# Hydrodynamic shocks in microroller suspensions

## TL;DR

This study investigates hydrodynamic shocks in microroller suspensions near a floor, revealing how collective effects and nonlocal interactions lead to finite-width shock fronts with unique dynamics, combining experiments, simulations, and models.

## Contribution

It introduces a continuum model explaining finite shock width due to nonlocal hydrodynamics and highlights the role of particle height in shock behavior.

## Key findings

- Density waves propagate in all directions.
- Shock fronts have finite width and move rapidly.
- Finite width is governed by particle height and nonlocal interactions.

## Abstract

We combine experiments, large scale simulations and continuum models to study the emergence of coherent structures in a suspension of magnetically driven microrollers sedimented near a floor. Collective hydrodynamic effects are predominant in this system, leading to strong density-velocity coupling. We characterize a uniform suspension and show that density waves propagate freely in all directions in a dispersive fashion. When sharp density gradients are introduced in the suspension, we observe the formation of a shock. Unlike Burgers' shock-like structures observed in other active and driven confined hydrodynamic systems, the shock front in our system has a well-defined finite width and moves rapidly compared to the mean suspension velocity. We introduce a continuum model demonstrating that the finite width of the front is due to far-field nonlocal hydrodynamic interactions and governed by a geometric parameter: the average particle height above the floor.