Microdroplet generation on demand by macro droplet impact on liquid-infused surface

Generating droplets of a length scale smaller than the capillary length scale with control over the volume is a difficult task because of the dominance of surface tension. This work demonstrates a simple method through which droplets of the desired volume with a diameter as small as ten times smaller than the capillary length scale are generated by macro droplet impact on a liquid-infused surface (LIS). The uniqueness of this approach lies in its capability to generate a diverse array of droplet sizes with a single nozzle without using any external tool. Simple gravity-driven droplet impact leads to the generation of smaller droplets, with volumes ranging from ~10 nl to ~5 µl. An aqueous droplet ~10 µl impacts a slightly inclined LIS made from soot particles and 10 cst silicone oil. By varying the release height, the impact velocity and Weber number (We) are varied, keeping the capillary number low (Ca << 1). So inertia and surface tension dictate the dynamics, making We a suitable parameter to control the dynamics. LIS offers the least lateral resistance for droplet retraction and sufficient adhesion to prevent droplets from bouncing off, as LIS has contact angle(CA) hysteresis <3° and high vertical adhesion (CA ~103°). This leads to a long liquid column during retraction which breaks into daughter droplets. Here experimentally and theoretically shown that the size of the generated droplet can be varied by We. Droplet/nozzle diameter ratios of 0.4 to 3.3 are achieved with a 0.55 mm nozzle, avoiding complex actuators.