The interaction of a laser-induced cavitation bubble with an elastic boundary and
its dependence on the distance between bubble and boundary are investigated experimentally.
The elastic boundary consists of a transparent polyacrylamide (PAA)
gel with 80% water concentration with elastic modulus E = 0.25 MPa. At this E-value,
the deformation and rebound of the boundary is very pronounced providing
particularly interesting features of bubble dynamics. It is shown by means of high-speed photography with up to 5 million frames s−1 that bubble splitting, formation
of liquid jets away from and towards the boundary, and jet-like ejection of the
boundary material into the liquid are the main features of this interaction. The maximum
liquid jet velocity measured was 960 m s−1. Such high-velocity jets penetrate
the elastic boundary even through a water layer of 0.35 mm thickness. The jetting
behaviour arises from the interaction between the counteracting forces induced by
the rebound of the elastic boundary and the Bjerknes attraction force towards the
boundary. General principles of the formation of annular and axial jets are discussed
which allow the interpretation of the complex dynamics. The concept of the Kelvin
impulse is examined with regard to bubble migration and jet formation. The results
are discussed with respect to cavitation erosion, collateral damage in laser surgery,
and cavitation-mediated enhancement of pulsed laser ablation of tissue.
The interaction of a laser-induced cavitation bubble with an elastic boundary is
investigated experimentally by high-speed photography and acoustic measurements.
The elastic material consists of a polyacrylamide (PAA) gel whose elastic properties
can be controlled by modifying the water content of the sample. The elastic
modulus, E, is varied between 0.017 MPa and 2.03 MPa, and the dimensionless
bubble–boundary distance, γ, is for each value of E varied between γ = 0 and γ = 2.2.
In this parameter space, jetting behaviour, jet velocity, bubble migration and bubble
oscillation time are determined. The jetting behaviour varies between liquid jet
formation towards or away from the elastic boundary, and formation of an annular
jet which results in bubble splitting and the subsequent formation of two very fast
axial liquid jets flowing in opposite directions. The liquid jet directed away from the
boundary reaches a maximum velocity between 300 ms−1 and 600 ms−1 (depending
on the elastic modulus of the sample) while the peak velocity of the jet directed towards the boundary ranges between
400 ms−1 and 800 ms−1 (velocity values averaged over 1 μs). Penetration of the
elastic boundary by the liquid jet is observed for PAA samples with an intermediate
elastic modulus between 0.12 and 0.4 MPa.
In this same range of elastic moduli and for small γ-values, PAA material is ejected
into the surrounding liquid due to the elastic rebound of the sample surface that
was deformed during bubble expansion and forms a PAA jet upon rebound. For
stiffer boundaries, the bubble behaviour is mainly characterized by the formation
of an axial liquid jet and bubble migration directed towards the boundary, as if
the bubble were adjacent to a rigid wall. For softer samples, the bubble behaviour
becomes similar to that in a liquid with infinite extent. During bubble collapse, however,
material is torn off the PAA sample when bubbles are produced close to the
boundary. We conclude that liquid jet penetration into the boundary, jet-like ejection
of boundary material, and tensile-stress-induced deformations of the boundary
during bubble collapse are the major mechanisms responsible for cavitation erosion
and for cavitation-enhanced ablation of elastic materials as, for example, biological
tissues.
We investigate experimentally the physical processes underlying pulsed cellular microsurgery and micromanipulation using nanosecond 532- and 1064-nm laser pulses focused at high numerical aperture. We find that the laser parameters employed for many microirradiation techniques are congruent with those leading to optical breakdown in water. We determine the size and shape of the laser-induced plasma, pressure of the emitted shock wave, and size and energy of the cavitation bubble formed by the expanding plasma. We discuss implications of the results for biophysical microirradiation procedures.
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