• Tan, Hua
• Shondhi, Sakib Sadat
• Alam, Md Nur E.

Abstract

<jats:title>Abstract</jats:title><jats:p>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 microhole. 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 microhole during the impact varies from 0 to 0.60 mm. The droplet release height is adjusted to vary the impact velocity from 0.49 to 1.40 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 microhole 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 microhole due to the off-center distance. A regime map has been produced to introduce the relation between Weber numbers and jet breakup conditions. We found the jetting mechanism and liquid volume penetrating through the microhole are directly influenced by the impact velocity of the droplet and the off-center distance. The jet ejection angle is dependent on Weber number and off-center distance. The jet pinch-off time was closely related to the capillary-inertial time scale. 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.</jats:p>

Topics

• impedance spectroscopy
• surface
• polymer
• additive manufacturing
• spray coating