We present a computer simulation study of a binary mixture of hard spherocylinders with different diameters ${(D}_1<D_2)$ and the same lengths ${(L}_1{=L}_2=L).\mathrm{}$ We first study a mixture of spherocylinders with lengths ${L=15D}_2$ and $D_1=0,$ which can be regarded as a mixture of rodlike colloids and ideal needles. We find clearly an entropy-driven isotropic-isotropic $\mathrm{}(I-I)\mathrm{}$ demixing transition in this mixture. In addition, we study a mixture of spherocylinders with diameter ratio $D_1{/D}_2=0.1\mathrm{}$ and we investigated the $I-I$ demixing transition as a function of the length $L$ of the particles. We observe a stable $I-I$ demixing for all values of $L$ in the range of $3<~{L/D}_2<~15,$ but we could not reach the limit $L=0,$ i.e., the hard-sphere mixture with diameter ratio of 0.1. Striking agreement is found for ${L/D}_2=15\mathrm{}$ with the results that follow from the second virial theory for infinitely elongated rods. For ${L/D}_2=2,$ we did not find a demixing transition till a total packing fraction of $\eta =0.581,$ which is higher than the packing fraction at which freezing occurs for a pure system of thick rods. Thus this result and the extrapolation of our finite-$L$ data to $L=0\mathrm{}$ gives us a fingerprint that the fluid-fluid demixing transition in the binary hard-sphere mixture with a diameter ratio of 0.1 is metastable with respect to freezing or does not exist at all at densities below close packing.

• Received 21 May 1997

DOI:https://doi.org/10.1103/PhysRevE.56.5594