N-heptane droplet vaporization using molecular dynamics

The vaporization of a n-heptane (C₇H₁₆) droplet is investigated using molecular dynamics (MD). This constitutes one of the first studies of droplet vaporization employing a polyatomic molecule. A torsion potential is employed for intramolecular interactions and a truncated Lennard-Jones (2.5σ) for all intermolecular and a select number of in-tramolecular interactions. During each integration step the structure of the molecule is maintained by constraining bond lengths and bond angles iteratively, using the RATTLE algorithm. Initial equilibration of the liquid- and gas-phase systems is done separately using an NVT simulation; velocity rescaling is applied for both the internal and translational temperatures. Four simulations are performed on systems composed of a total of 1526, 1529, 3031, and 3041 molecules under pressures of 1 and 2 atm, respectively. This corresponds to a single-species droplet vaporization process occurring in a superheated gaseous environment. Results in terms of molecular-time-averaged forces show noticeable departures from spherical symmetry in the droplet shape. This is attributed in part to the lack of symmetry of the C₇H₁₆ molecule, which translates to manifestations at the droplet scale. The Amsterdan method is employed to investigate droplet size histories. Relatively close agreement with D²-law behavior is reported, even though the Knudsen numbers are in an intermediate regime between the kinetic theory and continuum limits.