Ripping a Fluid Apart

Motion of a Viscoelastic Micellar Fluid around a Cylinder: Flow and Fracture

J.R. Gladden and A. Belmonte

PRL 98, 224501 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e224501

This duo from Penn state has experimentally demonstrated both the viscous and elastic regimes of a viscoelastic fluid. As with most fluid dynamics experiments, the photos are beautiful and fascinating.

Fluids flow. Solids deform or fracture. A viscoelastic fluid is a material that exhibits both types of response depending on how it is probed. In this Letter, Gladden and Belmonte have probed both types of response in a very simple way. The take a cylinder of diameter D and pull it through a layer of their fluid at constant velocity V. By varying the diameter and velocity, they observe three different types of response:

1) Flow: the fluid moves smoothly around the cylinder and recombines behind it.

2) Cut: the fluid still flows around the cylinder, but a cavity forms around it , and some air bubbles are trapped in its wake.

3) Tear: the cylinder rips through the fluid, leaving a trail of fin-shaped tears behind it, like a cylinder pulled through a thin plastic sheet.

These three types of response are displayed very effectively in Fig. 1. By plotting response as a function of V and D, the authors create a phase diagram for the fluid. There is a linear boundary between cut and flow, and a hyperbolic boundary between tear and the other two states. There is even a triple point.

The authors give an excellent discussion of their data. The boundary between flow and cut occurs when the time scale of fluid flow around the cylinder, D/V, exceeds the relaxation time of the fluid. The boundary between tear and the other two types of response has D*V constant. The authors use scaling arguments to determine the physical meaning of this constant, which is proportional to the tear strength of the fluid.

One more interesting observation is that when the cylinder is pulled fast enough, a crack forms in front of it. To analyze the stresses on the fluid, the authors used cross polarizers. They found a dipole pattern for the stress due to the moving cylinder, but the most fascinating picture from the paper is Fig. 6, the stress field due to a cubic probe. Amazing.

The pictures alone make this article worth reading. The authors analysis of the data is excellent.