Abstract
The behavior of a cavitation bubble is greatly influenced by its surroundings.
In an unbounded liquid a cavitation bubble grows and collapses
spherically but a nearby solid boundary changes everything. Now the
bubble collapses towards the wall and looses its spherical shape. During
collapse a thin jet is formed piercing through the center of the bubble
aimed towards the wall. Upon impacting on the boundary the jet spreads
out radially exerting a strong shear stress on the wall. The shear strength
drops with a -11=4 power law.
The strength of the jet depends strongly on the starting distance between
the bubble and the wall. The jet impact velocity increases the closer
the bubble gets to the wall up to a maximum for a standoff distance of
about » 0.6. For bubbles closer than that the jet velocity decreases.
This jet flow can be utilized to temporarily porate the membranes of
living cells; adherent cells are grown on the wall of a culture flask and exposed
to a single cavitation bubble. As the jet impacts on the cell monolayer
it detaches cells in a circular region around the point of impact. Surrounding
the cleared area there is a ring where the shear stress was to
weak to detach the cells but strong enough to rip small holes in the cell
membrane.
This permeabilization of the membrane can be detected by adding a
dye to the liquid such that only the porated cells will be stained. Afterwards
the cells are tracked for an entire day to make sure they survive the
treatment.
Interestingly enough when a bubble is created with a standoff distance
smaller than ° = 0.6 the amount of stained cells keeps increasing while
the circular detachment area shrinks. The cause for this is the bubble
growth on top off the surface which is also strong enough to porate cells.
This finding can be used for instance in microfluidics. If a bubble is
created in a thin liquid film between two parallel plates the bubble takes
on a flat pancake like shape. This quasi 2-dimensional bubble grows and
collapses in a circular fashion and no jets are formed. But as was shown before bubble growth across a surface can also porate cells and by growing
cells on one of the two walls in such a system this was also proven the case
in microfluidics.
Cells in suspension can also be porated but during bubble growth they
take on a ”tear” shape which is expected to be a result of entraining the
cell from the boundary layer into the main flow. A way to circumvent this
instability we created a second bubble on the other side of the cell. The
cell becomes compressed and simultaneously sheared yet it remains in
place.
Bubbles in confined geometries jet in the presence of a channel wall;
even when a small channel opening is present. By positioning the bubble
such that the jet does not impact on the wall but flows into the a channel
opening realizes a pump. This idea which was put forward for larger millimeter
sized bubbles by Khoo’s group (Khoo, et.al. 2005) has now been
for the first time realized on the microscale for lab on a chip devices.
Similar to the step from 3D to 2D the addition of second side wall
close the first side wall takes us from 2D to a quasi 1-dimensional bubble.
A bubble generated in such a long and thin channel only grows and
collapses in the lengthwise direction of the channel. A one dimensional
model does indeed describe the bubble dynamics quite accurately but
only if the temperature inside the bubble is taken into account.
This time another solid boundary will not induce jetting in the bubble.
A free interface close to the jet however does result in a jet. It is however
not a jet penetrating through the bubble but the result of the rapidly growing
bubble pushing liquid out of the open end of the channel. We demonstrate
on demand and reproducible jetting on the micrometer scale with
more than 100 m/s.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 12 Jun 2009 |
Place of Publication | Enschede |
Publisher | |
Print ISBNs | 978-90-365-2822-1 |
DOIs | |
Publication status | Published - 12 Jun 2009 |