Inserting test particles improves measurements of structure and thermodynamics of fluids
Inserting test particles improves measurements of structure and thermodynamics of fluids lead image
Modeling the molecular structure of fluids to better understand their properties and behaviors is of interest because it could have enormous industrial and biological significance. Typically, we characterize the microscopic structure of fluids by considering the distribution of particles around a reference particle, and this pairwise distribution allows access to the system’s thermodynamics.
An important version of this model uses hard spheres that only interact with each other by elastic collisions. This simple model embodies the strong repulsion of atoms when they are close together and approximately reproduces the structure in real monatomic liquids such as argon. In this model, the pressure can be directly related to the value of the pairwise distribution when the spheres are touching each other.
The conventional method for measuring the pairwise structure estimates this value from that of spheres that are nearly in contact. Now, authors report in The Journal of Chemical Physics demonstrating a method that provides the exact value at contact in simulations and colloidal experiments on two-dimensional hard disks.
The group used a relationship between the pairwise distribution and the probability of being able to insert an additional test particle at a fixed distance away from a particle of the fluid. They obtained the contact value by choosing the insertion distance to be exactly the diameter of a particle, so that the two particles touch. They also used this approach to study mixtures of particles with different sizes, and think that this method can be extended to investigate more complex fluids, such as those composed of hard rods or molecules made of fused hard spheres.
Source: “Communication: Contact values of pair distribution functions in colloidal hard disks by test-particle insertion,” by Adam Edward Stones, Roel P. A. Dullens, and Dirk G. A. L. Aarts, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5038668 .
