Zoom link is: https://kaust.zoom.us/j/3709860895

Abstract : 

This dissertation talks about the dynamics of the drop impact in two parts, the impact of the drop on the deep liquid pool with singular jet and sound emission, and the bouncing drop with filaments on the superhydrophoic solid surface.

  First, we use experiments and simulations to study drop impacts on a deep liquid pool, with a focus on fine vertical jetting and underwater sound emission from entrapped bubbles, during the rebounding of the hemispherical crater.  Both phenomena arise from the dimple formed at the bottom of the crater which can pinch off to entrap a small bubble. The much larger parametric complexity introduced by the use of two immiscible liquids, compared to that for the same liquid, leads to an extended variety of compound-dimple shapes. The fastest jet occurs from the rebounding of a telescope dimple shape without bubble pinch-off, at around 45 m/s, which leaves a toroidal micro-bubbles from the air-cusp at the base of the dimple. The finest jets have diameter of only 12 µm, and their normalized speeds are up to one order of magnitude greater than jets from bursting bubbles.  A new focusing mechanism for singular jetting from collapsing drop-impact craters is then proposed based on high-resolution numerical simulations. The fastest jet is confined in a converging conical channel with the entrained air sheet providing a free-slip outer boundary condition. Sound can be emitted from the oscillation of the entrapped dimple-bubble, while the tiny bubble from the initial impact is induced to oscillate with the entrapped bubble, triggering the double crest of the acoustic signal.  We track the compression of the bubble volume from the high-speed imaging and relate it to the hydrophone signal.

  In the second part, we investigate the impact of a polymeric drop on a superhydrophobic solid substrate with micropillar structure. The drop spreads on the substrate, wets the tops of the pillars, and rebounds out of the superhydrophobic soild surface.  Numerous liquid filaments are stretched from the liquid drop to the attached adjacent pillars, and minuscule threads would be left on the top of the pillars using the inclined superhydrophobic solid surface. The well-organized exposed polymer threads are left on the top of the pillars after solvent evaporation. The thickness of the deposition of filament bundles using the bouncing method are thinner than those formed by drop evaporation or drop rolling from SEM (scanning electron microscope) observation. 

Bio:  

Ziqiang Yang earned his Bachelor degree in Process Equipment & Control Engineering from China University of Petroleum (Beijing)in 2014. Then he obtained his Master's degree in Mechanical Engineering (Thermofluids direction) in Khalifa University of Science and Technology, Sas Al Nakhl Campus in Abu Dhabi, UAE in 2016. Now Ziqiang Yang is a PhD student at KAUST in Mechanical Engineering Program working under the supervision of Professor Sigurdur Thoroddsen. His research interests focus on using ultra-high-speed video imaging to study the dynamics of free-surface flows. This includes the breakup of drops and bubbles, singularities in hydrodynamics, viscoelastic fluids, fluid acoustics and tomo-PIV, coupled with an interest in computational fluid dynamics. And he is also interested in applying fluid mechanics in biological systems, like DNA separation & biomedical applications.