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Frank Smallenburg: „Liquid at zero temperature: network entropy defeats the crystal in patchy particles”
Universita' di Roma "La Sapienza", Italy

In a typical phase diagram, cooling down a liquid to sufficiently low temperature causes it to become metastable with respect to crystallization. Here, we introduce a simple model for patchy particles where, surprisingly, the liquid phase remains stable even in the zero-temperature limit. Hence, it is possible to continuously transform a diffusive liquid to a fully arrested (gel) state along an equilibrium route. The patchy particles under consideration here are based on the Kern-Frenkel model, with the additional limitation that each patch can only form a single bond, analogous to colloids functionalized by a limited number of DNA strands.

We use computer simulations and free energy calculations to study the phase behavior for particles with four attractive patches each. We show that increasing the flexibility of the bonds increases the number of ways a disordered, fully bonded network of particles can be formed, leading to a larger configurational entropy which stabilizes the fluid over the crystal phases. Finally, we use event-driven molecular dynamics simulations to investigate the change in dynamics in the fluid phase as the system is cooled. The self-diffusion slows down exponentially as the temperature decreases, following an Arrhenius-like behavior that can be understood directly based on the number of broken bonds in the network.