Interfacial transport processes involved in the surfactant facilitated wetting of liquids on solid surfaces and non-wetting on superhydrophobic surfaces

The control of the wetting properties of aqueous solutions on surfaces is critical to the implementation of many industrial technologies. Aqueous solutions are often required to rapidly wet hydrophobic solid and liquid (oil) surfaces. Surfactants, dissolved above the critical micelle concentration, are useful in quickly reducing aqueous/solid and aqueous/oil tensions to facilitate spreading. In other applications, aqueous droplets are required to roll over surfaces, and surfaces engineered with textures which trap air between grooves as the drop moves over the surface retain large droplet contact angles and reduced friction, which causes rolling.;The first part of this dissertation studies the transport of surfactant from an aqueous micellar solution to an oil phase, initially without surfactant, which is placed in contact with the water. Surfactant monomer diffuses and adsorbs from the aqueous phase onto the interface, and subsequently desorbs into the oil. The decrease in the surfactant monomer concentration in the vicinity of the surface disturbs the monomer-micelle equilibrium causing the micelles to break down to replenish the sublayer with monomer. The increase results in a more rapid reduction in interfacial tension. However, when the micellar concentration is too low, the micelle diffusion flux required to replenish the monomer underneath the surface cannot be achieved, and a zone is formed (just underneath the oil-water interface) from which micelles completely disappear. This micelle-free zone, which retreats from the surface, represents a barrier to the enhanced surfactant flux to the surface. A fluorescing dye is trapped in the micelles to provide a fluorescence contrast so that the micelle-free zone can be located. A Confocal Laser Scanning Microscope is used to spatially resolve the micelle-free zone, and measure the movement of the zone from the interface. A diffusion limited transport model is also developed which predicts the location of the micelle-free zone as a function of time, and compares well with the experiments.;Part two of the dissertation focuses on studying the (superhydrophobic) non-wetting behavior demonstrated by aqueous droplets on surfaces consisting of a periodic array of micron-sized posts. Boundary integral hydrodynamic solutions for the two dimensional, inertialess, gravity-driven movement of a droplet over this microtexture are obtained to understand the flow on the length scale of the topography. Two regimes are identified: In one, the advancing line spreads relatively easily over the top of a post, sticks to the back of the post, develops increasing curvature and finally jumps to the next post. This cycle repeats until the drop becomes tethered to the back of a post and achieves equilibrium. In the second, the advancing line again cycles between wetting, sticking and jumping, but penetrates the grove between the posts before jumping. This behavior, which precedes the full wetting regime, occurs when the contact angle of the post material is reduced or, the pitch between posts becomes large.