Flow and Film Thickness of Bingham Plastic Liquids on a Rotating Disk

The free surface film flow of Bingham plastic liquids on a rotating disk has been analyzed theoretically. It is shown that the Bingham plastic material in the inner core of radius rr remains.stressed below the yield value and behaves as an elastic solid. The fluid in the region r > rc is further separated by a yield surface into a region of shear flow adjacent lo the surface of the disk and a region of plug flow near the free surface. The film thickness profile and yield surface were evaluated numerically. The results show that it is impossible to produce radially uniform thin films from liquids which have a yield stress using the spin coating deposition (SCD) technique. The results further provide a rheological explanation of the 1922 observations of Walker and Thompson on the uniformity of films produced on substrates by SCD. An experimental film thickness profile of a liquid with a yield stress shows good qualitative comparison with predicted profiles