We present experimental evidence of the existence of an interfacial instability between two miscible liquids of similar (but non-identical) viscosities and densities under horizontal vibration. A stably stratified two-layer system is composed of the same binary mixture with different concentrations placed in a confined cell (with length twice as large as the height). Unlike the case of immiscible fluids, here, the interface is a transient layer of small but non-zero thickness. In the experiments, the frequency and amplitude were varied within the ranges 2-24 Hz and 1.5-16 mm, respectively. When the value of the oscillatory forcing increases, the amplitudes of the interface perturbations grow continuously, forming a saw-tooth frozen structure. This evolution is also examined numerically. In addition to the solutions of full 3-D Navier-Stokes equations, an averaging approach with separation of time scales is used for situations in which the forcing period is very small compared to the natural time scales of the problem. The simulation of averaged equations provides the explanation of the instability development, the calculations of the full nonlinear equations shed light on the decay of a wavy pattern. The results of numerical modelling perfectly support the experimental observations.
We explore the peculiar behaviour of an interface between two miscible liquids of similar (but non-identical) viscosities and densities under horizontal vibration with a frequency less than 25 Hz. Significant differences in the structure of the formed patterns were found between microgravity and ground experiments. In a gravity field, a spatially periodic saw-tooth frozen structure is generated in the interface which dissipates at long times. By contrast, under the low gravity conditions of a parabolic flight, the long lived pattern consists of a series of vertical columns of alternating liquids.
We analyze the dynamical response of an isothermal liquid bridge to a step change in the mass force magnitude by numerically solving the three-dimensional Navier-Stokes equations. We study the free surface oscillations caused by both axial and lateral pulses of the mass force. The oscillation amplitude and the dynamical stability limit are calculated for different values of the parameters characterizing the fluid configuration. We examine the stability of one of the liquid bridges to be analyzed in the Japanese and European Research Experiment on Marangoni Instabilities experiment on board of the International Space Station (ISS). We study the response of that liquid bridge to real g-jitter on board of the ISS.
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