Reentrant Network Formation in Patchy Colloidal Mixtures under Gravity

D. de las Heras, Lucas L. Treffenst├Ądt, and Matthias Schmidt
Phys. Rev. E, 93, 030601(R), (2016)     DOI: 10.1103/PhysRevE.93.030601
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We study a two-dimensional binary mixture of patchy colloids in sedimentation-diffusion-equilibrium using Monte Carlo simulation and Wertheim's theory. By tuning the buoyant masses of the colloids we can control the gravity-induced sequence of fluid stacks of differing density and percolation properties. We find complex stacking sequences with up to four layers and reentrant network formation, consistently in simulations and theoretically using only the bulk phase diagram as input. Our theory applies to general patchy colloidal mixtures and is relevant to understanding experiments under gravity.

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Snapshots of the simulations

Colloidal dispersions of patchy particles form the focus of very active current research in soft condensed matter science due to their potential applications, such as the design of custom materials with on-demand optical and mechanical properties, their ability to act as models of protein interactions, and due to a wide interest in general mechanisms of self-assembly. Powerful theoretical and simulation tools to understand the bulk behavior of patchy colloids have been established in recent work. On the experimental side, it has become possible to synthesize a broad variety of different types of patchy colloidal particles, and mass production of patchy colloids is promising to become widely available in the near future.

Carrying out systematic sedimentation experiments is a primary tool to investigate central properties of colloidal systems. Examples include the investigation of the equation of state and the observation of phase separation. However, as gravity can have a very strong effect on colloids, which typically possess several different components, the connection between the bulk properties and the sedimentation behavior of a given colloidal systems is far from obvious.

Here, we investigate the general relationship of the influence of gravity and network-formation in colloidal systems. We use a specific binary model mixture of patchy colloids under gravity, which we investigate theoretically and with computer simulation. We demonstrate that knowledge of the bulk phase behavior is sufficient to understand the very complex sedimentation behavior. Moreover, we show that gravity can induce novel phenomenology, such as reentrant network formation, even in relatively simple patchy colloidal systems.

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