Dynamic wetting of soft materials and applications of dynamic tensiometry

Abstract

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.