Time-resolved analysis of cavitation induced by CW lasers in absorbing liquids

Abstract

We present novel results on thermocavitation using a CW medium-power near infrared laser (lambda=975 nm) focused into a saturated copper nitrate saline solution. Due to the large absorption coefficient at the laser wavelength, the solution can be heated to its superheat limit (T(sh) approximately 270-300 degrees C). Superheated water undergoes explosive phase transition around T(sh) producing approximately half-hemispheric bubbles (gamma approximately 0.5) in close contact with the substrate. We report the temporal dynamic of the cavitation bubble, which is much shorter than previously reported under similar conditions. It was found that the bubble radius and pressure wave amplitude emitted on bubble collapse decreases exponentially with the power laser. Thermocavitation can be a useful tool for the generation of ultrasonic waves and controlled ablation for use in high-resolution lithography.

Optic cavitation with CW lasers: A review

“…As power is increased, both R max and the bubble lifetime decrease almost linearly, at least in this power range. In previous works, 30 we showed that the reduction in bubble radius scales exponentially. Although the experimental conditions are different herein, there is no physical reason why this dependence should be different.…”

“…As reported before, a strong shock wave is emitted upon collapse, whose amplitude scales linearly with the bubble radius. 30 In fact, Vogel et al, 31 found that for laser-induced pulsed cavitation, up to 70%-90% of the input energy is converted into mechanical energy. Although these results are valid for pulsed illumination in pure water, acoustic losses also dominate in thermocavitation.…”

“…The frequency and amplitude were measured with a hydrophone and the signal displayed on a digital oscilloscope; more experimental details can be found in Ref. 30. The first striking feature is that the shockwave amplitude is larger when the beam is focused well above or below the glass-solution interface but the cavitation frequency is quite small.…”

“…It is important to mention that heating up to the spinodal limit (or cavitation temperature) is not instantaneous, the time to reach it is called thermocavitation time τ (Ref. 30). The inset picture in Figure 4(a) shows the typical signals that were monitored: (i) laser pulse (green line) and (ii) HP signal (yellow line); each vertical line corresponds to a single cavitation event for 67 mW of laser power.…”

“…Since the total bubble lifetime is just ∼300 µs (for the lower power), this means that another bubble must be formed after the first cavitation occurs, i.e., bubble formation is a quasi-periodic process with frequency dependent on the beam power as noted earlier. 30 Likewise, for the case of 124 mW, the cavitations' frequency must be faster. Thus, our simulations explain why the cavitation frequency is intensity-dependent.…”