OFF-CENTERED IMPACT OF DROPLET ON A MICROHOLED HYDROPHILIC SUBSTRATE

Sakib Sadat Shondhi

doi:10.7273/000006970

Droplet impact is central to numerous technological applications including spray coating, inkjet printing, additive manufacturing, etc. Droplet impact on solid surfaces leads to diverse outcomes, such as splashing, spreading, receding, jetting, and rebounding. In recent years, there is growing research interest on the droplet impact on mesh or holed surfaces. In this work, we investigate the dynamics associated with the off-centered impingement of a water droplet on a partially wetting substrate pierced with a single micro-hole via high-speed video photography. A 0.5-mm-diameter punch is used to produce circular holes of ~630 μm diameter on a plastic film substrate of 240 μm thickness. A combination of a 10μL pipette tip and a syringe pump is used to dispense droplets of 2.8 mm in diameter. A high-speed camera with magnification lens is used to capture the entire process of droplet impact over the micro hole. An in-house MATLAB code has been developed for the processing and data analysis of the captured images. In our study, the distance (referred to as off-center distance hereafter) between centers of droplet and micro hole during the impact varies from 0 to 1.80 mm. The droplet release height is adjusted to vary the impact velocity from 0.49 to 1.49 m/s. Our results show that with sufficiently high impact velocity as the droplet spreads radially over the substrate, a downward jet driven by the impact inertia pinches off from the micro hole and then breaks up into one or multiple droplets due to Plateau–Rayleigh instability. The jet is ejected with an angle from the axis of the micro hole due to the off-center distance. A regime map has been produced to introduce the relation among the Weber numbers, off-center distances, and jet breakup conditions. We found the jetting mechanism and liquid volume penetrating through the micro hole are directly influenced by the impact velocity of the droplet and the off-center distance. The development of the ejection angle is dependent on Weber number and off-center distance, and the unique pattern of the angle development has been thoroughly discussed. In exploring the maximum jet length prior to its breakup, our study aligns the jet's pinch-off timing with the capillary-inertial timescale, revealing a nuanced dependency that underscores the delicate balance between inertia and surface tension. Our data indicate that the maximum spreading factor βmax generally follows the power law with the Weber number (We) as βmax ~ We0.22. We also established a relationship between the number of satellite droplets and impact velocity. The volume of ejected single or satellite droplets was found to be dependent on the Weber number and off-center distance. We also found that the total volume of the ejected jet depends on the Weber Number differently for different ranges of We and off-center distances. In this work, we have formulated scaling laws for jet speed, length, breakup, and the volume of liquid transmitted through the micro hole under varying off-centered impacts.

OFF-CENTERED IMPACT OF DROPLET ON A MICROHOLED HYDROPHILIC SUBSTRATE

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