FIG. 2.

Drop coalescence process leading to colloidal catapulting: (a) Coalescence of water drops (r_d = 99 μm) on a polystyrene particle (r_p = 148 μm), which is surrounded by air and supported by a silicon substrate. Since the particle-substrate contact is already slightly reduced at 0.3 ms (more apparent in the video), the particle departure from the substrate must have started at 0.25 ± 0.05 ms, at which point the merged drop starts to exert an upward force to pull the particle away from the substrate. (b) 2D simulations of symmetric drop coalescence on a stationary particle, where the unit vectors represent the capillary-inertial velocity u_ci and the time is reduced by the capillary-inertial time with $t^{*}=t/{\tau }_{\mathit{c}\mathit{i}}$. The numerical parameters are adopted to match the experiments in (a): $Oh=0.0127,\hspace{0.17em}{r}_d/r_p=0.67$, and θ = 90°. A small mobility parameter of ${\gamma }^{*}={10}^{-9}$ is chosen to effectively pin the contact line. Other properties including the water and air properties at 20 °C can be found in Liu et al.² (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4955085.1] Download video file ^{(178.2KB, mov)} DOI: 10.1063/1.4955085.1 [URL: http://dx.doi.org/10.1063/1.4955085.2] Download video file ^{(376.8KB, mov)} DOI: 10.1063/1.4955085.2