Peculiar encounter between self-propelled droplet and static droplet: swallow, rerouting, and recoil

Understanding the outcome and mechanism of droplet–droplet collision is important in digital microfluidics, especially for the encounter between the self-propelled and static droplets. Observation of the collision process between a self-propelled droplet and another static liquid droplet on a poly(methyl methacrylate) substrate. The former is a mixture of dimethylformamide and 10 wt% Silwet L-77, while the latter can be either miscible or immiscible with the former. Four types of interesting outcomes after the encounter are identified: (i) swallow-and-run (liquid paraffin), (ii) swallow-run-pinning (olive oil), (iii) rerouting-and-run (water), and (iv) recoil-and-run (glycerol). The swallow behavior is observed as the two droplets are immiscible, but the rebound behavior is acquired as they are miscible. The former can be explained by the two spreading coefficients, while the latter may be attributed to the buildup of the gradient of the interfacial tension upon contact.