The microscopic role of deformation in the dynamics of soft colloids

Abstract

Soft colloids allow to explore high density states well beyond random close packing. An important open question is whether softness controls the dynamics under these dense conditions. While experimental works reported conflicting results [1][2][3], numerical studies so far mostly focused on simple models that allow particles to overlap, but neglect particle deformations, thus making the concept of softness in simulations and in experiments very different. To fill this gap here we propose a new model system consisting of polymer rings with internal elasticity. At high packing fractions the system displays a compressed exponential decay of the intermediate scattering functions and a super-diffusive behavior of the mean-squared displacement. These intriguing features are explained in terms of the complex interplay between particle deformations and dynamic heterogeneities, which give rise to persistent motion of ballistic particles. We also observe a striking variation of the relaxation times with increasing particle softness clearly demonstrating the crucial role of particle deformation in the dynamics of realistic soft colloids.In recent years, colloidal particles have emerged as useful model systems which provide access to phases and states with no counterpart in atomic and molecular systems [4][5][6]. In addition they have allowed to establish new mechanisms to control phase behaviour [7][8][9] and to deepen our understanding of the glass and jamming transition [1,10,11]. A crucial parameter controlling colloidal behaviour is particle softness, which can be quantified by the ratio between elastic and thermal energy [12]. Hence, particle internal elasticity is the key ingredient to distinguish hard particles like sterically stabilized polymethylmethacrylate (PMMA) colloids from soft and ultrasoft ones such as microgels, emulsions or star polymers to name a few. Several experimental works reported that softness controls the dependence of the structural relaxation time τ α on temperature T or on the packing fraction ζ -the so called fragility. A system is called fragile when the τ α dependence is described by a Vogel-Fulcher-Tamman law [13], meaning that its variation is large over small changes of T or ζ; contrarily strong systems are characterized by an Arrhenius behaviour, implying a mild variation of τ α on varying the control parameter. While the pioneering study of Mattsson and coworkers [1] proposed a link between elasticity and fragility, there is still no consensus on this issue. Recent work based on a simple theoretical model have confirmed that such a link exists [2], but this picture has been later challenged by experiments on colloids of different softness [3]. To gain microscopic knowledge on this matter, we usually resort on simulations of simple repulsive models, as for example systems interacting with the Hertzian potential [14], which is found to describe microgel particles behavior at moderate packing fractions [15], but is expected to fail in denser conditions where soft colloids tend to shrink, ...

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