Monte Carlo study of the isotropic–nematic interface in suspensions of spherocylinders Tanja Schilling 1 , Richard Vink 1 , and Stefan Wolfsheimer 1 Johannes Gutenberg Universit¨ at Mainz, 55099 Mainz, Germany Abstract. The isotropic to nematic transition in suspensions of anisotropic col- loids is studied by means of grand canonical Monte Carlo simulation. From mea- surements of the grand canonical probability distribution of the particle density, the coexistence densities of the isotropic and the nematic phase are determined, as well as the interfacial tension. 1 Introduction On change of density, suspensions of rod-like particles undergo a phase tran- sition between an isotropic fluid phase, where the particle orientations are evenly distributed, and an anisotropic fluid phase (called “nematic” phase), where the particle orientations are on average aligned. Fig. 1 shows a sketch of these phases. In the 1940s, this phenomenon was explained by Lars Onsager in a theory based on infinitely elongated hard spherocylinders [1]. Onsager showed that the basic mechanism of the transition is the interplay between positional and orientational entropy. The size of the excluded volume – i.e. the volume around one particle, which another particle cannot enter, because it would produce an overlap – depends on the angle between the two particles’ axis. If the particles lie parallel, the excluded volume is minimized. Hence particles which are aligned, gain accessible volume and therefore positional entropy, but they loose orientational entropy. At a certain density the balance between isotropic nematic Fig. 1. Sketches of the isotropic phase (left) and nematic phase (right): In the isotropic phase, particle positions and orientations are disordered. In the nematic phase, particle orientations are aligned.