We develop a methodology for direct molecular-level simulation of adiabatic expansion of gas through a small orifice to a vacuum. The gas attains supersonic speeds, cools, and nucleates. The proposed approach combines equations of frictionless fluid dynamics with molecular dynamics simulation in an expanding periodic box. There are two key components of the proposed algorithm: (i) a time-reversible integrator tailored to an expanding system, and (ii) an iterative procedure employed to satisfy the condition of steady flow. For a conical nozzle (opening angle of 60°), the simulations with argon and water vapor predict cluster sizes in agreement with the experiment. Clusters of irregular shapes observed in the experiment [J. Lengyel et al. Phys. Rev. Lett. 2014, 112, 113401] are not reproduced. The role of friction, turbulence, and sonic boom originating at the sharp nozzle edge is discussed.
Expansion of water vapor through
a small orifice to a vacuum produces
liquid or frozen clusters which in the experiment serve as model particles
for atmospheric aerosols. Yet, there are controversies about the shape
of these clusters, suggesting that the nucleation process is not fully
understood. Such questions can be answered by molecular dynamics simulations;
however, they require microsecond-scale runs with thousands of molecules
and accurate energy conservation. The available highly parallel codes
typically utilize domain decomposition and are inefficient for heterogeneous
systems as clusters in a dilute gas. In this work, we present an implementation
of molecular dynamics on graphics processing units based on the Verlet
list and apply it to several systems for which experimental data are
available. We reproduce sufficiently sized clusters but not the experimentally
observed clusters of irregular shape.
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