Irregular Motion of a Falling Spherical Object Through Non-Newtonian   Fluid
  The falling of an object through a non-Newtonian fluid is an interesting problem, depending on the details of the rheology of the fluid. In this paper we report on the settling of spherical objects through two non-Newtonian fluids: Laponite and hair Gel. A falling object's behavior in passing through a thixotropic colloidal suspension of synthetic clay, Laponite, has been reported to have many behavioral regimes. Here we report observation of a new regime where irregular motion is observed. We argue that this irregular motion may be interpreted as onset of chaos. Observation of this regime depends on the size of the falling sphere, relaxation time of fluid and concentration of particles in the suspension. Similar experiments in Gel, a yield stress polymeric fluid, do not reveal such behavior. 
Samira Hasani, Nahid Maleki-Jirsarei and Shahin Rouhani
cond-mat.soft