Utilizing Dynamic Tensiometry to Quantify Contact Angle Hysteresis and Wetting State Transitions on Nonwetting Surfaces

Goniometric techniques traditionally quantify two parameters, the advancing and receding contact angles, that are useful for characterizing the wetting properties of a solid surface; however, dynamic tensiometry, which measures changes in the net force on a surface during the repeated immersion and emersion of a solid into a probe liquid, can provide further insight into the wetting properties of a surface. We detail a framework for analyzing tensiometric results that allows for the determination of wetting hysteresis, wetting state transitions, and characteristic topographical length scales on textured, nonwetting surfaces, in addition to the more traditional measurement of apparent advancing and receding contact angles. Fluorodecyl POSS, a low-surface-energy material, was blended with commercially available poly(methyl methacrylate) (PMMA) and then dip- or spray-coated onto glass substrates. These surfaces were probed with a variety of liquids to illustrate the effects of probe liquid surface tension, solid surface chemistry, and surface texture on the apparent contact angles and wetting hysteresis of nonwetting surfaces. Woven meshes were then used as model structured substrates to add a second, larger length scale for the surface texture. When immersed into a probe liquid, these spray-coated mesh surfaces can form a metastable, solid–liquid–air interface on the largest length scale of surface texture. The increasing hydrostatic pressure associated with progressively greater immersion depths disrupts this metastable, composite interface and forces penetration of the probe liquid into the mesh structure. This transition is marked by a sudden change in the wetting hysteresis, which can be systematically probed using spray-coated, woven meshes of varying wire radius and spacing. We also show that dynamic tensiometry can accurately and quantitatively characterize topographical length scales that are present on microtextured surfaces.United States. Air Force Office of Scientific Research (W 911NF-07-D-0004

Exploring the kinetics of switchable polymer surfaces with dynamic tensiometry

Switchable polymer multilayer coatings consisting of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) were prepared via Layer-by-Layer (LbL) assembly and post-functionalized with poly(ethylene glycol methyl ether) (PEG). This resulted in a soft polar coating that reversibly and repeatedly rearranges from hydrophobic to hydrophilic (or vice versa) when contacted with water (or air). Goniometry is used to quantify the forward surface rearrangement in the form of transient measurements of the water contact angle. By examining the time evolution of the water contact angle at various temperatures, the apparent activation energy for the forward surface rearrangement (E[subscript a,f]) can be determined. Further insight can be gained into the kinetics of this surface reconstruction process by utilizing dynamic tensiometry to measure the evolution in the contact angle of a liquid meniscus at several rates and temperatures as it advances or recedes over the multilayer films. A simple first-order thermally-activated rate process is shown to describe the forward and reverse surface reconstruction and enables the shape of the measured tensiometric force curves during repeated immersion and emersion to be predicted quantitatively. Using this model we show that the character of this switchable surface coating can appear to be hydrophobic or hydrophilic depending on a single dimensionless parameter which incorporates the characteristic time-scale for temperature-dependent surface rearrangement, the speed of immersion and the capillary length of the liquid meniscus.Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio). Propulsion DirectorateUnited States. Air Force Office of Scientific ResearchUnited States. Army Research Office (Contract W911NF-07-D-0004)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762

Designing Robust Hierarchically Textured Oleophobic Fabrics

Commercially available woven fabrics (e.g., nylon- or PET-based fabrics) possess inherently re-entrant textures in the form of cylindrical yarns and fibers. We analyze the liquid repellency of woven and nanotextured oleophobic fabrics using a nested model with n levels of hierarchy that is constructed from modular units of cylindrical and spherical building blocks. At each level of hierarchy, the density of the topographical features is captured using a dimensionless textural parameter D[subscript n][superscript *]. For a plain-woven mesh comprised of chemically treated fiber bundles (n = 2), the tight packing of individual fibers in each bundle (D[subscript 2][superscript *] ≈ 1) imposes a geometric constraint on the maximum oleophobicity that can be achieved solely by modifying the surface energy of the coating. For liquid droplets contacting such tightly bundled fabrics with modified surface energies, we show that this model predicts a lower bound on the equilibrium contact angle of θE ≈ 57° below which the Cassie–Baxter to Wenzel wetting transition occurs spontaneously, and this is validated experimentally. We demonstrate how the introduction of an additional higher order micro-/nanotexture onto the fibers (n = 3) is necessary to overcome this limit and create more robustly nonwetting fabrics. Finally, we show a simple experimental realization of the enhanced oleophobicity of fabrics by depositing spherical microbeads of poly(methyl methacrylate)/fluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) onto the fibers of a commercial woven nylon fabric.United States. Army Research Office (W911NF-13-D-0001

Sustainable Drag Reduction in Turbulent Taylor-Couette Flows by Depositing Sprayable Superhydrophobic Surfaces

We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at Re=8.0×10[superscript 4]. We show that the mean skin friction coefficient C[subscript f] in the presence of the superhydrophobic coating can be fitted to a modified Prandtl–von Karman–type relationship of the form (C[subscript f]/2)[[superscript -1/2] = Mln (Re(C[subscript f]/2)[[superscript 1/2]) + N + (b/Δr)Re(C[subscript f]/2)[superscript 1/2] from which we extract an effective slip length of b ≈ 19  μm. The dimensionless effective slip length b[superscript +] = b/δ[subscript ν], where δ[subscript ν] is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as b[[superscript +] ~ Re[superscript 1/2] in the limit of high Re.United States. Office of Naval Research (Contract 3002453814

Dynamic wetting of soft materials and applications of dynamic tensiometry

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 171-184).Surfaces and interfaces pervade our world and understanding the phenomena that occur at them is imperative for a wide range of commercial and industrial applications. This thesis focuses on investigating the influence of physical and chemical parameters on surface wettability and characterizing interfacial phenomena in a range of solid-liquid systems. In particular, a surface characterization technique (dynamic tensiometry) has been extended to provide further insight into the wetting properties of liquid-repellent surfaces, and the efficacy of engineered surfaces for applications in drag reduction, oleophobic fabric design and fog harvesting is detailed. Goniometric techniques traditionally quantify two parameters, the advancing and receding contact angles, that are useful for characterizing the wetting properties of a solid surface; however, dynamic tensiometry can provide further insight into the wetting properties of a surface. A framework for analyzing tensiometric results will be detailed that allows for the determination of wetting hysteresis, wetting state transitions, and characteristic topographical length scales on textured, nonwetting surfaces, in addition to the more traditional measurement of apparent advancing and receding contact angles. Switchable polymer multilayer coatings were prepared that reversibly and repeatedly rearrange from hydrophobic to hydrophilic (or vice versa) when contacted with water (or air). By examining the time evolution of the water contact angle at various temperatures, the apparent activation energy for the forward surface rearrangement (Ea,f) can be determined. Further insight can be gained into the kinetics of this surface reconstruction process by utilizing dynamic tensiometry to measure the evolution in the contact angle of a liquid meniscus at several rates and temperatures as it advances or recedes over the multilayer films. Next, the efficacy of engineered surfaces for three applications is explored. First, the ability of a superhydrophobic surface to reduce skin friction in turbulent Taylor-Couette flow is investigated. A reduction in the wall shear stress measured at the rotating inner cylinder is demonstrated by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of skin friction reduction becomes progressively larger as Re increases with a decrease of 22% observed at Re = 80, 000. I next detail a framework for designing robust hierarchically textured oleophobic fabrics. The liquid repellency of woven and nano-textured oleophobic fabrics is analyzed using a nested model with n levels of hierarchy that is constructed from modular units of cylindrical and spherical building blocks. For a plain-woven mesh comprised of chemically treated fiber bundles (n = 2), the tight packing of individual fibers in each bundle imposes a geometric constraint on the maximum oleophobicity that can be achieved solely by modifying the surface energy of the coating. I demonstrate how the introduction of an additional higher order micro /nano-texture on the fibers (n = 3) is necessary to overcome this limit and create more robustly non-wetting fabrics. Finally, previous work on fog harvesting is expanded at both the lab and pilot scales. The methodology for coating lab scale meshes is scaled up, allowing standard fog collectors (SFCs) to be coated, which are currently being deployed in the field for real world testing. Furthermore, a lab scale fog harvesting apparatus is used to investigate how mesh wire geometry affects the prevalence of mesh clogging and observe that thin rectangular wires show promise in reducing the effect of clogging for a given fog mesh spacing.by Justin Alan Kleingartner.Ph. D

Fog Water Collection Effectiveness: Mesh Intercomparisons

To explore fog water harvesting potential in California, we conducted long-term measurements involving three types of mesh using standard fog collectors (SFC). Volumetric fog water measurements from SFCs and wind data were collected and recorded in 15-minute intervals over three summertime fog seasons (2014–2016) at four California sites. SFCs were deployed with: standard 1.00 m[superscript 2] double-layer 35% shade coefficient Raschel; stainless steel mesh coated with the MIT-14 hydrophobic formulation; and FogHa-Tin, a German manufactured, 3-dimensional spacer fabric deployed in two orientations. Analysis of 3419 volumetric samples from all sites showed strong relationships between mesh efficiency and wind speed. Raschel mesh collected 160% more fog water than FogHa-Tin at wind speeds less than 1 m s[superscript –1] and 45% less for wind speeds greater than 5 m s[superscript –1]. MIT-14 coated stainless-steel mesh collected more fog water than Raschel mesh at all wind speeds. At low wind speeds of < 1 m s[superscript –1] the coated stainless steel mesh collected 3% more and at wind speeds of 4– 5 m s[superscript –1], it collected 41% more. FogHa-Tin collected 5% more fog water when the warp of the weave was oriented vertically, per manufacturer specification, than when the warp of the weave was oriented horizontally. Time series measurements of three distinct mesh across similar wind regimes revealed inconsistent lags in fog water collection and inconsistent performance. Since such differences occurred under similar wind-speed regimes, we conclude that other factors play important roles in mesh performance, including in-situ fog event and aerosol dynamics that affect droplet-size spectra and droplet-to-mesh surface interactions. Keywords: Fog mesh, Fog water collection efficiency, Raschel mesh, Hydrophobic coatingNational Science Foundation (U.S.) (Grant OCE-1333976)Geological Survey (U.S.). Climate and Land Use Change Mission AreaNBD NanotechnologiesNational Science Foundation (U.S.). Small Business Innovation Research Program (Award 2015-33610-23832