Drop deformation by laser-pulse impact

Abstract

A free-falling absorbing liquid drop hit by a nanosecond laser-pulse experiences a strong recoil-pressure kick. As a consequence, the drop propels forward and deforms into a thin sheet which eventually fragments. We study how the drop deformation depends on the pulse shape and drop properties. We first derive the velocity field inside the drop on the timescale of the pressure pulse, when the drop is still spherical. This yields the kinetic-energy partition inside the drop, which precisely measures the deformation rate with respect to the propulsion rate, before surface tension comes into play. On the timescale where surface tension is important the drop has evolved into a thin sheet. Its expansion dynamics is described with a slender-slope model, which uses the impulsive energy-partition as an initial condition. Completed with boundary integral simulations, this two-stage model explains the entire drop dynamics and its dependance on the pulse shape: for a given propulsion, a tightly focused pulse results in a thin curved sheet which maximizes the lateral expansion, while a uniform illumination yields a smaller expansion but a flat symmetric sheet, in good agreement with experimental observations. IntroductionA laser pulse interacting with an absorbing liquid body can deposit a finite amount of energy, concentrated both in time and space, which eventually triggers a dramatic hydrodynamic response. Focused nanosecond pulses have for instance been used to induce cavitation in liquids confined in capillary tubes (Vogel et al. 1996;Sun et al. 2009;Tagawa et al. 2012), or jetting and spraying in sessile drops (Thoroddsen et al. 2009). These situations involving a liquid close to a wall result in localized flows. By contrast, we consider here the situation of a mobile liquid body: the impact of a nanosecond laser pulse onto an absorbing unconfined liquid drop, which, as first described by Klein et al. (2015), has a global hydrodynamic response to the pulse: the drop propels forward at a speed of several meters per second, strongly deforms and eventually fragments (see Fig. 1). This dynamics is similar to that following a mechanical impact such as on a solid substrate or a pillar, which has been studied thoroughly (see e.g. Clanet et al. 2004;Yarin 2006;Villermaux & Bossa 2011;Kolinski et al. 2012;Riboux & Gordillo 2014;Josserand & Thoroddsen 2016), including a few studies on the fragmentation of the drop (Villermaux 2007;Xu et al. 2007;Villermaux & Bossa 2009, 2011Riboux & Gordillo 2014). A laser proves to be an adequate tool to vary the extension of the impact without arXiv:1512.02415v1 [physics.flu-dyn] 8 Dec 2015

Expansion Dynamics after Laser-Induced Cavitation in Liquid Tin Microdroplets

“…introducing the capillary time scale here as τ c = π ρR 3 0 /12σ and the relevant Weber number as We = ρR 0Ṙ (t=0) 2 /σ. The above solution is identical in form to that obtained previously for a disk-type expansion of a droplet following ns-laser pulse impact [11,23].…”

Power-law scaling of plasma pressure on laser-ablated tin microdroplets

“…Hence, as long as we can neglect small variations of the mass M and size R 0 of the irradiated droplet, the problem of the analytic derivation of the scaling of U with E od is reduced to the derivation of the analogous scaling for the local (at the pole) pressure impulse j p0 . Before tackling this issue, we provide some additional information on the angular dependence of the ablation pressure that might be helpful for a general analysis of the hydrodynamic response of liquid droplets to laser pulses 11,32 . Fig.…”

Droplet deformation by short laser-induced pressure pulses

“…The velocity field (2.25) derived from the pressure impulse is plotted in figure 10, where we show the θ− component for r = 0.5 at different times (solid lines). For comparison, the incompressible velocity field as derived in Gelderblom et al (2016) is plotted as the black dashed line. When the pulse duration is short ( figure 10a) the velocity field at r = 0.5 for t = 1 is zero, since the momentum has not yet propagated far enough into the droplet.…”

“…However, we can obtain a first order approximation of the droplet shape at early times, i.e. when the deviations from a spherical shape are still small, by advecting material points on the interface as described in Gelderblom et al (2016).…”