ID 152

Drops and jets formed during low-velocity droplet impact onto hydrophobic substrates

 

Abstract:

Disintegration of droplets during the event of drop impact on solid surfaces should be prevented when a clean drop deposition is desired, i.e., in inkjet printing applications. Common disintegration mechanisms include partial rebound, fingering breakup, prompt and corona splash, etc. All these mentioned mechanisms usually occur at medium and high We. However, jets and secondary droplets can even be formed when a low-velocity droplet impacts onto hydrophobic substrates. In this work, we report the accompanying jets and droplets related phenomena during the receding phase of water droplets impacting on polydimethylsiloxane (PDMS) substrates with two different mixing ratios (10:1 and 40:1) of the base and curing agent. Within the range of low We=3~14 in the experiment, both the high-speed, tiny jets and relatively low-speed, thick jets were observed. From the high-speed videos, it can be seen that the difference between tiny jets and thick jets is caused by two different kinds of air cavity retracting behavior, the former of which resulting in air bubble entrapment while the other not. Thick jets phenomenon dominates during this particular range of Weber numbers. Moreover, secondary droplets ejection was found for the low We conditions. This secondary droplet ejection is different from the droplet generation induced by partial rebound, which occurs at much higher We, in the aspect of the size of the secondary droplets.