On The Measurement And Dynamic Behavior Of Micro-jetting Induced From Bubble Bursting At A Superhydrophobic Surface

A thin layer of air attached to the underwater superhydrophobic surface separates the superhydrophobic surface substrate from the liquid phase,offering a nearly free-shear gas-liquid interface.Owing to this,the superhydrophobic surface has extensive and important applications in drag reduction of marine vehicles and anti-biofouling.How to enhance the stability of the air layer remains the bottleneck and challenge of the superhydrophobic surface engineering application,and unraveling the failure mechanism of the air layer is therefore of urgent importance.In this paper,a single-camera-binocular high-speed shadowgraph system was constructed and applied to obtain the dynamic evolution of the gas-liquid interface,and then the development and impingement of micro-jetting induced from bubble bursting at a superhydrophobic surface were studied.The flow mechanisms of the two sub-processes before and after the micro-jetting impinging on the wall are very dynamically similar to those of the Worthington jet and the stable impinging jet,respectively.Before the micro-jetting contacting the superhydrophobic surface,the average velocity U_jet of the micro-jetting increases with the increase in Oh,and decreases with the increasing bubble diameter d_eq.The curve（r-t）of the bubble spreading radius（r）with time（t）can be divided into four stages according to the different dominant forces.After the micro-jetting impinging the superhydrophobic surface,the air layer in the peripheral area of the stagnation point and even the superhydrophobic coating would be destroyed,and thus the surface super-wettability would be lost.This is a highly-likely undiscovered new mechanism of the instability of the air layer on the underwater superhydrophobic surface.During the micro-jetting impacting on the superhydrophobic surface,impacting liquid film always radially expanded to some degree at different bubble sizes（2mm～4.7mm）,which causes the fluctuation of the spreading radius in transition region（the third stage）.With the expansion of the outer interface of the bubble and the weakening of the impingement of the micro-jetting,the impacting liquid film will expand rapidly and then disappear gradually.Numerical results based on the VOF coupled with Level-set method show that the micro-jetting profile impinging on the superhydrophobic surface is different from the stable impinging jet,confirming the existence of the micro-jetting impinging liquid film.This paper contributes to deepening the understanding of the micro-jetting dynamics process induced from bubble bursting at superhydrophobic surfaces,expectedly promoting the progress of the underwater superhydrophobic technology.