(224b) Removal of Particles From Surfaces Using Non-Newtonian Fluids

The removal of contaminants is an important step in production process of many technologically-driven industries, such as high-precision optical lenses, magnetic storage media, solar cells, semiconductor devices, and photographic films. The present study examines a novel cleaning process effective for micron or smaller particulate contaminants, involving the use of semidilute aqueous polymer solutions rinsed off by an impinging water jet. Unlike many existing removal processes, this continuous process generates limited and nonhazardous aqueous waste.

We have investigated the particle removal mechanism with a model system of silica particles on glass substrates. First, we constructed an experimental set-up to image particle removal in situ using differential interference contrast microscopy, which enables particle imaging down to the nanometer scale. Using a high-speed camera, we determined the relevant timescales for removal is on the order of milliseconds. We also developed a quantitative technique to evaluate removal efficiency by using fluorescently dyed silica particles and a fluorescent 2D scanner.

We found that effective particle removal requires the cleaning solution to be highly elastic, including high molecular weight polymer solutions, Boger fluids, and worm-like micelle solutions. Newtonian solutions such as low molecular weight polymer solutions and glycerol-water show very low particle removal abilities. There is a direct correlation between the extensional viscosities of the cleaning solution and the particle removal efficiency. It is also necessary for the rinsing flow to contain an extensional component. A purely shear flow shows limited efficiency while a siphoning process shows very high efficiency. Thus, we propose that when the microstructures of the cleaning solutions are stretched, a dramatic increase in tensile resistance coupled with extensional flow is responsible for achieving particle removal. This is further supported by high-speed videos showing that the particles are ?plucked away? from the surface instead of ?rolling away.?