Quasi-periodic and irregular motion of a solid sphere falling through a thixotropic yield-stress fluid

Abstract

We report the observation of the oscillatory and irregular motion of solid spheres settling under the influence of gravity in a thixotropic yield-stress fluid, namely, a suspension of Laponite. The size of the ball and the aging time of the Laponite suspension are found to be two important parameters that determine whether oscillations occur. The irregular motion may be related to the existence of an unstable flow region and shear banding as is concluded from comparisons with rheological measurements, namely, the flow curve and creep tests, using the same Laponite suspensions.

A Numerical Investigation of Different Behavioral Regimes of the Free-Falling Solid Objects in Laponite Suspension

“…The irregular oscillating motion has been observed in Hasani et al (2011) and Fazilati et al (2017) experimental tests, is one of the most amazing regimes of the free falling sphere in Laponite gel.…”

“…In conclusion, when a spherical solid sphere falls under the gravity into the Laponite gel, based on experimental background, four regimes for velocity variation have been seen (Fazilati et al, 2017). Here we numerically investigated and modeled three common regimes.…”

“…Here we played a bit with experimental data of Fazilati et al (2017) and have obtained the flow curve of the fluid around the sphere due to its pseudo-oscillatory motion, and then compared it with the flow curve of Laponite gel which Fazilati et al (2017) derived from the rheological test. It was determined that in a certain range of shear rates (in the shear stress control rheological test) the viscosity of the fluid becomes negative (in equivalent to the slope of the flow curve), that it is to say an unknown instability took place in the fluid (Fielding, 2016;Maki and Renardy, 2012;Cheng, 2003).…”

“…In another interesting work, Hasani et al (2011) and Fazilati et al (2017), experimentally studied the free falling motion of a sphere in Laponite suspension and noticed a pseudo-oscillatory regime for certain range of aging time and diameter of the sphere. Due to the complexity of Laponite, the origin of this peculiar behavior is still unclear.…”

“…Particles settling in Laponite has been reported to exhibit amazing motion regimes (Fazilati et al, 2017;Ferroir et al, 2004;Bonn et al, 2002). In a simplistic look at the issue of free fall, especially in Newtonian fluids, two regimens are expected to emerge one including a steady state fall with a constant velocity and the other one a fall that quickly ends up to rest and causes the settling object almost suspended on the surface of the fluid.…”