Experimental methods for in situ studies of morphology development during flow: Instability of thin films.

Rheo-optical methods are briefly reviewed. Light-scattering techniques have been chosen as a case. A hypothesis concerning the breakup of layer samples was tested by comparing with data obtained by rheo-optical measurements and electron microscopy. The samples were made by sandwiching a thin PS film between two plaques of PMMA sheets making up a disk. The sandwich sample was run at various shear rates in a parallel plate glass geometry in a rheo-optical setup built into a Rheometrics mechanical spectrometer. The time dependence of the breakup process was evaluated from the light-scattering measurements and sustained by micrographs recorded at selected points. The stability time was determined as the time between inception of shear and the point at which the sample scattered light. At low shear rates a high scattering of stability time was observed which was believed to be dominated by imperfections in the interfacial region. A film breakup mechanism involving creation of parallel layers of thin, filmlike formations parallel with the original film is observed. These "layers" then break up to a final state encompassing nearly spherical drops spread over a thickness much larger (by orders of magnitude) than the original film thickness.