Melting a Granular Glass by Cooling

Abstract

Driven granular systems readily form glassy phases at high particle volume fractions and low driving amplitudes. We use computer simulations of a driven granular glass to evidence a re-entrance melting transition into a fluid state, which, contrary to intuition, occurs by reducing the amplitude of the driving. This transition is accompanied by anomalous particle dynamics and super-diffusive behavior on intermediate time-scales. We highlight the special role played by frictional interactions, which help particles to escape their glassy cages. Such an effect is in striking contrast to what friction is expected to do: reduce particle mobility by making them stick. PACS numbers:Friction implies stability. A solid block only remains static on an inclined plane because there is friction with the underlying surface. Similarly, a heap made of granular particles, in general, will have a higher angle of stability with increased friction coefficient [1]. Inside the heap, the particle volume fraction will generally be lower and the number of inter-particle contacts smaller, while the heap nevertheless remains stable [2]. A similar stabilization occurs when the grains are driven into a fluid state. Under shear, the jamming transition from a freely flowing state to a yield-stress fluid occurs at lower volume fractions as compared to the frictionless case [3][4][5].Here, we present simulations of a driven granular system were friction acts opposite to what is expected from these simple examples. We show how friction can lead to anomalous particle dynamics that very efficiently fluidize the system. As a result the system undergoes a re-entrance melting transition from a glassy to a fluid state by lowering the amplitude of driving.Experimentally, a variety of driving mechanisms have been proposed to characterize the dynamical properties of dense granular systems. Among those are shaking [6,7] fluid-or air-flow [8][9][10], cyclic shear [11,12] or temperature oscillations [13,14]. We describe a two-dimensional system, similar to the setup used in Refs. [6,9,10,15]. At high densities and low enough driving amplitude these systems readily form glassy states, where structural relaxation is completely suppressed [16]. Interestingly, Ref.[15] also reports anomalous particle dynamics deep in the glassy phase, and suspects friction to play a central role in this process. Unfortunately, it is rather difficult experimentally to quantitatively characterize or tune the frictional interactions between the particles [17] or between particles and container walls. The connection between friction and particle dynamics is therefore unclear. Our simulations have the goal to elucidate such a connection.Model -We consider a monolayer of N = 2500 disks. One half of the particles ("small") have radius R s = 0.5d and mass m s = ρ(4π/3)R 2 , where L is the size of the simulation box. Unless otherwise stated we fix φ = 0.825, which is only slightly below the random close packing value of φ c = 0.84. Periodic boundary conditions are used in both directions...

(Ir)reversibility in dense granular systems driven by oscillating forces

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Connecting the Drops: Observing Collective Flow Behavior in Emulsions

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How nano‐scale roughness impacts the flow of grains influenced by capillary cohesion

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