Yohann Trivino | Searcharxiv

arXiv2025

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

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.

cond-mat.soft↗

arXiv2025

A soft particle dynamics method based on shape degrees of freedom

In this paper, we present a 2D numerical model developed to simulate the dynamics of soft, deformable particles. To accommodate significant particle deformations, the particle surface is represented as a narrow shell composed of mass points that interact through elasto-plastic force laws governing their linear and angular relative displacements. Particle shape changes are controlled by these interactions, in conjunction with a uniform particle core stiffness. We calibrate and verify this model by comparing the deformation of constrained beams under load with theoretical predictions. Subsequently, we explore the diametral compression of a single particle between two walls, focusing on the influence of the particle core stiffness and shell plasticity. Our findings indicate that increased core stiffness reduces particle volume change and promotes the development of faceting through flat contact areas with the walls. To further illustrate the model's capabilities, we apply it to the uniaxial compaction of a granular material composed of core-shell particles. We show that, depending on the core stiffness and shell plasticity, the compaction leads to either a significant reduction of particle volumes or an improved pore filling due to particle shape changes. At high compaction, particle shapes vary: elastic particles without core stiffness become mostly elongated, elastic particles with core stiffness form polygonal shapes, while plastic particles develop elliptical or highly irregular forms. Finally, we simulate the tensile fracture of a tissue composed of elastic or plastic cells, illustrating the model's potential applicability to soft tissues that undergo both large cell deformations and fracture.

physics.comp-ph↗