Glass Transitions and Rotator Phase Emergence in Rough Hard-Particle Liquids across Friction

Overview

Liquids are ubiquitous in industrial and chemical applications. They are typically composed of geometrically rough colloidal particles that exhibit complex dynamical behavior arising from microscopic surface structure. Using large-scale molecular dynamics simulations of two-dimensional hard particles with tunable surface roughness, modeled through an effective friction parameter, we characterize how density and roughness jointly govern dynamical and structural transitions. We establish the first functional form for the divergence of rotational relaxation times, showing that they follow a power-law divergence with an exponent independent of friction. The divergence occurs more gradually in lower-friction systems, while higher-friction systems exhibit an earlier onset of dynamical arrest. Consistent with this, the rotational glass transition density is higher for lower-friction systems, as particles in higher-friction systems interlock more readily due to asperity interactions. We find that friction plays a negligible role in dynamical behavior below a critical density, where particles behave similarly to smooth systems. Beyond this threshold, particles abruptly begin to occupy gaps between asperities, producing a rapid change in dynamical response. This critical density decreases with increasing surface roughness, reflecting the larger asperity sizes in higher-friction systems. To probe regimes beyond direct dynamical accessibility, we introduce a method that combines Voronoi-based cage constructions with a free-volume technique to infer dynamics from static structure at very high densities. This approach provides new insight into relaxation behavior deep in the glassy regime. Most importantly, we identify and characterize a previously unreported intermediate “rotator phase,” in which particles retain rotational freedom while translational motion is arrested. This phase arises from a gearing-like effect between asperities. The extent of this effect depends strongly on surface roughness: lower-friction systems exhibit greater rotational motion within cages, while higher-friction systems suppress this behavior due to stronger interlocking.

Competition history

• CSEF 2026 Physics & Astronomy (Senior Division) · Entry S-17-22

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