Unsteady convective flow of a preheated water-in-oil emulsion droplet impinging on a heated wall

Abstract

This work proposes a mechanism of deformation of an emulsion droplet upon collision with a wall, considering the vortex motion of a liquid inside the droplet. This motion leads to an increase in dissipative energy losses, affects spreading, corona splashing, and droplet relaxation at different liquid and wall temperatures, ranging from 20 °C to 80 °C, and influences the equilibrium shape of the drop during the liquid relaxation. For We = 100–900 and Re = 100–4000, a physical model is presented for the maximum spreading diameter of the emulsion droplet; it takes into account the heating of the boundary viscous layer and the development of temperature gradients along the droplet height, convective mixing of the liquid layers, and translational and vortex flow motion along the radius and height of the droplet. The process of corona splashing of the emulsion droplet has been studied, and the influence of the viscosity gradient due to the intermittent near-wall water film formation on the dynamics of the “corona” has been revealed. These differences led to the formation of an air gap, which in the case of an emulsion drop caused the development of a corona at lower We compared to homogeneous liquids. The duration of the liquid relaxation before capillary wetting was affected by the potential barrier of the contact line of the droplet, which depended on the vortex component of the velocity field as well as on the temperatures of the interacting media. Altering the initial thermal boundary conditions changed the relaxation time up to 60%.