  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.
