Droplet formation by rapid expansion of a liquid

Molecular dynamics of two- and three-dimensional liquids undergoing a homog eneous adiabatic expansion provides a direct numerical simulation of the at omization process. The Lennard-Jones potential is used with different force cutoff distances; the cluster distributions do not depend strongly on the cutoff parameter. Expansion rates, scaled by the natural molecular time uni t (about a picosecond), are investigated from unity down to 0.01; over this range the mean droplet size follows the scaling behavior of an energy bala nce model which minimizes the sum of kinetic plus surface energy. A second model which equates the elastic stored energy to the surface energy gives b etter agreement with the simulation results. The simulation results indicat e that both the mean and the maximum droplet size have a power-law dependen ce upon the expansion rate; the exponents are - 2d/3 (mean) and - d/2 (maxi mum), where d is the dimensionality. The mean does not show a dependence up on the system size, whereas the maximum does increase with system size, ant i furthermore, its exponent increases with an increase in the force cutoff distance, A mean droplet size of 2.8/eta(2), where eta is the expansion rat e, describes our high-density three-dimensional simulation results, and thi s relation is also close to experimental results from the free-jet expansio n of liquid helium. Thus. one relation spans a cluster size range from one atom to over 40 million atoms. The structure and temperature of the atomic clusters are described.