Experimental observations and modeling of nanoparticle formation in laser-produced expanding plasma

Abstract

Interaction of a laser beam with a target may generate a high velocity expanding plasma plume, solid debris, and liquid nano- and microparticles. They can be produced from plasma recombination, vapor condensation or by a direct expulsion of the heated liquid phase. Two distinct sizes of particles are observed depending on the temperature achieved in the plasma plume: Micrometer-size fragments for temperatures lower than the critical temperature, and nanometer-size particles for higher temperatures. The paper presents experimental observations of fragments and nanoparticles in plasma plumes created from gold targets. These results are compared with theoretical models of vapor condensation and microparticle formation.

Droplet evolution in expanding flow of warm dense matter

“…This task requires one to choose a model for nucleation, or to input results from MD calculations. Nucleation of clusters from a supersaturated vapor is the situation of nucleation whose kinetics is the easiest to model theoretically [35], but still choices have to be made [13], that are beyond the scope of this paper. Any model for nucleation will depend crucially on surface tension, so we make the remark here that estimating the surface tension for small droplets is delicate because of its enhancement at small sizes [36,37].…”

“…Our model is a priori out of thermal equilibrium (T l = T g ), so the density of the liquid is not determined yet. It seems reasonable to assume pressure equilibrium between the droplet and the gas [13], because we expect that a few collisions are sufficient for the droplet to "experience" the gas pressure, and adjusting the liquid pressure to it requires only a small density change, due to the very low compressibility of the liquid.…”

“…On one hand, Molecular Dynamics (MD) codes [14][15][16][17][18][19][20] compute the dynamics of each particle separately, and have given powerful insight into the processes of fragmentation, phase explosion, and the different mechanisms for ablation, but they are inherently limited to only treat the early times (< 1ns, [21]), and with a small number of particles (∼ 10 7 ). On the other hand, hydrodynamic codes [13,[22][23][24][25] can model experiments completely, but they deal with mesoscopic fluid cells, and often rely on crude approximations concerning the molecular and kinetic processes involved. Complex hydrodynamic codes including a treatment of the kinetics of phase change processes and surface effects in each cell are under develop-ment [13,26], but providing a complete and reliable description of a whole WDM experiment is still a challenge.…”

“…On the other hand, hydrodynamic codes [13,[22][23][24][25] can model experiments completely, but they deal with mesoscopic fluid cells, and often rely on crude approximations concerning the molecular and kinetic processes involved. Complex hydrodynamic codes including a treatment of the kinetics of phase change processes and surface effects in each cell are under develop-ment [13,26], but providing a complete and reliable description of a whole WDM experiment is still a challenge.…”

“…Recently, there has been significant progress in the observation of those two-phase flows, from the early ablation and plume expansion stages in the the ps and ns timescales [8][9][10][11] to the late µs timescale evolution including "post-mortem" analysis of the clusters [12,13].…”