Computer simulation of microscopic liquid drops

Molecular dynamics computer simulations of small liquid drops are reported. Systems of between 60 and 1300 particles have been studied, each comprising of a central drop that is allowed to come to equilibrium with its surrounding vapour. Two common potential models have been used for the interactions between particles, the Lennard-Jones 12-6 form and the Stockmayer potential (LJ 12-6 plus a point dipole moment). Simulations for the Stockmayer fluid have been performed with µ2/ϵσ3 = 1 and µ2/ϵσ3 = 3. For both potentials, relatively long cut-offs have been employed so that “tail” corrections are unnecessary. Results are given for the size and density profiles of such drops over a range of temperatures. The Kelvin equation for the excess vapour pressure outside a curved interface is investigated as a means of determining the surface tension, γ, for a given fluid. Calculations using simulation results, in conjunction with the known coexistence vapour pressure, lead to a value of γ which is consistent with previous measurements of this quantity. This technique is seen to work best with large drops near the triple point, and though such simulations are currently quite expensive the increasing power of computers could make this a viable method for finding the surface tension. The surface width of the liquid vapour interface in these systems is analysed as a function of temperature. Comparisons of the width with theoretical calculations for this quantity show that a significant discrepancy exists. This difference can be explained in terms of the contribution of surface oscillations (capillary waves) to the width. The dielectric properties of Stockmayer drops have been studied via measurements of the mean square moment within spheres about the centre of mass of the system. It is found that values of the static dielectric constant, ϵs, for the “bulk” liquid within the drops are consistent with previous results obtained by Adams and Adams for homogeneous systems, at least for µ2/ϵσ3 = 1. For the more strongly polar fluid, with µ2/ϵσ3 = 3, it may be that systems of more than 450 particles are necessary to determine the liquid value of ϵs, but more work is required on this point. In addition to determining within the drops, it is shown that measured data for the mean square moment as a function of radius can be explained in terms of a (scalar) dielectric constant that depends on r. A dielectric profile of similar width and position to the density profile is obtained. Measurements have also been made of the time dependent correlations of the total moment within spherical volumes about the centre of mass. In principle the dynamic dielectric constant can be obtained from these functions. However, it has not been possible to obtain consistent results for ϵ(ω) from the data reported here and some of the reasons for this are examined.