Confined cavitation: an experimental study

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.