| Non-Newtonian or viscoelastic fluids do many things which Newtonian fluids cannot do. Examples include rod climbing, the tubeless siphon, and cusp-like tails on rising bubbles. In this talk I will describe the even more peculiar behavior of a class of aqueous micellar solutions, in which the micelles take the form of long tubes (often called wormlike micelles). I will present our recent observations of the nontransient oscillations of bubbles (and spheres) rising (and falling) in a wormlike micellar fluid. By constrast, in Newtonian and polymer fluids a steady state is eventually reached, either monotonically as in the Newtonian case, or with transient oscillations as with some polymers. I will discuss several proposed constitutive models for wormlike micellar fluids which may explain our observations, and present some preliminary results for an ordinary differential equation model for the falling sphere, coupled to a modified Johnson-Segalman-Oldroyd model which accounts for some of the shear-dependent dynamics of the micelles. |