  Polydisperse fluids are encountered everywhere in biological and industrial
processes. These fluids naturally show a rich phenomenology exhibiting
fractionation and shifts in critical point and freezing temperatures. Here, we
study the impact of size polydispersity on the basic nature of Lennard-Jones
(LJ) liquids, which represent most molecular liquids without hydrogen bonds,
via two- and three-dimensional molecular dynamics computer simulations. A
single-component liquid constituting spherical particles and interacting via
the LJ potential is known to exhibit strong correlations between virial and
potential energy equilibrium fluctuations at constant volume. This correlation
significantly simplifies the physical description of the liquid, and these
liquids are now known as Roskilde-simple (RS) liquids. We show that this simple
nature of the single-component LJ liquid is preserved even for very high
polydispersities (above 40% polydispersity for the studied uniform
distribution). We also investigate isomorphs of moderately polydisperse LJ
liquids. Isomorphs are curves in the phase diagram of RS liquids along which
structure, dynamics, and some thermodynamic quantities are invariant in
dimensionless units. We find that isomorphs are a good approximation even for
polydisperse LJ liquids. The theory of isomorphs thus extends readily to
multi-component systems and can be used to improve even further the
understanding of these intriguing systems.
