Atomic Dynamics of Explosive Boiling of Liquid-Argon films Using Molecular Dynamics Simulations

Xiang Gu, H. M. Urbassek



Using molecular-dynamics simulation, we study the explosive boiling of thin liquid Ar films adsorbed on a metal surface. This process might be induced by heating the metal substrate by an ultrafast laser.


I. Atomistic Overview


Fig.1 Atomistic Overview for Ar films of thickness (a) 4ML, (b) 16ML at 275 ps after the substrate started heating. Colors denote reduced temperature.


After sudden heating of the substrate, thin films (4ML, Fig. 1a) completely fragment into a mixture of small clusters and monatomics. In thicker films, however, only the part close to the heated surface vaporizes, and the resulting high vapor pressure drives the top part of the film as a large droplet away from the surface. This liquid droplet does not fragment but cools by evaporating off a few atoms.



II. Behavior of atoms in the vicinity to the heating surface


We subdivide the surface-near part of the expanding Ar films into three regions, each comprising 60 atoms, which are named the bottom, middle and top slab, respectively, in agreement with their increasing distances to the surface. In the following, temperatures and densities are given in reduced units based on the Lennard-Jones potential for Ar.


(1)     The bottom slab reaches a highest temperature of T* ≈ 2.2, and enters the metastable region at very low densities n* < 0.1.

(2)     The middle slab reaches only lower peak temperature at T* ≈ 1.8, and enters the metastable region already at higher densities, n* = 0.2~0.3.

(3)     The top slab is heated merely to T* ≈ 1.3 and enters the metastable region above the critical temperature at n* = 0.5~0.6.


The differences in the evolution of these slabs are easily rationalized by considering the heat conduction which heats the top part later and to less high temperatures than the bottom part. This effect is enhanced by the work of expansion which has been delivered before the top part has been heated.


                                    (a)                                                                    (b)

Fig.2 State evolution of 4ML liquid film (a) and 16ML liquid film (b)


III. Cluster Distribution


After explosive boiling, the films are fragmented into a multitude of clusters. Data of abundance distribution f(m) are plotted at the final simulation time of t = 600 psThe distribution of small clusters, m ≤ 10, obeys a power law of the form f(m) = A.m(where A ≈ 300 and τ ≈ 2.5); this value of the power exponent τ is quite close to that predicted by Fisher, τ = 7/3, for the cluster distribution of a system in thermal equilibrium at the critical point of the gas-liquid phase transition. This fact is further evidence that our system passed close to the critical point into the coexistence region, and there the break-up of the liquid into a highly fragmented cluster-rich phase started.


Fig.3  Cluster abundance distribution for Ar films of thickness

(a) 4ML                                                             (b) 16ML