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Oscillations of a solid sphere falling through a wormlike micellar fluid.
Anandhan Jayaraman1, Andrew Belmonte
1The W. G. Pritchard Laboratories, Department of Mathematics, Penn State University, University Park, Pennsylvania 16802, USA.
Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|October 26, 2005
Summary
Larger spheres falling through wormlike micellar fluids exhibit oscillations due to flow instabilities. This unsteady motion is linked to the fluid's complex rheology and critical velocity gradients.
Area of Science:
- Fluid dynamics
- Rheology
- Soft matter physics
Background:
- Wormlike micellar fluids exhibit complex rheological behaviors.
- Sphere falling in fluids typically reaches terminal velocity.
- Understanding instabilities in complex fluids is crucial for various applications.
Purpose of the Study:
- To experimentally investigate the falling motion of spheres in wormlike micellar fluids.
- To identify conditions leading to unsteady motion and oscillations.
- To correlate observed phenomena with fluid properties and flow parameters.
Main Methods:
- Experimental setup involving falling spheres of varying sizes and densities.
- Measurement of sphere trajectories and velocities.
- Analysis of fluid rheology and velocity gradients.
Main Results:
- Smaller/lighter spheres reached terminal velocity.
- Larger/heavier spheres exhibited oscillations during descent.
- Onset of oscillations correlated with a critical velocity gradient scale, Gamma(c) ≈ 1 s⁻¹.
Conclusions:
- The observed oscillations are attributed to flow instabilities in wormlike micellar fluids.
- Instability is linked to the fluid's complex rheology and the formation/breaking of flow-induced structures.
- Critical velocity gradient provides a key parameter for predicting unsteady motion.