Dynamics of Low-Viscosity Liquids Interface in an Unevenly Rotating Vertical Layer

The behavior of two immiscible low-viscosity liquids differing in density and viscosity in a vertical flat layerundergoing modulated rotation is experimentally studied. The layer has a circular axisymmetric boundary. Inthe absence of modulation of the rotation speed, the interphase boundary has the shape of a short axisymmetriccylinder. A new effect has been discovered, under the influence of rotation speed modulation, the interface takeson a new dynamic equilibrium state. A more viscous liquid covers the end boundaries of the layer in the form ofthin films, which have the shape of round spots of almost constant radius; with increasing amplitude of thevelocity modulation, the wetting boundary expands. It is found that upon reaching the critical amplitude ofoscillations, the film of a viscous liquid loses stability, and the outer edge of the wetting spot collapses and takeson a feathery structure. It is shown that this threshold is caused by the development of the Kelvin–Helmholtzoscillatory instability of the film. The spreading radius of a spot of light viscous liquid and its stability are studieddepending on the rotation rate, amplitude, and frequency of rotation speed modulation. The discovered averagedeffects are determined by different oscillatory interaction of fluids with the end-walls of the cell, due to differentviscosities. The effect of films forming can find application in technological processes to intensify mass transfer atinterphase boundaries.