Hadi Mohammadigoushki

Sedimentation of a sphere in wormlike micellar fluids

In this paper, we report a detailed experimental investigation of sedimentation of a sphere through wormlike micellar fluids by a combination of rheometry, particle tracking velocimetry, and particle image velocimetry techniques. Beyond a critical threshold, a sphere never reaches a terminal velocity and instead exhibits oscillatory motion in the axial direction similar to previous reports [Jayaraman and Belmonte, Phys. Rev. E 67, 065301R (2003); Chen and Rothstein, J. Non-Newtonian Fluid Mech. 116, 205–234 (2004)]. Although this phenomenon has been reported in the past, there is little understanding of how various parameters affect sphere motion and whether it follows any scaling laws. In this work, we systematically varied parameters such as sphere density, sphere size, temperature, and concentration of surfactant and salt for the cetyltrimethylammonium bromide/sodium salicylate system over a wide range of inertia and elasticity. It is shown that a Deborah number, defined here as characteristic shear ra...

Size segregation in sheared two-dimensional polydisperse foam.

We report experiments on simple shear of a monolayer of bidisperse and polydisperse bubbles in a Couette device. The bubbles segregate according to their sizes, with larger ones in the middle of the gap and smaller ones closer to the walls, when the shear rate and the bubble size ratio are each above a threshold. The spatial distribution of the larger bubbles becomes flatter across the gap as its area fraction increases. To explain these observations, we adapt a model for monodisperse emulsions that predicts the spatial distribution of droplets as an outcome of the competition between migration away from the walls and shear-induced diffusion. The dense packing of bubbles in our foam intensifies bubble-bubble interaction, which manifests itself both in lateral migration due to wall repulsion and in collision-induced diffusion. After accounting for this difference via an effective capillary number based on the deformation of the bubbles, the model predicts the observed bubble distributions accurately.

Inertio-elastic instability in Taylor-Couette flow of a model wormlike micellar system

In this work, we use flow visualization and rheometry techniques to study the dynamics and evolution of secondary flows in a model wormlike micellar solution sheared between concentric cylinders, i.e., in a Taylor-Couette cell. The wormlike micellar solution studied in this work contains cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). This system can be shear banding and highly elastic, nonshear banding and moderately elastic, or nearly Newtonian as the temperature is varied over a narrow range. The effect of elasticity on transitions and instabilities is probed by changing the temperature over a wide range of elasticity (El  ≪ 1, El  ≈ 1, and El  ≫ 1). Elasticity is defined as the ratio of the Weissenberg number to the Reynolds number. For shear banding wormlike micelle solutions where El ≫ 1, a primary transition from the base Couette flow to stationary vortices that are evenly spaced in the axial direction of the shear cell and are characterized by an asymptotic wave-length is obse...

Bubble migration in two-dimensional foam sheared in a wide-gap Couette device: Effects of non-Newtonian rheology

We report experiments on the migration of a large bubble in an otherwise monodisperse two-dimensional (2D) foam sheared in a wide-gap Couette device. The bubble migrates away from the walls toward an equilibrium position between the center of the gap and the inner cylinder. This differs from the situation in a narrow-gap Couette device, where the equilibrium position is at the center of the gap [Mohammadigoushki and Feng, Phys. Rev. Lett. 109, 084502 (2012)]. The shift in equilibrium position is attributed to the non-Newtonian rheology of the foam, which is brought out by the nonhomogeneous shearing in a wide-gap geometry. Two aspects of the rheology, shear-thinning and the first normal stress difference, are examined separately by comparing with bubble migration in a xanthan gum solution and a Boger fluid. Shear-thinning shifts the equilibrium position inward while the normal stress does the opposite. Bubble migration in the 2D foam is the outcome of the competition between the two effects.