Jet ejection following drop impact on micropillared hydrophilic substrates

Droplet-wall interactions are well known to generate a wide variety of outcomes such as spreading, splashing, receding, jetting, and rebounding. In this paper, we focus on the evolution of jets that form during the recoil of impinging drops on partially wetting hydrophilic substrates composed of cylindrical micropillars. The impact of the millimeter-sized drops of water-glycerol mixtures on the microstructured hydrophilic substrates is investigated by high-speed video photography. Impact velocity and fluid viscosity are varied to characterize the jets. Wetting angles are maintained in the range of 43.6 degrees <= theta <= 51.4 degrees. A regime map is constructed to convey the jet behavior at a glance. We find that jet speed, height, and diameter scale linearly with the Weber number. We also find that the jet originates from the inertial collapse of an air cavity formed during the recoil phase of the drop following impact on the microstructured substrate. The relationship between the size of the top jet drop and jet velocity obeys the scaling law of [Ganan-Calvo, Phys. Rev. Lett. 119, 204502 (2017)] for jets induced by capillary surface singularities. No jet is observed for sufficiently high drop viscosity.