Investigation of Jet Formation Following Droplet Impact on Microholed Hydrophilic Substrate

Md Nur E Alam

doi:10.7273/000005014

Droplet impingement on solid hydrophobic and superhydrophobic surfaces produces diverse phenomena, including spreading, splashing, jetting, receding, and rebounding. In microholed surfaces, downward jets through the hole can be caused by high impact inertia and the cavity collapse formed during the droplet's recoiling. With enough impact inertia, the jets can pinch off the substrate and eventually form a smaller droplet or break up into several satellite droplets. In this work, the dynamics of downward jets through the micro-holes that form during the impact of water droplets impinging upon hydrophilic substrates were investigated experimentally. The small circular holes of ~600 µm diameter were created in the 200-µm thick plastic film substrates using a 0.5 mm diameter punch. A 10µL-pipette tip with a syringe pump was used to produce the millimeter-sized droplets. Great care has been taken to ensure that the microscopic droplets can directly impact over the micro-holes. The entire process of water droplets impacting micro-holed substrate was studied in detail using two high-speed video photography cameras. A MATLAB code has been developed to quickly process a large number of captured images from experiments to characterize the droplet impact and jet dynamics, such as the spreading factor, jet formation, jet length, breakup time, and satellite droplet volume. We found the jetting mechanism and volume penetrating through the microhole are directly influenced by the impact velocity of the droplet. When the impact velocity is lower than a threshold value, the jet driven by the impact inertia will not be able to pinch off from the substrate. Above the threshold velocity, the jet pinches off to form single or multiple droplets. The pinch-off time was closely related to the capillary-inertial time scale. For relatively high impact velocity, the jet stretched longer and broke up into multiple droplets due to Plateau–Rayleigh instability.We found the maximum length of the jet before its breakup is related to the impact velocity and Weber Number. A regime map has been produced to introduce the relation between Weber numbers and jet breakup conditions. The volume of the 1st satellite droplet was independent of the Weber number with an average normalized value of 0.018. We also found that the total volume of the ejected jet depends on the Weber Number differently for different ranges of We. Moreover, the maximum spreading factor (βm), maximum jet length (Lm), and necking radius were found to follow scaling laws as βm ~We0.22, Lm ~ 7.44 Ui, and rn/lc ~ τα, respectively. Also, the maximum and average speeds of the jet were found to vary linearly with the impact speed of the droplet. Significantly, this study has reported an upward jetting for a specific range of Weber Numbers. The upward jet speed, width, and time required to form the jet follow the particular trend with Weber Number. However, the top droplet speed and diameter require further investigation to determine the trend they follow.

Investigation of Jet Formation Following Droplet Impact on Microholed Hydrophilic Substrate

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