Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water-air interface

TL;DR

This study explores how increasing laser power on a microbead at a water-air interface causes flow pattern transitions, revealing a hydrodynamic instability that could inform the design of self-propelling colloids.

Contribution

It demonstrates the azimuthal instability in thermocapillary flows around a heated microbead and links flow pattern changes to surface impurity effects and hydrodynamic instabilities.

Findings

Flow transitions from axisymmetric to vortex pairs with increased power

Surface boundary conditions shift from no-slip to stress-free at high power

Vortex pair formation indicates a hydrodynamic instability

Abstract

We investigate the radial thermocapillary flow driven by a laser-heated microbead in partial wetting at the water-air interface. Particular attention is paid to the evolution of the convective flow patterns surrounding the hot sphere as the latter is increasingly heated. The flow morphology is nearly axisymmetric at low laser power P. Increasing P leads to symmetry breaking with the onset of counter-rotating vortex pairs. The boundary condition at the interface, close to no-slip in the low-P regime, turns about stress-free between the vortex pairs in the high-P regime. These observations strongly support the view that surface-active impurities are inevitably adsorbed on the water surface where they form an elastic layer. The onset of vortex pairs is the signature of a hydrodynamic instability in the layer response to the centrifugal forced flow. Interestingly, our study paves the way for the design of active colloids able to achieve high-speed self-propulsion via vortex pair generation at a liquid interface.