Non-isothermal droplet spreading/dewetting and its reversal

Abstract : Axisymmetric non-isothermal spreading/dewetting of droplets on a substrate is studied , wherein the surface tension is a function of temperature, resulting in Marangoni stresses. A lubrication theory is first extended to determine the drop shape for spread-ing/dewetting limited by slip. It is demonstrated that an apparent angle inferred from a fitted spherical cap shape does not relate to the contact-line speed as it would under isothermal conditions. Also, a power law for the thermocapillary spreading rate versus time is derived. Results obtained with direct numerical simulations (DNSs), using a slip length down to O(10 −4) times the drop diameter, confirm predictions from lubrication theory. The DNS results further show that the behaviour predicted by the lubrication theory that a cold wall promotes spreading, and a hot wall promotes dewetting, is reversed at sufficiently large contact angles and/or viscosity of the surrounding fluid. This behaviour is summarized in a phase diagram, and a simple model that supports this finding, is presented. Although the key results are found to be robust when accounting for heat conduction in the substrate, a critical thickness of the substrate is identified above which wall conduction significantly modifies wetting behaviour.
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Yi Sui, Peter D. M. Spelt. Non-isothermal droplet spreading/dewetting and its reversal. Journal of Fluid Mechanics, Cambridge University Press (CUP), 2015, in press. ⟨hal-01163707⟩

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