Predicting the dynamics of an electrically charged drop impacting a solid surface is of great importance in various industrial applications such as spray coating and painting. An impacting drop decelerates immediately before the touch-down by the lubrication pressure of the intervening air. The pressure build-up forms a dimple resulting in the entrapment of bubbles under the drop. We measured and visualized the dimple and bubble formations using high-speed interferometry and back-light imaging techniques by focusing on the effect of the electrostatic charge on the surface deformation. We clarified that the dimple size decreased, and the kink of the dimple became more flattened with increasing the charge, which resulted in forming a micro-bubble band around a central bubble. By comparing the dimple profile obtained with interferometry and bubble images, we revealed that an air layer, thinner than 266 nm, raptures into small pieces forming a microbubble band while a thicker air layer collapses into a bubble. Replacing a substrate from conductive to dielectric hinders the decrease in dimple volume, confirming Maxwell stress acting between the drop bottom surface and the substrate plays a crucial role in determining the bubble entrapment.
Venturi tube is a microbubbles generator with simple and robust design. Air was injected into the liquid flow, transported through the throat of the Venturi tube and break up as microbubbles in the diverging area. In high-speed liquid, cavitation occurred along with the air transported to the throat of the Venturi tube. One of the factors that affected cavitation existence is dissolved gas level in the liquid. Therefore, this study aims to compare the microbubble collapse phenomena under different dissolved gas level to further understand the effect of cavitation to microbubbles formation. In this experiment, two different dissolved oxygen level (DO) were used to represent the dissolved gas level in the liquid. The result showed that although breakup mechanism was generally the same for both DO level, higher DO at high liquid speed had more wrinkles on its bubbles surfaces during the bubbles breakup process. Pressure measurement results at higher liquid speed demonstrated that DO 8.5 mg/L had slightly higher inlet pressure than DO 6.5 mg/L for the same gas flow condition. Calculation on bulk cavitation number and pressure loss coefficient suggested cavitation occurrence at higher liquid speed and higher air flow ratio. Higher DO have slightly higher pressure loss coefficient under the same bulk cavitation number. On the other hand, bubbles number distribution results showed that higher DO condition produced more bubbles than lower DO specifically at higher liquid speed with low and medium air flow ratio. Calculation of SMD for different DO at high liquid speed resulted in a slightly smaller bubbles diameter at low and medium air flow ratio of higher DO while the value is almost the same for high air flow ratio. At low liquid speed, higher DO have smaller SMD value for all air flow ratio cases.