Flow-Driven Formation of Solid-like Microsphere Heaps

TL;DR

This study investigates how microsphere heaps form in microfluidic channels under flow, revealing a critical velocity for formation and a relationship between flow speed and heap stability, with implications for colloidal assembly.

Contribution

It introduces a flow-driven method to create and analyze solid-like microsphere heaps, highlighting the critical flow velocity and the dependence of heap stability on flow conditions.

Findings

Heaps form only above a critical flow velocity.

The steady-state angle of repose increases with flow velocity.

Heaps are destroyed by thermal rearrangements when flow stops.

Abstract

We observe the formation of heaps of repulsive microspheres, created by flowing a colloidal microsphere suspension towards a flat-topped ridge placed within a quasi two-dimensional microfluidic channel. This configuration allows for both shear and normal forces on the microspheres in contact with the ridge. The heaps, which form against the ridge, are characterized by two distinct phases: a solid-like bulk phase in the interior, and a highly-fluctuating, liquid-like state which exists along its leading edge. We observe that heaps only form above a critical flow velocity, (v_c), and that they are destroyed by thermal rearrangements when the flow ceases. We monitor the dynamics of heap formation using fluorescence video microscopy, measuring the heap volume and the angle of repose in response to microsphere deposition and erosion processes. We find that the steady-state angle of repose, (\theta_f), increases as a function of inflow velocity, (v_\infty), with a functional form (\theta_f \propto \sqrt{v_\infty - v_c}).