On interfacial tension measurement with a spinning drop in gyrostatic equilibrium

Depending on operating speed, gyrostatic equilibrium is not approached closely enough for reliable tension measurement. Flow visualization experiments show that virtually rigid rotation gives way to secondary flows around the drop as spin velocity is decreased. Flows are attributable to a gravity-induced, internal, hydrodynamic shear layer, to a misalignment-induced Taylor column, to externally excited inertial oscillations, and to mass transfer effects. Bizarre shapes of ultralow tension drops are probably caused by these effects, which should be avoided by design or operation of the instrument. Relevant criteria are given.