Droplet Impact on a Micro-structured Hydrophilic Surface: Maximum Spreading, Jetting, and Partial Rebound

Abstract

• Droplet impact on a hydrophilic microstructured substrate was studied. • Various impact outcomes were observed in our experiments. • The effects of impact velocity and fluid viscosity on impact phenomena were studied. • The scaling law for the maximum spreading diameter was reported. • The satellite droplets produced in jetting and partial rebound were analyzed. In this paper, we perform an experimental study of droplet impact on a partially wetting hydrophilic substrate composed of cylindrical micro-pillars. Water and glycerol are mixed at different ratios to primarily change liquid viscosity and keep surface tension approximately constant. We show that our microstructured hydrophilic surface can exhibit many of the same impact outcomes as hydrophobic surfaces including spreading, recoiling, jetting, and partial rebound. A regime map is constructed to convey the overall effects of the impact velocity and viscosity on the impact outcomes. Our data indicate that the maximum spreading factor β max generally follows the power law with the Weber number We as β max ∼ We 0.25 . However, the scaling relation of β max ∼ We 0.2 Re 0.04 provides a better correlation for β max because the viscous dissipative effect due to flow through the micro-pillars on the substrate becomes increasingly important for more viscous fluids. The rapid jet caused by the collapse of the air cavity in the recoil phase grows in a self-similar pattern. The relationship between the size of the top jet droplet and jet velocity is found to obey the same scaling law originally proposed for the bubble bursting jet. The partial rebound occurs only for low viscosity fluids with relatively high impact velocity. The size of the rebounding droplet emitted by the breakup of inertially stretched thick liquid thread in the partial rebound is found to be nearly independent of the impact velocity. The elapsed time between droplet impingement and partial rebound event scales with the capillary time.