Droplet orthogonal impact on nonuniform wettability surfaces

The vast majority of prior studies on droplet impact have focused on collisions of liquid droplets with spatially homogeneous (i.e., uniform-wettability) surfaces. But in recent years, there has been growing interest on droplet impact on nonuniform wettability surfaces, which are more relevant in practice. This paper presents first an experimental study of axisymmetric droplet impact on wettability-patterned surfaces. The experiments feature millimeter-sized water droplets impacting centrally with on a flat surface that has a circular region of wettability (Area 1) surrounded by a region of wettability (Area 2), where (i.e., outer domain is less wettable than the inner one). Depending upon the droplet momentum at impact, the experiments reveal the existence of three possible regimes of axisymmetric spreading, namely (I) interior (only within Area 1) spreading, (II) contact-line entrapment at the periphery of Area 1, and (III) exterior (extending into Area 2) spreading. We present an analysis based on energetic principles for , and further extend it for cases where (i.e., the outer domain is more wettable than the inner one). The experimental observations are consistent with the scaling and predictions of the analytical model, thus outlining a strategy for predicting droplet impact behavior for more complex wettability patterns

Solid/Liquid Interactions on Wettability-Modified Surfaces

This thesis explores the intricate domain of solid-fluid interaction on wettability-modified surfaces, addressing novel and underexplored aspects in the field. It investigates the fundamental studies that contribute to a deeper understanding with a specific focus on parametric investigations that have been lacking in existing literature. The first part of the thesis unfolds with an examination of single droplet impact on non-uniform wettability surfaces. The investigation involves experimental characterizations of axisymmetric droplet impact on a circular area with a specific wettability, surrounded by a region exhibiting different wettability properties. Three distinct regimes of droplet spreading: I) Interior spreading, II) Contact-line entrapment, and III) Exterior spreading are identified depending on the droplet momentum. A theoretical analysis, based on energetic principles, is also presented and compared to experimental observations to enhance further understanding. The second part of the thesis concentrates on understanding the impact of liquid-jet impingement on superhydrophobic metal meshes, probing into the influence of flow parameters, mesh geometry, and liquid properties. The investigation delves further into the different regimes of jet impact—prebreakthrough, breakthrough, and post-breakthrough, offering comprehensive insights into the dynamics of these phases. In the final segment, the focus shifts towards the fabrication of superoleophobic surfaces capable of repelling very low surface tension liquids. The study successfully designs and fabricates surfaces that can repel very low surface tension liquids like octane (21.14 mN m−1) and arrest the spreading of even lower surface tension liquids like heptane (19.74 mN m−1) using a facile and straightforward technique. By unraveling the complexities of these solid-fluid interaction studies, this thesis provides a robust foundation for the design of engineering devices for diverse technological domains, particularly in microfluidic applications like lab-on-a-chip technology. The potential applications of this research span high-rate fluidic transport, enhanced condensation heat transfer, water capture, area-selective cooling, and hydrodynamic drag reduction

A new methodology for measuring solid/liquid interfacial energy

Hypothesis The interfacial energy between a solid and a liquid designates the affinity between these two phases, and in turn, the macroscopic wettability of the surface by the fluid. This property is needed for precise control of fluid-transport phenomena that affect the operation/quality of commercial devices/products. Although several indirect or theoretical approaches can quantify the solid/liquid interfacial energy, no direct experimental procedure exists to measure this property for realistic (i.e. rough) surfaces. Makkonen hypothesized that the frictional resistance force per unit contact-line length is equal to the interfacial energy on smooth surfaces, which, however, are rarely found in practice. Consequently, the hypothesis that Makkonen’s assumption may also hold for rough surfaces (which are far more common in practice) arises naturally. If so, a reliable and simple experimental methodology of obtaining for rough surfaces can be put forth. This is accomplished by performing dynamic contact-angle experiments on rough surfaces that quantify the relationship between the frictional resistance force per unit contact-line length acting on an advancing liquid ( ) and the surface roughness in wetting configurations. Experiment We perform static and advancing contact-line experiments with aqueous and organic liquids on different hydrophilic surfaces (Al, Cu, Si) with varying Wenzel roughnesses in the range 1-2. These parameters are combined with the liquid’s known surface tension to determine . Findings rises linearly with the surface roughness. Analysis based on existing theories of wetting and contact-angle hysteresis reveals that the slope of vs. Wenzel roughness is equal to the solid/liquid interfacial energy, which is thus determined experimentally with the present measurements. Interfacial energies obtained with this experimental approach are within 12% of theoretically predicted values for several solid/liquid pairs, thereby validating this methodology