Dynamics of elastic dumbbells sedimenting in a viscous fluid: oscillations and hydrodynamic repulsion

TL;DR

This study investigates how elastic dumbbells settling in a viscous fluid exhibit oscillations, hydrodynamic repulsion, and complex dynamics due to elasticity, revealing effects beyond rigid particles and providing insights into many-particle systems.

Contribution

It demonstrates that elasticity significantly alters sedimentation dynamics, leading to non-periodic oscillations and universal solutions, which were not previously characterized in such systems.

Findings

Elasticity causes non-periodic tumbling and length changes.

Horizontal hydrodynamic repulsion occurs regardless of spring constant.

System converges to a universal time-dependent solution over time.

Abstract

Hydrodynamic interactions between two identical elastic dumbbells settling under gravity in a viscous fluid at low-Reynolds-number are investigated within the point-particle model. Evolution of a benchmark initial configuration is studied, in which the dumbbells are vertical and their centres are aligned horizontally. Rigid dumbbells and pairs of separate beads starting from the same positions tumble periodically while settling down. We find that elasticity (which breaks time-reversal symmetry of the motion) significantly affects the system's dynamics. This is remarkable taking into account that elastic forces are always much smaller than gravity. We observe oscillating motion of the elastic dumbbells, which tumble and change their length non-periodically. Independently of the value of the spring constant, a horizontal hydrodynamic repulsion appears between the dumbbells - their centres of mass move apart from each other horizontally. The shift is fast for moderate values of the spring constant k, and slows down when k tends to zero or to infinity; in these limiting cases we recover the periodic dynamics reported in the literature. For moderate values of the spring constant, and different initial configurations, we observe the existence of a universal time-dependent solution to which the system converges after an initial relaxation phase. The tumbling time and the width of the trajectories in the centre-of-mass frame increase with time. In addition to its fundamental significance, the benchmark solution presented here is important to understand general features of systems with larger number of elastic particles, at regular and random configurations.