The effect of the spatial variation of the evaporative flux on the deposition from a sessile droplet #

Stephen K Wilson, Hannah-May D’Ambrosio, Alexander W Wray, Brian R Duffy

10:50 Wednesday in 3Q16.

Part of the Evaporation session.

Abstract #

The evaporation of sessile droplets occurs in numerous physical contexts, with applications in nature, industry, and biology, including coating, agricultural spraying, and chemical decontamination. As a consequence of the wide variety of everyday applications, the deposition from an evaporating sessile droplet containing dispersed solid particles has been the subject of extensive investigation in recent years, particularly regarding the ring deposit that often forms near the contact line (the “coffee-ring”) [1]. Of key interest in many industrial applications, such as in inkjet printing and nano-fabrication, is the ability to control the shape of the final deposit, often with a desire for uniform deposits. In this talk we investigate the effect of spatial variation of the local evaporative flux on the deposition from a pinned evaporating droplet. In particular, we consider a one-parameter family of evaporative fluxes which includes diffusion-limited and spatially-uniform fluxes, as well as fluxes which are largest at the centre of the droplet, as special cases. For a thin axisymmetric droplet we determine the resulting flow due to the evaporation, the evolution of the concentration of dispersed solid particles within the droplet, and the evolution of the mass of deposit on the substrate. Three qualitatively different deposit types are observed depending upon the spatial variation of the local evaporative flux, namely, ring deposits, paraboloidal deposits, and deposits at the centre of the droplet. In addition, we numerically investigate the particle paths within the droplet for purely advective transport to examine the role of the free surface on the deposition of particles from an evaporating droplet.

[1] Wilson, S.K., D’Ambrosio, H.-M. “Evaporation of sessile droplets” Ann. Rev. Fluid Mech. 55, 481-509 (2023)