re-entrant geometry, or by the roughening of a hydrophilic surface followed by a conformal coating of a hydrophobic material. Intrinsically hydrophobic surfaces have poor thermophysical properties, such as thermal conductivity, and thus are not suitable for heat transfer applications. Re-entrant geometries, although versatile in applications where droplets are deposited, break down during spatially random nucleation and flood the surface. Chemical functionalization of rough metallic substrates, although promising, is not utilized because of the poor durability of conformal hydrophobic coatings. Here we develop a radically different approach to achieve stable superhydrophobicity. By utilizing laser processing and thermal oxidation of copper (Cu) to create a high surface energy hierarchical copper oxide (CuO), followed by repeatable and passive atmospheric adsorption of hydrophobic volatile organic compounds (VOCs), we show that stable superhydrophobicity with apparent advancing contact angles ≈160° and contact angle hysteresis as low as ≈20° can be achieved. We exploit the structure length scale and structure geometry-dependent VOC adsorption dynamics to rationally design CuO nanowires with enhanced superhydrophobicity. To gain an understanding of the VOC adsorption physics, we utilized X-ray photoelectron and ion mass spectroscopy to identify the chemical species deposited on our surfaces in two distinct locations: Urbana, IL, United States and Beijing, China. To test the stability of the atmosphere-mediated superhydrophobic surfaces during heterogeneous nucleation, we used high-speed optical microscopy to demonstrate the occurrence of dropwise condensation and stable coalescence-induced droplet jumping. Our work not only provides rational design guidelines for developing passively durable superhydrophobic surfaces with excellent flooding-resistance and self-healing capability but also sheds light on the key role played by the atmosphere in governing wetting

Nanoengineered condenser surfaces for enhancing transport in thermal desalination by air gap membrane distillation

Thermal desalination is a technique that uses heat or thermal energy to desalinate water, unlike reverse osmosis. Membrane distillation (MD) is a type of thermal desalination technology having various configurations. Air gap membrane distillation (AGMD) is one of the more energy efficient MD configurations, being especially advantageous over other configurations at high salinity. However, the large mass transfer resistance of the air gap dramatically reduces the permeate flux, impairing performance. Higher condensation performance can be achieved by using a smaller air gap size, but typical film-wise condensation flow patterns flood the air gap at the optimal gap size (\u3c1 mm). Experiments show that dropwise and jumping-droplet condensation regimes, achieved using hydrophobic and superhydrophobic condensing surfaces respectively, can improve droplet shedding, allowing for thinner gap sizes. A systemlevel numerical model is used to demonstrate that these surfaces could thereby enable improved energy efficiency (2.1× increase of gained output ratio) while avoiding flooding at gap sizes as small as 0.2 mm. Superhydrophobic surfaces with directional jumping of droplets, specifically in the direction of gravity, are also tested and compared to droplets that jump normal to the condensing surface. Novel condensing surfaces that include a combination of the superhydrophobic and superhydrophilic patterns create flow regimes having pathways for faster permeate removal. Other condensing surfaces, including SLIPS (slippery liquidinfused porous surfaces) and laser-ablated superhydrophobic patterned surfaces are tested to the check the extent to which they improve the permeate removal rate while exhibiting different condensation regimes that merit further exploration

Dynamics of Thin Surfactant Films

Gravitational drainage from plane vertical films of various ordinary surfactants, cationic, anionic, non-ionic and superspreaders are studied theoretically and experimentally using a microinterferometric technique. The ordinary surfactants showed ordered interferometric color bands and the film thickness decreased linearly in time and surface elasticity was calculated from the measured time dependent film thickness. The superspreaders showed complicated dynamic turbulent-like interferometric patterns and had an order of magnitude longer life time before bursting compared to their “cousin” non-superspreaders. The stabilization of the superspreader films and the nonlinear decrease of the film thickness with time is attributed to significant disjoining pressure measured using gravitational drainage of vertical films and is associated with the van der Waals repulsion of the fluffy surfaces of the film formed by long superspreader bilayers hanging from the free surfaces. Two surfactant mixture solutions at different mixing ratios were used to find a relation between the lifetime of planar vertical film and foamability of surfactant solution. The results show that solutions with longer lifetimes in planar film drainage reveal a higher foamability. Also, the foamability of the mixed surfactant systems was found to be greater than the foamability of each of the individual components. The higher foamable surfactant solution was added to gypsum slurry during the manufacturing of dry wallboard leading to increased fluidity of the gypsum-foam slurry, thereby reducing the total water required for the process. Gravitational drainage from thin planar surfactant films in the presence of inorganic salts was experimentally studied. Strong ion-specific effects of the counter ions were found to affect the stability and the rate of drainage of the planar foam films as a function of concentration of the inorganic salts. The counter-ions can either stabilize (below the critical concentration) or destabilize the foam films. Ionic surfactant solutions were also used to study gravitational drainage from thin vertical planar films supported on a frame with the upper and lower parts being electrodes. The imposed electric field resulted in various additional physical phenomena. The interplay of these phenomena stabilized the film drainage irrespectively of polarity. Similar effects were observed with foams

Cloaking Dynamics on Lubricant-Infused Surfaces

Lubricant-infused surfaces (SLIPSs/LISs) enable omniphobicity by reducing droplet pinning through creation of an atomically smooth liquid-liquid interface. Although SLIPSs/LISs provide efficient omniphobicity, the need for lubricant adds additional barriers to heat and mass transport and affects three-phase contact line dynamics. Here, evaporation dynamics of microscale water droplets on SLIPSs/LISs are investigated using steady and transient methods. Although steady results demonstrate that evaporation on SLIPSs/LISs is identical to solid functional surfaces having equivalent apparent contact angle, transient measurements show significant increases in evaporation timescale. To understand the inconsistency, high-speed optical imaging is used to study the evaporating droplet free interface. Focal plane shift imaging enables the study of cloaking dynamics by tracking satellite microdroplet motion on the cloaked oil layer to characterize critical timescales. By decoupling the effect of substrate material and working fluid via experiments on both microstructured copper oxide and nanostructured boehmite with water and ethanol, it is demonstrated that lubricant cloaking cannot be predicted purely by thermodynamic considerations. Rather, coalescence dynamics, droplet formation, and surface interactions play important roles on establishing cloaking. The outcomes of this work shed light onto the physics of lubricant cloaking, and provide a powerful experimental platform to characterize droplet interfacial phenomena

Droplet evaporation on functional surfaces

© 2018 International Heat Transfer Conference. All rights reserved.Droplet evaporation is an important phenomenon governing many man-made and natural processes. Characterizing the rate of evaporation with high accuracy has attracted the attention of numerous scientists over the past century. Traditionally, researchers have studied evaporation by observing the change in the droplet size in a fixed time interval. However, the transient nature coupled with the significant mass-transfer governed gas-dynamics occurring at the droplet three-phase contact line make the classical method crude. Furthermore, the intricate balance played by the internal and external flows, evaporation kinetics, thermocapillarity, binary-mixture dynamics, curvature, and moving contact lines make the decoupling of these processes impossible with classical transient methods. Here, we use our recently developed spatially-steady method to characterize the rate of evaporation of sessile droplets on functional surfaces. By utilizing a piezoelectric dispenser to feed microscale droplets ( ≈ 9 µm) to a larger evaporating droplet at a prescribed frequency, we can both create variable-sized droplets on any surface, and study their evaporation rate by modulating the piezoelectric droplet addition frequency. Using the spatially-steady technique, we studied water evaporation of droplets having base radii ranging from 30 µm to 270 µm on surfaces of different functionalities (45 ≤ a,app ≤ 162, where a,app is the apparent advancing contact angle) under different substrate temperature conditions (30℃ ≤ s ≤ 60℃, where s is the functional surface temperature). Our work shows that the rate of evaporation increases linearly for increasing droplet size, and the surface functionality halts its important role at elevated temperatures