Bubble nucleation and pressure generation during laser cleaning of surfaces

Bubble nucleation and growth dynamics, on a nanosecond time scale, induced by pulsed laser heat ing of a liquid-solid interface are studied experimentally. A surface-plasmon probe is implemented as a novel, highly sensitive method for the study of submicroscopic bubbles, providing accurate information on the nucleation thresholds, growth velocities, and transient pressure generation by rapid bubble growth. Owing to the higher sensitivity of the surface plasmon probe to small bubbles, it is demonstrated that bubble nucleation sets in at a lower liquid superheating than previously determined with the use of optical reflectance or piezoelectric transducer measurements. A comparison of experimentally determined bubble growth velocities with computational results confirms that bubble growth is governed by the heat transfer from the solid surface into the liquid. Reconstructed surface plasmon resonance-curves from transient signals are used to estimate the fractional volume and number density of bubbles in the superheated liquid layer. Further, a surface plasmon probe is utilized for the absolute measurement of bubble-growth-induced pressure amplitudes on a nanosecond time scale. The measurements yield peak pressure amplitudes in the range of similar to 1-5 MPa with a pressure pulse width of similar to 40 ns. Additionally, the phase of an acoustic pulse is observed to change upon reflection at the liquid-solid interface if bubbles are present, providing a direct proof for laser-induced bubbles.