THE MAGNIFICATION FACTOR IN SPINNING DROP INTERFACIAL TENSIOMETRY

Abstract

The accuracy with which the interfacial tension between two immiscible fluid phases can be determined by the spinning drop method is limited by the accuracy with which the spinning drop diameter can be found. Thus, it is important to know the factor by which the cylindrical sample tube and its content magnifies the on-axis drop or bubble. Here, we analyze the image formation and magnification factor of the spinning drop by a non-paraxial ray tracing method of geometric optics. Our analysis shows that when the drop and the walls of the lube are concentric, the magnification factor is exactly the refractive index of the denser fluid surrounding the drop and is independent of the refractive index and wall thickness of the sample tube. Our analysis also reveals that the image, a virtual one, is formed concentric with the drop.