Cavitation and acoustic emission around laser-heated microparticles

Abstract

We studied transient cavitation bubble formation and acoustic emission around individual laser-heated microparticles using subnanosecond time-resolved microscopy. Microcavitation bubbles were observed as early as 0.5 ns after the particles were heated by a 30 ps laser pulse. The bubbles expanded to a few micrometers in size and collapsed on the time scale of 0.1–1 μsec. We discuss microcavitation as the origin of anomalously large photoacoustic effects and nonlinear optical responses observed in laser-heated colloidal suspensions, as well as a mechanism for cellular damage in biologic tissue containing pigment particles.

Explosive Boiling of Water Films Adjacent to Heated Surfaces: A Microscopic Description

“…For the 12-layer system ( Figure 2) and the 24-layer system (Figure 3), the H 2 O molecules near the surface vaporize and push large aggregates of H 2 O molecules away from the surface, a scenario that could be relevant to the bubble formation around laser irradiated melanosomes. 10 The physics of the process observed in Figures 1-3 can be clarified by examining the temperature evolution within the H 2 O film during the course of the simulation. Contour plots showing the temperature for the three systems as a function of time and distance from the top Au layer are shown in Figure 4.…”

“…9 In the limit of an infinitely thick overlayer, the outward force from the phase explosion would result in a compressive pressure wave propagating through the medium as observed in the experiments of the melanosomes in the transparent medium. 10 Complementary information on the film dynamics can be obtained from density contour plots shown in Figure 5. In the initial structure of the water film, there are pairs of layers in which the O atoms have approximately the same height above the metal surface.…”

“…10 The full implications of the heat transfer along with the outgoing pressure wave will be investigated in simulations that maintain the external pressure and allow for the outward flow of energy.…”

“…As a first step, we have chosen to use a nearly instantaneous temperature increase in the metal substrate. A value of 1000 K is chosen for the temperature because it ensures we are above the fluence threshold as observed in bubble formation, 10 it is well above the critical temperature of H 2 O and it is below the melting temperature of Au. Because the rate of heating and the final temperature can be important, we are investigating the influence of these parameters on the film separation process.…”

“…10 In these experiments, the melanosomes that consist of absorbing pigment particles are embedded in a transparent liquid medium. Within 0.5 ns after the laser pulse, bubbles are visible only around the melanosomes and not within the liquid medium, indicating that the gas formation was localized adjacent to the surface of the absorbing particles.…”