Magnetophoresis of paramagnetic metal ions in porous media.

We report a numerical investigation of the magnetophoresis of solutions containing paramagnetic metal ions. Using a simulated magnetic field of a superconducting magnet and the convection-diffusion model, we study the transport of transition metal salts through a porous medium domain. In particular, through a detailed comparison of the numerical results of magnetophoretic velocity and ion concentration profiles with prior published experiments, we validate the model.

Cavitation Rheology of Model Yield Stress Fluids Based on Carbopol.

Measuring the surface tension of yield stress fluids has remained a critical challenge due to limitations of the traditional tensiometry techniques. Here, we overcome those limits and successfully measure the surface tension and mechanical properties of a model yield stress fluid based on Carbopol gels via a needle-induced cavitation (NIC) technique. Our results indicate that the surface tension is approximately 70 ± 3 mN/m, and is independent of the rheology of yield stress fluid over a wide range of yield stress values σ = 0.

Probing self-assembled micellar topologies via micro-scale diffusive dynamics of surfactants.

Hypothesis: Surfactants spontaneously self-assemble in aqueous solutions and are critical in energy, biotechnology, and the environment. The self-assembled micelles may experience distinct topological transitions beyond a critical counter-ion concentration, yet the associated mechanical signatures are identical. By monitoring self-diffusion dynamics of individual surfactants in micelles via a non-invasive H NMR diffusometry, we may distinguish various topological transitions overcoming challenges associated with traditional microstructural probing techniques.

Helical Locomotion in Yield Stress Fluids.

We report three stages for locomotion of a helical swimmer in yield stress fluids. In the first stage, the swimmer must overcome the material's yield strain to generate rotational motion. However, exceeding the first threshold is not sufficient for locomotion.

Kinetics of shear banding flow formation in linear and branched wormlike micelles.

We investigate the flow evolution of a linear and a branched wormlike micellar solution with matched rheology in a Taylor-Couette (TC) cell using a combination of particle-tracking velocimetry, birefringence, and turbidity measurements. Both solutions exhibit a stress plateau within a range of shear rates. Under startup of a steady shear rate flow within the stress plateau, both linear and branched samples exhibit strong transient shear thinning flow profiles.

Linear branched: flow of a wormlike micellar fluid past a falling sphere.

We report experiments on flow of wormlike micellar solutions past a falling sphere. By increasing the salt-to-surfactant concentration ratio, and beyond a viscosity peak, wormlike micelles experience a transition from linear to branched microstructure. Two viscoelastic wormlike micelles with salt to surfactant concentrations on each side of the viscosity peak are considered.

Nuclear Magnetic Resonance Diffusometry of Linear and Branched Wormlike Micelles.

Diffusion studies using nuclear magnetic resonance (NMR) spectroscopy were conducted on two model surfactant solutions of cetyltrimethylammonium bromide/sodium salicylate (CTAB/NaSal) and cetylpyridinium chloride/sodium salicylate (CPCl/NaSal). By increasing the salt-to-surfactant concentration ratio, these systems display two peaks in the zero-shear viscosity and relaxation time, which are indicative of transitions from linear to branched micellar networks. The goal of this work is to assess the sensitivity of NMR diffusometry to different types of micellar microstructures and identify the mechanism(s) of surfactant self-diffusion in micellar solutions.

Transient evolution of flow profiles in a shear banding wormlike micellar solution: experimental results and a comparison with the VCM model.

In this paper we investigate the flow of a shear banding wormlike micellar fluid based on cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). The flow is studied in a custom-built Taylor-Couette (TC) cell via a combination of particle tracking velocimetry and in situ rheology. The spatiotemporal evolution of the velocity profile across the rheometer gap is tracked after an imposed step in the shear rate.

A flow visualization and superposition rheology study of shear-banding wormlike micelle solutions.

In this paper, we use rheometry and flow visualization to study the dynamics of the interface between shear bands in a wormlike micellar solution sheared between concentric cylinders, i.e., in a Taylor-Couette (TC) cell, and to evaluate the stress diffusion coefficient and the stress correlation length in the Johnson-Segalman model.

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.

Anomalous coalescence in sheared two-dimensional foam.

We report an experimental study on shearing a monolayer of monodisperse bubbles floating on liquid in a narrow-gap Couette device. The bubbles in such a "bubble raft" coalesce only if the shear rate exceeds a threshold value. This is in contrast to the conventional wisdom that bubbles and drops coalesce for gentler collisions, at shear rates below a critical value.

Size-differentiated lateral migration of bubbles in Couette flow of two-dimensional foam.

We report experiments on lateral migration of bubbles in a two-dimensional foam sheared in a narrow-gap Couette device. A larger bubble in an otherwise monodisperse bubble raft migrates toward the center of the gap as long as the bubble size ratio and the shear rate are each above a threshold. The migration speed is roughly two orders of magnitude higher than that of a single bubble, and increases with the shear rate and the size ratio.