Rupture time of droplets impacted by a burst of picosecond laser pulses

Liquid Sn droplets were irradiated with shaped bursts of picosecond laser pulses. The shapes of the deforming droplets following the impact of the recoil pressure induced by these bursts were imaged using a high-speed shadowgraph system. The rupture time ${\tilde{t}}_b$ of the droplet expanding as a thin fluid film was measured for each case. A Rayleigh-Taylor instability analysis is done in order to determine the dependences governing ${\tilde{t}}_b$. The evidence supports the hypothesis that the initial perturbations of the developing Rayleigh-Taylor instabilities are on the order of the ablation depth and that there is a lower cutoff wavelength of these initial perturbations of $\sim 10\mu \mathrm{m}$.

Figure 1

The functional layout of the experiment. The deforming droplets were imaged at ${90}^{\circ }$ relative to the laser axis.

Figure 2

An example sequence illustrating the rim expansion and sudden rupture of the expanding sheet into a web of ligaments $[N_p=8$, ${\tilde{E}}_B=2.4\times {10}^5$, 20 MHz, rising saw-tooth (RST) [see Fig. 3]]. The timing from left to right is $\tilde{t}=0.071$, 0.13, 0.20, 0.27, 0.41, and 0.59. The third frame displays $R\left(t\right)$, which is defined as the path length along the sheet from the sheet center to the rim.

Figure 3

Examples of burst profiles implemented in this work for (a) double pulse, (b) flattop (FT), (c) rising sawtooth (RST), (d) falling sawtooth (FST), (e) Gaussian, (f) double burst. Each peak represents one laser pulse of the shown dimensionless pulse energy ${\tilde{E}}_p$ at the time from the beginning of the burst $\tilde{t}$.

Figure 4

Example cases of varying $W{e}_s$. The markers represent the measured $R\left(t\right)$ and the dashed lines represent the corresponding fit of (1) to measure $W{e}_s$. The perforation times ${\tilde{t}}_b$ for the plotted cases are also shown. The highlighted area marks the range where ${\tilde{t}}_b$ is observed across all measured cases.

Figure 5

Example cases showing first hole perforation, where (a) $N_p=27$ and ${\tilde{E}}_B=2.3\times {10}^5$, and (b) $N_p=8$ and ${\tilde{E}}_B=3.3\times {10}^5$.

Figure 6

The breakup time ${\tilde{t}}_b$ plotted as a function of $W{e}_s$ and compared with (21). Rupture of the sheet does not appear for ${\tilde{t}}_b⪆0.385$, which corresponds to the time of the maximum rim radius. The highlighted area marks the variation in Eq. (21) for ${\eta }_0=60–80$ nm. The coefficient of determination for the logarithms of experimental values compared to Eq. (21) is $R^2=0.90$.