Growth dynamics of breath figures on phase change materials: a numerical study

Abstract

We present a numerical investigation of the effect of droplet motion on the growth dynamics of breath figures during condensation on phase change material. Breath figures are a micro-scale pattern of droplets that form when droplets condense on a cold surface. The numerical model considers the growth of droplets due to condensation, droplet coalescence, random droplet movement, and surface wettability. We study the dynamics of breath figures in terms of the time of evolution of the mean radius of droplets $\langle R \rangle$, surface coverage $\varepsilon ^2$, and the droplet size distribution $n_{\mathrm{s}}$. We demonstrate that the droplets’ movement significantly changes the distribution of droplets condensing on a phase change material by increasing coalescence. We observed four growth regimes on phase change materials due to the movement of droplets. First, in the initial regime, $\langle R \rangle \sim t^{\alpha _1}$, intermediate regime $\langle R \rangle \sim t^{\alpha _2}$, coalescence-dominated regime $\langle R \rangle \sim t^{\alpha _3}$, and late regime $\langle R \rangle \sim t^{\alpha _4}$. The growth exponents are $\alpha _1 \approx 1/2$, $\alpha _2 \approx 1$, and $\alpha _4 = 1/3$. While the growth exponent $\alpha _3$ depends on the contact angle of the surface $\theta$. Furthermore, we show the scaling of the droplet size distributions at different times.