Quantifying the destructuring of a thixotropic colloidal suspension using falling ball viscometry

TL;DR

This study investigates how falling spheres behave in aging Laponite suspensions, revealing size-dependent dynamics and providing a model to quantify suspension destructuring over time.

Contribution

The paper introduces a mathematical model that captures the rapid structural changes in thixotropic Laponite suspensions caused by falling balls, enabling quantification of destructuring rates.

Findings

Larger balls do not reach terminal velocity during experiments.

The model accurately predicts velocity over time for various conditions.

Fitting the model estimates the destructuring rates of the suspension.

Abstract

The settling dynamics of falling spheres inside a Laponite suspension is studied. Laponite is a colloidal synthetic clay that shows physical aging in aqueous suspension due to the spontaneous evolution of inter-particle electrostatic interactions. In our experiments, millimeter-sized steel balls are dropped in aqueous Laponite suspensions of different ages (i.e., time elapsed since sample preparation). The motion of the falling balls are captured using a high-speed camera and the velocities of their centroids are estimated from the images. Interestingly, we observe that balls of larger diameters fail to achieve terminal velocity over the entire duration of the experiment. We propose a mathematical model that accounts for rapid structural changes (expected to be induced by the falling ball) in Laponite suspensions whose aging time scales are much slower than the time of fall of the ball. For a range of ball sizes and Laponite suspension ages, our model correctly predicts the time-dependence of the ball velocity. Furthermore, fits to our model allow us to estimate the rates of destructuring of the thixotropic suspensions due to the passage of the falling ball.