Interfacial phenomena in immiscible liquids subjected to vibrations in microgravity

Abstract

We consider the response to periodic forcing between 5 Hz and 50 Hz of an interface separating immiscible fluids under the microgravity conditions of a parabolic flight. Two pairs of liquids with viscosity ratios differing by one order of magnitude are investigated. By combining experimental data with numerical simulations, we describe a variety of dynamics including harmonic and subharmonic (Faraday) waves, frozen waves and drop ejection, determining their thresholds and scaling properties when possible. Interaction between these various modes is facilitated in microgravity by the relative ease with which the interface can move, altering its orientation with respect to the forcing axis. The effects of key factors controlling pattern selection are analysed, including vibrational forcing, viscosity ratio, finite-size effects and residual gravity. Complex behaviour often arises with features on several spatial scales, such as Faraday waves excited on the interface of a larger columnar structure that develops due to the frozen wave instability – this type of state was previously seen in miscible fluid experiments but is described for the first time here in the immiscible case.

Faraday waves on band pattern under zero gravity conditions

“…Under zero g, when the stabilizing effect of gravity is absent, the amplitude of the frozen waves thus diverges, forming bands oriented perpendicularly to the vibration direction [see Figs. 1(c) and 1(d)] as observed in CO 2 [5,9,10], H 2 [11,12], liquid mixtures of FC-40 and silicone oil [13], and miscible mixtures of water-isopropanol with water [14]. This orienting effect of vibrations was predicted theoretically in [15].…”

“…In the experimental studies performed with near critical fluid under zero gravity or liquid mixtures, it was shown that under vibrations a periodical pattern (bands) can develop in the system [5,13,14]. Depending on the parameters of the vibrations, an instability can be observed in the bands, which leads to the development of Faraday waves.…”

“…Thus, according to (1) the critical value of the vibration velocity amplitude tends to zero when gravity vanishes; i.e., in the case of zero gravity, a KH instability could take place at any values of aω. However, recent calculations carried out for containers of finite length [13] have demonstrated the existence of a nonzero-critical vibration intensity. These results are not in contradiction with the zero-critical vibration as reported in [5], where, for infinite layers, the long wave perturbations are the most dangerous and the critical value of aω for these perturbations is equal to zero.…”

Effect of initial interface orientation on patterns produced by vibrational forcing in microgravity

“…Such columnar structures have the effect of rotating large portions of the interface into a nearly perpendicular orientation with respect to the forcing, which, in turn, allows Faraday waves to form when their threshold is surpassed. The interfacial structures resulting from this type of interaction are complex, with features on multiple length and time scales (Shevtsova et al 2016;Salgado Sánchez et al 2019b). These two-scale interfacial patterns were first observed in parabolic flight experiments (Shevtsova et al 2016) and, although the wavenumber is dependent on the vibrational velocity (Gaponenko et al 2015a) in accordance with predictions of linear theory, there are a number of additional factors that play an important role in selecting the experimental patterns.…”

A review of fluid instabilities and control strategies with applications in microgravity

“…The curvature of the underlying surface, amplified by the vibroequilibria effect, means that the frozen waves are generally not of uniform amplitude across the interface. Furthermore, they grow rapidly, quickly transforming into nonlinear columnar structures in many cases [51,100,101,134,136,149]. Because such columnar structures effectively rotate large portions of the interface into a nearly perpendicular orientation with respect to the forcing, they can support Faraday waves.…”