Influence of doublets on self-organization and viscoelasticity of model repulsive 2D binary colloids

We study a two-dimensional binary mixture with different interaction strengths serving as a simplified model inspired by experimental colloidal systems. The investigations are carried out by molecular dynamics simulations focusing on self-organization, dynamics, and viscoelasticity. The reference system is composed of spherical colloidal particles interacting with a short-ranged repulsive version of the Lennard-Jones pair potential, the Weeks-Chandler-Andersen (WCA) potential, with the interaction strength ${ϵ}_{11}$ ; the second component, referred to as "doublets," interacts via the same potential, but with the interaction strength ${ϵ}_{22}=4{ϵ}_{11}$ , while the cross-species interaction has the strength ${ϵ}_{12}=2{ϵ}_{11}$ . We focus on the effect of doublets on system properties. To accurately compute the dynamic moduli, we fit the shear-stress autocorrelation function (SACF) to a double Kohlrausch-Williams-Watts (KWW) stretched (or compressed) exponential function, which provides a satisfactory approximation for both the fluid and solid phases. We perform simulations at fixed pressure, varying the fraction of doublets and the temperature. We observe that the doublets increase the melting temperature, diminish the diffusion of the reference species, and strongly affect the viscoelastic properties. Our results, arising from the competition between stronger pair interactions and the reduced density due to the presence of doublets, can be (partly) interpreted by effective hard disk diameters and associated effective packing fractions. These findings provide clear signatures that may be qualitatively compared to experiments with colloidal suspensions.