Berkeley Fluids Seminar

University of California, Berkeley

Bring your lunch and enjoy learning about fluids!

October 22, 2014

Hadi Mohammadigoushki (Chemical Eng., UC Berkeley)

Instabilities in Wormlike Micellar Solutions Sheared in Taylor-Couette cell

Simple Newtonian liquids are structured at the molecular scale. The characteristic times of molecular motions being in the order of the pico-second, therefore, fluid properties are not affected by usual flows. In contrast, complex fluids are structured materials with a scale typically from 10nm to 100 μm. Such a length scale provides with characteristic times in the order of 10-3 to 100 s that can be excited by the flow. This often results in a new organization of the structure of the fluid. For example, wormlike surfactant micelles with characteristic lengths can be stretched, disentangled and entangled. In many cases, homogeneous flow is unstable above a critical shear rate, and the system separates into ''shear bands''. Shear-banding is ubiquitous in complex fluids and in wormlike micelles, it is associated with a stress plateau that separates two increasing branches in the flow curve. In this talk, we report on our recent study on interfacial instability of wormlike micellar solutions sheared in a costume made Taylor-Couette cell. The computer controlled TC cell allows us to rotate both cylinders separately. Wormlike micellar solutions contain Water, CTAB, and NaNo3 with different compositions are highly elastic and exhibit shear banding within a range of shear rate. We visualized the flow field in θ -z as well as r-z planes, using multiple cameras. When subject to small shear rates, flow is stable and azimuthal, but it becomes unstable above a certain threshold for shear rate. This shear rate coincides with the shear rate associated with the onset of shear banding. Visualizing θ-z plane shows that this instability is characterized by stationary bands equally spaced in z direction. Meanwhile, increasing of the shear rate results to larger wave lengths. Above a threshold of shear rate fluids show a chaotic behavior that is reminiscent of elastic turbulence. We also studied the effect of ramp speed on the onset of instability and reported a minimum acceleration below which the critical Weissenberg number for onset of instability is unaffected. Moreover, visualizations in r-z direction reveals that the interface between bands undergo dynamical transitions and finally reached an undulated state.

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