Bubble Bouncing on Solid/Free Surfaces

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.
Introduction
Experimental Technique Laser Induced Fluorescence

• 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

• 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 (M0
  0)

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
Bubble Trajectories
Results and Conclusions
Experimental Technique Shadowgraph
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.
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.
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,