Gravitational aggregation regimes: critical dissipation threshold, optimal rigidity and fractal transition

TL;DR

This study uses 3D Discrete Element Method simulations to explore how contact mechanics and self-gravity influence aggregation regimes, revealing critical dissipation thresholds, optimal rigidity, and structural transitions relevant to planetesimal formation.

Contribution

It introduces a systematic analysis of the interplay between dissipation, stiffness, and structure in gravitational aggregation using DEM simulations, highlighting regimes and transitions.

Findings

Identification of a critical dissipation threshold (~500) for aggregation acceleration.

Discovery of an optimal normalized stiffness (~1e6) minimizing aggregation time.

Mapping of structural transitions from compact to ramified fractal structures.

Abstract

I present a three-dimensional Discrete Element Method study of self-gravitation and contact mechanics in cold granular assemblies. The model couples direct Newtonian attraction between every particle pair with a linear visco-elastic normal contact law. Particles are treated as non-cohesive spheres; the normal force is parameterized to reproduce a prescribed restitution coefficient. Rotations are integrated using quaternions to avoid singularities. By normalizing the stiffness kn by kstar = G*m^2/R^3 and time by the free-fall time t_ff, I perform systematic parameter campaigns over dissipation (gamma) and normalized stiffness ktilde = kn/kstar. Results reveal three aggregation regimes. For low gamma the particles remain largely dispersive; above a critical gamma of about 5e2 aggregation accelerates until plateaus are reached in the aggregation time T_agg divided by t_ff. For stiffness ktilde on the order of 1e6 the aggregation time reaches a clear minimum. The cluster fraction C/Ntot shows a non-monotonic dependence on ktilde, with optimal cohesion at intermediate rigidity and peripheral isolation at extreme stiffness. Mapping the fractal dimension F across (gamma, ktilde) demonstrates transitions from compact structures (F about 3) to ramified structures (F below 2). These findings quantify how microscopic contact laws govern both the kinetics and microstructure of gravity-driven aggregation, providing a predictive framework for planetesimal formation and for calibrating DEM models against laboratory and micro-gravity experiments.