Isobaric molecular simulation of boiling

Despite widespread applications of boiling, its nanoscale features e.g., nucleation, microfilm boiling, adjoint pressure, non-evaporative layer etc. are not yet well explored. In past decades, molecular dynamics evolved out to be a reliable tool to investigate these nanoscale features. However, most of these studies are carried out in isochoric condition, which fails to mimic the actual scenario. In the present study, a moving piston-based system is prepared using LAMMPS, and a constant force is applied to it to maintain a constant pressure. Further, a viscous force is added against the piston motion proportional to its velocity to avoid piston oscillations. The interaction force between the piston and water is critical for a piston-based system because a traditional Lenard-Jones force will impart an additional adjoint pressure to the fluid. As the attractive part of the Lenard-Jones force field is responsible for the adjoint pressure, a repulsive-only force field is implemented in this study by chopping off the attractive part of the force field. To validate the system, water in liquid and vapor form is placed underneath the piston. The density of the system is studied at different temperatures and was found to be close to the ideal density values. The measured pressure of the vapor system also seems stable with small-amplitude oscillations over the time range (10 ns) of the simulation. However, for the liquid phase, measurement of the exact pressure is not possible because of high fluctuations. Further, the system is validated in terms of the saturation temperature for two different pressures (1 and 10 bar). To understand the effect of the piston-fluid force interaction, a liquid water pool is equilibrated at 125˚ C and 1 bar pressure with both traditional and repulsive-only force field. Using the latter, the piston rises monotonously, however, with the former, the piston stays stable over the liquid pool without any sign of phase-change.