Delayed Gravitational Collapse of Attractive Colloidal Suspensions

TL;DR

This paper develops a theoretical and numerical model to describe the delayed gravitational settling in colloidal gels, capturing the initial resistance and subsequent collapse behavior relevant for industrial applications.

Contribution

It introduces a viscoelastic continuum model based on bond density to explain the delay in settling and identifies emergent time and length scales governing gel dynamics.

Findings

Model reproduces the delay in settling observed experimentally.

Predicts formation of dense layer on top of the gel during settling.

Provides a foundation for studying erosion and industrial geometries.

Abstract

Colloidal gels have strong industrial relevance as they can behave liquid- and solid-like. The latter allows them to support the buoyant weight against gravity. However, the system is intrinsically out-of-equilibrium, which means that the colloids must eventually settle out of the suspension. The process of settling has been captured theoretically, but the presence of a delay time during which the gel appears relatively unaffected by gravity has not. Here, we modify existing frameworks to capture this delay, by treating the gel as a continuum with viscoelastic response that is based on the local bond density. We can solve our model numerically to obtain the evolution of the colloid density profile and recover qualitatively the accumulation of a dense layer on top of the settling gel, as is experimentally observed in depletion gels. This numerical study is complemented by a theoretical analysis that allows us to identify an emergent time and length scale that set the dynamics of the gel. Our model provides a solid foundation for future studies that incorporate hydrodynamic erosion and tackle industrially relevant geometries.