Defects and Frustration in the Packing of Soft Balls

TL;DR

This paper introduces the Hookean-Voronoi energy model for soft ball packings, analyzes equilibrium configurations, and explores how system size influences the energy landscape and stability of ordered versus disordered packings.

Contribution

It develops a minimal energy model for deformable soft particles, characterizes equilibrium conditions, and investigates the transition from ordered to disordered packings as the number of particles increases.

Findings

Regular hexagonal packing minimizes energy.

Large systems tend to settle in low-energy, quasi-ordered states.

Higher energy disordered states dominate as system size grows.

Abstract

This work introduces the Hookean-Voronoi energy, a minimal model for the packing of soft, deformable balls. This is motivated by recent studies of quasi-periodic equilibria arising from dense packings of diblock and star polymers. Restricting to the planar case, we investigate the equilibrium packings of identical, deformable objects whose shapes are determined by an $N$-site Voronoi tessellation of a periodic rectangle. We derive a reduced formulation of the system showing at equilibria each site must reside at the ``max-center'' of its associated Voronoi region and construct a family of ordered ``single-string'' minimizers whose cardinality is $O(N^2)$. We identify sharp conditions under which the system admits a regular hexagonal tessellation and establish that in all cases the average energy per site is bounded below by that of a regular hexagon of unit size. However, numerical investigation of gradient flow of random initial data, reveals that for modest values of $N$ the system preponderantly equilibrates to quasi-ordered states with low energy and large basins of attraction. For larger $N$ the distribution of equilibria energies appears to approach a $\delta$-function limit, whose energy is significantly higher than the ground state hexagon. This limit is possibly shaped by two mechanisms: a proliferation of moderate-energy disordered equilibria that block access of the gradient flow to lower energy quasi-ordered states and a rigid threshold on the maximum energy of stable states.