Computer simulations of attractive one-patch colloids

Colloids can be modified chemically to create patches on their surface that result in directional attractions [1, 2]. In computer simulations, attractions can be modelled with Kern-Frenkel Square-well interactions [3]. Interestingly, already the simplest patchy architectures can result in intriguing phenomena such as the growth of tubular structures [4]. The interplay between the interaction type, range and the patch size is crucial, e.g. Janus colloids can form a wild variety of different structures in and out of thermodynamic equilibrium [5–7]. Here, we focus on Janus colloids with one attractive hemisphere [8]. To calculate their equilibrium phase behaviour we use free energy calculations and a range of different computational techniques [9]. Using successive umbrella sampling we found a gas-liquid critical point with a re-entrant behaviour that is typical for these particles. At low temperatures, the fluid consists of small clusters or micelles that are extremely hard to equilibrate. To determine the free energy of this phase we use cluster size distribution calculations. In order to find crystal structures, we employ Monte Carlo variable shape box simulations [10]. In addition, we discovered interesting structures that can not be readily identified with this method, such as a crystal with a very large unit cell and a phase of wrinkled bilayer sheets that can compete and coexist with both the fluid and crystal phases. Micelles and bilayers can form for non-spherical one-patch colloids as well: dumbbells made of overlapping spheres can form extended sheets that fold into large micelles.

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