Andrew Belmonte
W. G. Pritchard Laboratories, Department of Mathematics, Penn State University
| Among the instabilities which distinguish viscoelastic fluids from Newtonian (Navier-Stokes) fluids, those involving free surfaces are often the most striking. For instance, an air bubble rising in a viscoelastic fluid can have a steady cusp-like tail. I will describe two different experiments initially inspired by such cusped bubbles. The first concerns the steady state shape of a polymer fluid drop falling through a viscous medium. We observe a dimpled shape, which becomes unstable to an internal tip-streaming; at larger volumes the drop is toroidal, and stable. The second experiment focuses on the stretched funnel-shaped surface formed by the dynamic wetting of a solid sphere sinking into a viscoelastic fluid. For a slowly sinking sphere we observe a buckling instability, and other instabilities are observed as the sinking speed is increased. Mathematical models for these observations will also be presented. |
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