EXPERIMENTAL METHODS FOR IN-SITU STUDIES OF MORPHOLOGY DEVELOPMENT DURING FLOW - THE CASE OF INSTABILITY OF THIN-FILMS STUDIED BY LIGHT-SCATTERING

Rheo-optical methods are briefly reviewed. Light-scattering techniques have been chosen as a case. A hypothesis concerning the breakup of la yer samples was tested by comparing with data obtained by rheo-optical measurements and electron microscopy. The samples were made by sandwi ching a thin PS film between two plaques of PMMA sheets making up a di sk. The sandwich sample was run at various shear rates in a parallel p late glass geometry in a rheo-optical setup built into a Rheometrics m echanical spectrometer. The time dependence of the breakup process was evaluated from the light-scattering measurements and sustained by mic rographs recorded at selected points. The stability time was determine d as the time between inception of shear and the point at which the sa mple scattered light. At low shear rates a high scattering of stabilit y time was observed which was believed to be dominated by imperfection s in the interfacial region. A film breakup mechanism involving creati on of parallel layers of thin, filmlike formations parallel with the o riginal film is observed. These ''layers'' then break up to a final st ate encompassing nearly spherical drops spread over a thickness much l arger (by orders of magnitude) than the original film thickness.