Acoustic streaming in 2D freely suspended smectic liquid crystal film

TL;DR

This study investigates how low-frequency acoustic waves induce in-plane vortex flows in 2D smectic liquid crystal films, demonstrating control over flow parameters and confirming theoretical models.

Contribution

It provides the first experimental measurement of vortex parameters in acoustic streaming within 2D smectic films using a novel tracking method.

Findings

Vortices form only above a threshold acoustic pressure.

Flow parameters are controllable via acoustic pressure, frequency, and geometry.

Experimental results align with existing theoretical predictions.

Abstract

We study a horizontal streaming excited by means of a low frequency and intensity acoustic wave in 2D freely suspended film of thermotropic smectic liquid crystal. Acoustic pressure induces fast periodic transverse oscillations of the film, which produce in-plane stationary couples of vortices slowly rotating in opposite directions owing to hydrodynamic nonlinearity. The parameters of vortices are measured using a new method, based on tracking disk-shaped island defects. The horizontal motion occurs only when the amplitude of acoustic pressure exceeds threshold value, which can be explained by Bingham-like behavior of the smectic film. The measurements above threshold are in a good agreement with existing theoretical predictions. We have demonstrated experimentally that in-plane flow is well controlled by changing the acoustic pressure, excitation frequency and geometry of the film. The observations open a way to use the phenomenon in various applications.