1. Bubble Bouncing on Solid/Free Surfaces M.R. Brady, D.P. Telionis – Engineering Science and Mechanics P.P. Vlachos – Mechanical Engineering R.-H. Yoon, S.B. Yazgan – Mining Engineering Virginia Tech The rise of a buoyant bubble and its interaction with solid and free surfaces was experimentally investigated using a combination of shadowgraphs and Laser Induced Fluorescence (LIF). These optical techniques were used to track the bubble position, and measure the velocity field around the bubble in a time resolved manner. The results are quantified as a function of bubble size and surfactant concentration of the fluid medium. Forces on a particle or bubble/added mass system 1.Lubrication Forces Squeeze film lubrication (Panton) (viscous adhesion) Pad finds very large repulsive pressures if pushed against plate; but very large attractive forces if pulled up and away 2.Stokes drag 3.Added mass (ideal flow) 4.Bubble Elasticity (Barnocky & Davis) 3  R 2 U/2h = C 3 U/h L h Introduction Equation of Motion General case of a particle or bubble 0 in a flow (without lubrication or elasticity) To include lubrication, integrate pressure relation note: With lubrication and for the case of a bubble (M 0 = 0): Stokes and lubrication forces are always opposing motion. They are dissipative effects. Experimental Technique: Laser Induced Fluorescence Laser Induced Fluorescence is the effect of seeding a flow with fluorescent particles, and tracking their movement with high speed video. The incident laser light is subtracted out the image with a filter, leaving only the fluorescent particles. The resulting velocity field around the bubble is then calculated through a cross correlation based software. Images and corresponding velocity fields (zoomed in) for 1.5mm bubble in pure water hitting a free surface Experimental Technique: Shadowgraph High-speed images (1000fps) were recorded for the buoyancy-driven bouncing bubble. The independent quantities were bubble size, ionic concentration and hydrophobicity of the contact surface. A sample time series of shadowgraphs is shown below for a 1.5mm bubble. The substrate had a 75 degree contact angle with a water droplet in a fluid medium of pure water, Bubble Trajectories The vertical component of the center of mass of the bubbles were tracked and their distance from the origin is shown below. The first case shows the behavior for a 5e-4 M Sodium Silicate solution and the second case for pure water. Results and Conclusions The three figures below show how the terminal velocity and the Coefficient of Restitution (ratio of post-bounce momentum to pre-bounce momentum) scale with relevant nondimensional quantities. The significant increase in velocity in pure water compared to a surfactant concentration can be attributed to surfactant molecules attaching to the bubble, as shown in the drawing. The surfactant molecules create a no-slip condition in the region around the wake of the bubble and increase the dissipative viscous drag on the bubble.