Submitted to the Department of Chemical Engineering.
Thesis (Ph.D.)--Stanford University, 2013.
The introduction and subsequent removal of highly elastic solutions from surfaces has allowed industry to effectively remove particulate contaminants from high-grade silicon. The advantage of this continuous process over conventional techniques is the noninvasive removal of the particulates, while generating limited nonhazardous aqueous waste. Our group investigated the use of polymeric liquids to effectively eliminate particles without damaging the delicate surfaces. To investigate this removal, we studied two different flow types (syphoning and rinsing) of various rheological fluids to understand the governing physics that allow for removal. A significant effort was necessary to gain an understanding of the rinsing flow type. An impinging jet of water was used to rinse coating fluids of varying rheology to investigate the flow structures of the resulting hydraulic jumps qualitatively and quantitatively. We observed the interactions of the two-fluid system during the transient growth of the flow profile, finding rheological dependencies on the magnitude, velocity, and topography. The presence of shear thinning of the coating liquid influenced the overall velocity of the radial growth, while determining the geometry of the driving front. The dependence of these results on the local rheology was supported by experiments on Newtonian fluids of various shear viscosities. The presence of elasticity was seen to dampen the disturbances of the hydraulic jump, influence the overall jump height, and vary the radial growth of the jump. These observations were supported by experiments of Boger fluids with varying elasticity. To better understand the flow conditions necessary for particle removal, we present a simple theoretical model showing that the presence of large elongational viscosities are exploited by local flow gradients. Specifically, we simplify the complicated siphoning flow field near the substrate to a radially dependent shear flow. The radial position where the local shear rate becomes leading order with the characteristic polymer relaxation time is shown to correlate well with the onset of removal of the particulates from the surface. This correlation is attributed to the substantial polymer contributions to the stress field that result. Studying the interaction of inhomogeneous droplets and jets that are miscible with their surroundings, such as glycerol drops falling into water, was a natural extension of the rinsing flow work. This basic flow problem is central to numerous industrial and natural processes, such as mixing of cleansing liquids and creating biocompatible implants for drug delivery. Although the interactions of immiscible drops and jets show similarities to miscible systems, the small, transient interfacial tension associated with miscible systems create distinct outcomes such as intricate droplet shapes, break-up resistant jets, and spreading sessile drops. Experiments were conducted to understand several basic multiphase flow problems involving miscible liquids including the free-surface pendant drops or "sagging blobs" resulting from drop impaction. Using high-speed imaging of the morphological evolution of the flows, we show that these processes are controlled by interfacial tensions. Miscible jets, which allow the creation of fibers and tubular shapes from inelastic materials that are otherwise difficult to process due to capillary breakup were also investigated. This work shows that stabilization from the diminishing interfacial tensions of the miscible jets allow various elongated morphologies to be formed. When combined with a mechanism to freeze these fibers, highly oriented materials, such as anisotropic collagen fibers used as scaffolds for regeneration of anisotropic tissues, can be created.