Impact of a compound droplet on a solid surface: The effect of the shell on the core

Abstract

The dynamic behavior of compound droplets impacting a solid surface was studied via experiments over $\kappa$ (defined as the ratio of the compound droplet shell thickness h to the diameter D₀ of compound droplet) ranging from 0 to 0.34, We ranging from 25 to 325 and Re ranging from 165.3 to 3405.2. The spreading diameter ratio, the maximum spreading dynamic contact angle and spreading speed of the core were investigated. Four modalities of the core of compound droplets were observed on the solid surface, including a) core rebound, b) no rebound, c) core splitting rebound, d) core splitting. The results revealed that the thickness of the shell, We, and the viscosity of the shell have a significant effect on the rebound and spreading processes of the core of the compound droplet. The high viscosity oil shell is conducive to its spreading. As the thickness of the oil shell increases, its cushioning effect on the water core also increases. In addition,$\kappa =0.02254{\mathrm{We}}^{0.503},\kappa =-0.336e^{\left(-\frac{\mathrm{We}}{121.056}+0.319\right)}$ and $\kappa =0.06086e^{-\frac{\mathrm{We}}{121.056}}+0.07716e^{-\frac{\mathrm{We}}{121.70598}}+0.01595$ were used to divide the modal boundary of the compound droplet core. Further analysis reveals the correlation between We, Re, ${\beta }_m$ and.$\kappa ,\left(12+We\right){\beta }_m=8+3\left(1-cos\left(22.78+84.57\kappa \right)\right){\beta }_m^3+0.955\frac{{\mathrm{We}}^{1.05}}{\mathrm{Re}}{\beta }_m^{6.5}.$