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.