Impact of a falling droplet on a sessile droplet on a surface with controllable wettability: Effects of surface wettability and Weber number.

HYPOTHESIS

By systematically varying the wettability of solid surface, and Weber number (We) of falling water droplet, the influence of these factors on droplet dynamics can be determined.

EXPERIMENTS

The morphological evolution and droplet dynamics of a free-falling water droplet impacting an identically sized sessile water droplet on a dielectrowetting substrate with controllable wettability were primarily studied. The effects of We of the free-falling water droplet and the wettability of dielectrowetting surface on the dynamics of the free-falling water droplet impacting the sessile water droplet are discussed using dimensionless parameters such as the neck diameter growth factor, β, and the contact diameter growth factor, β.

FINDINGS

The results indicate that the neck diameter growth factor, increases exponentially with t and β∝t, and the growth rate, k, increases as We of the falling droplet and surface wettability increase. The middle height factor decreased linearly with t during the neck diameter growth stage. The absolute value of the linear decrease rate of δ is also close to the exponential increase rate of β for different We, which satisfies β∝δ. As the surface wettability decreases and We increases, β increases according to β∝We. While τ∝We, the variation trend is opposite to that of β. These results offer valuable perspectives on the underlying mechanisms controlling the dynamics of a free droplet impacting a sessile droplet and contribute to optimization of processes involving droplet interactions, such as spray cooling.