Influence of thermocapillary flow induced by a heated substrate on atomization driven by surface acoustic waves

J. Muñoz, J. Arcos, O. Bautista, F. Méndez

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Abstract

Thermocapillary flow on the mechanism of interfacial destabilization prior to atomization of a sessile Newtonian droplet subjected to surface acoustic waves (SAWs) is analyzed. We assumed that an interfacial temperature distribution is induced on the free surface of the millimeter-sized water droplet since the droplet is on a heated substrate. Given the dependence of surface tension on interfacial temperature, shear stresses combined with SAWs lead to the development of thermocapillary flow. The evolution equation for a small-scale droplet under the combined influence of SAW atomization and thermocapillary flow is derived via an asymptotic approach to the hydrodynamic equations, arising the acoustic capillary and Marangoni numbers. In this limit, our simplified droplet model can predict capillary instability leading to atomization once a critical amplitude is reached for the induced capillary waves at the liquid droplet. In doing so, our model also represents the influence of the thermocapillary effect on the interfacial deformation of the droplet and shows how the Marangoni flow promoted by a heated substrate counteracts the acoustic stress, leading to a virtually uniform droplet aspect ratio and thus larger aerosol diameters compared to the isothermal case. These results are supported by the development of a novel analytical expression that has allowed us to estimate the characteristic aerosol size under thermocapillary flow and SAW excitation, and to postulate thermocapillary flow as a new valuable means of explaining the regulation of the characteristic aerosol size at SAW atomization.

Original languageEnglish
Article number012119
JournalPhysics of Fluids
Volume35
Issue number1
DOIs
StatePublished - 1 Jan 2023

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