Evaporative instability in pulsed laser-heated droplets

Propagation of high energy lasers through clouds: modeling and simulation

A model for 10.6 µm high energy laser beam interaction with a uniform, monodisperse cloud of water droplets is developed. The model includes droplet and vapor heating as well as droplet shattering in the "fast regime" as defined in Appl. Opt.28, 3671 (1989)APOPAI0003-693510.1364/AO.28.003671. The cloud dynamics feed back on the laser via changes in the complex refractive index. In one space dimension, the model is solved exactly, including an explicit formula for the front of the cleared channel. Numerical simulations are conducted for the axisymmetric three-dimensional case. Model predictions and limitations are discussed.

Deformation pathways and breakup modes in acoustically levitated bicomponent droplets under external heating

Controlled breakup of droplets using heat or acoustics is pivotal in applications such as pharmaceutics, nanoparticle production, and combustion. In the current work we have identified distinct thermal acoustics-induced deformation regimes (ligaments and bubbles) and breakup dynamics in externally heated acoustically levitated bicomponent (benzene-dodecane) droplets with a wide variation in volatility of the two components (benzene is significantly more volatile than dodecane). We showcase the physical mechanism and universal behavior of droplet surface caving in leading to the inception and growth of ligaments. The caving of the top surface is governed by a balance between the acoustic pressure field and the restrictive surface tension of the droplet. The universal collapse of caving profiles for different benzene concentration (<70% by volume) is shown by using an appropriate time scale obtained from force balance. Continuous caving leads to the formation of a liquid membrane-type structure which undergoes radial extension due to inertia gained during the precursor phase. The membrane subsequently closes at the rim and the kinetic energy leads to ligament formation and growth. Subsequent ligament breakup is primarily Rayleigh-Plateau type. The breakup mode shifts to diffusional entrapment-induced boiling with an increase in concentration of the volatile component (benzene >70% by volume). The findings are portable to any similar bicomponent systems with differential volatility.

Explosive boiling of water droplets irradiated with intense CO(2)-laser radiation: numerical experiments

The results of numerical calculations of water-droplet explosions initiated by intense CO(2)-laser radiation are presented. The theoretical model for this process is based on the solution of the value of the thermal-boundary problem in an inhomogeneously heated droplet, including the kinetic equation describingvapor generation in a superheated liquid. The main characteristics of droplet explosions (e.g., degree of explosive evaporation and time of explosion) are calculated. It is established that these characteristics depend on the heating rate of the droplet and on its radius. The results point to the fact that two droplet-heating regimes can be distinguished-slow heating and rapid heating-based on the behavior of the explosive boiling process. This division represents the competition of real physical processes in an irradiated droplet and makes it possible to separate the basic, specific features of the explosion process.

Response of two-phase droplets to intense electromagnetic radiation

The response of two-phase droplets to intense radiant heating is studied to determine the incident power that is required for causing explosive boiling in the liquid phase. The droplets studied consist of strongly absorbing coal particles dispersed in a weakly absorbing water medium. Experiments are performed by confining droplets (radii = 37, 55, and 80 microm) electrodynamically and irradiating them from two sides with pulsed laser beams. Emphasis is placed on the transition region from accelerated droplet vaporization to droplet superheating and explosive boiling. The time scale observed for explosive boiling is more than 2 orders of magnitude longer than published values for pure liquids. The delayed response is the result of energy transfer limitations between the absorbing solid phase and the surrounding liquid.