arXiv subjects

Hadi Mohammadigoushki

Publications and source records attributed to Hadi Mohammadigoushki.

16 recordsLinked to original sources

arXiv2026

Drag and Yielding of Rotating Bodies in Yield-Stress Fluids

We investigate the settling dynamics of rotating objects in a yield stress fluid by combining controlled experiments with numerical simulations. Experiments were conducted using cylinders and spheres of varying surface roughness, rotated within a Helmholtz coil and immersed in a Carbopol based yield stress fluid. Complementary numerical simulations employed a viscoplastic Herschel Bulkley model to capture the coupled effects of sedimentation and rotation. To parameterize the problem, we define rotation rate to characterize rotation and the Bi to characterize sedimentation. Measurements of the drag coefficient show a strong dependence on both surface roughness and rotation rate. Flow visualization reveals that enhanced rotation generates a plastic deformation zone in the orthogonal plane and promotes wall slip, while at a stagnation point flow develops in the wake, gradually weakening and disappearing as rotation increases. In addition, the plastic drag coefficient decreases with increasing Bi and approaches an asymptotic plateau at high Bi. Numerical simulations reproduce the general scaling of drag with and but consistently underpredict experimental values, likely due to wall slip and nonlinear effects such as the stagnation point flow not present in the model. The onset of sedimentation (yield limit) was also measured and found to increase with increasing rotation and to depend on surface roughness. Finally, simulations highlight scaling relations for drag coefficient, providing new insight into the interplay of sedimentation, rotation, and viscoplastic rheology.

physics.flu-dyn↗

arXiv2025

Magnetically Assisted Separation of Weakly Magnetic Metal Ions in Porous Media. Part 2: Numerical Simulations

We present a numerical investigation of the magnetophoresis of metal ions in porous media under static, nonuniform magnetic fields. The multiphysics simulations couple momentum transport, mass diffusion, and magnetic field equations, with the porous medium modeled using two distinct approaches: a Stokes-based formulation incorporating effective diffusivity, and a Brinkman-based formulation that explicitly accounts for permeability and medium-induced drag. Comparison with recent experimental data [Nwachuwku et al. Submitted, 2025] reveals that the Stokes model partially fails to capture key trends, while the Brinkman model, with permeability accurately reproduces observed transport behavior on various porous media. Our simulations predict that both paramagnetic (MnCl2) and diamagnetic (ZnCl2) ions may form field-induced clusters under magnetic gradients over a range of concentrations of 1mM-100mM and magnetic field gradients of up to 100 T2/m. The dominant driving force is found to be the magnetic gradient (Kelvin) force, while the paramagnetic force from concentration gradients contributes minimally. In binary mixtures, hydrodynamic interactions between paramagnetic and diamagnetic clusters significantly alter transport dynamics. Specifically, paramagnetic clusters can pull diamagnetic clusters along the magnetic field gradient, enhancing diamagnetic migration and suppressing the motion of paramagnetic species. These findings highlight the importance of porous media modeling and interspecies interactions in predicting magnetophoretic transport of ionic mixtures.

physics.chem-ph↗

arXiv2025

Magnetically Assisted Separation of Weakly Magnetic Metal Ions in Porous Media.Part 1: Experiments

We report experiments on the magnetophoresis of paramagnetic (MnCl2) and diamagnetic (ZnCl2) metal ions in porous media under the influence of a non-uniform magnetic field generated by a permanent magnet. Experiments were carried out in a range of initial ion concentrations (1-100 mM), porous media particle sizes (63 um and 500 um), and varying mixture ratios of metal ion concentrations. For single-ion magnetophoresis, paramagnetic MnCl2 migrated toward the magnet surface, with an enrichment of approximately 2-4 percent near regions of high magnetic field. Conversely, diamagnetic ZnCl2 moved away from regions of highest magnetic field gradients, with depletion levels of 0.5-1.8 percent relative to the initial concentration. Our results demonstrate that magnetophoresis is directly proportional to porous media particle size, increasing with larger particle sizes, a trend attributed to the reduced drag forces experienced by the ions in media with larger particles. Interestingly, in binary mixtures, both MnCl2 and ZnCl2 migrated toward regions of highest magnetic field, contrary to their individual behaviors. The magnetophoretic effect of MnCl2 was diminished with increasing concentrations of ZnCl2, indicating interactions between the two ions. These findings suggest that both metal ions undergo field-induced cluster formation, with cluster sizes in the micrometer range, in both single and binary ion systems. In binary mixtures, the two ions appear to interact, potentially forming mixed clusters containing both MnCl2 and ZnCl2.

cond-mat.mtrl-sci↗

arXiv2025

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 material's yield strain to generate rotational motion. However, exceeding the first threshold is not sufficient for locomotion. Only when the viscous forces are sufficiently strong to plastically deform the material to a finite distance away from the swimmer, net locomotion will occur. Once locomotion is underway in the third stage, the yield stress retards swimming at small pitch angles. Conversely, at large pitch angles, yield stress dominates the flow by enhancing swimming speed. Flow visualizations reveal a highly localized flow near the swimmer in yield stress fluids.

physics.flu-dyn↗

arXiv2025

A Study of Non-linear Flows and Shear Banding in Wormlike Micelles Under Varying Elasticity, Flow Curvature, and Surfactant Chemistry

We report the flow dynamics of two shear-banding wormlike micellar solutions with distinct surfactant chemistries in a Taylor-Couette (TC) setup following a startup shear. The solutions, formulated with CTAB/NaSal and CPyCl/NaSal, exhibit comparable bulk rheology and equilibrium microstructural properties. By varying the TC gap size, we systematically examine elasticity number over a range of 128000-4470000, while flow curvature spans from 0.022 to 0.171. Under a step shear into the stress plateau, both solutions exhibit a pronounced stress overshoot that intensifies with increasing elasticity number and flow curvature, followed by the development of growing flow heterogeneities. Beyond a critical threshold of elasticity number and curvature, the CTAB/NaSal solution exhibits transient flow reversal, whereas the CPyCl/NaSal solution, despite developing similar heterogeneities, does not undergo flow reversal under any tested conditions. We hypothesize that this difference arises from the stronger electrostatic repulsions between CPyCl head groups due to their larger size, coupled with the higher dissociation rate of Cl counterions. Additionally, the quasi-steady velocity profiles are significantly influenced by elasticity number and flow curvature. Wall slip at the outer cylinder exhibits a non-monotonic dependence on these parameters: it is negligible at low values of elasticity number and flow curvature, peaks at intermediate values, and diminishes at high values. Our findings highlight how surfactant chemistry, elasticity number, and flow curvature control shear-banding flows in wormlike micelles.

physics.flu-dyn↗

arXiv2025

Magnetophoresis of Weakly Magnetic Nanoparticle Suspension Around a Wire

We present a combined experimental and numerical study into the magnetophoresis behavior of weakly magnetic nanoparticle suspensions in the vicinity of a wire under a non-uniform magnetic field and negligible inertia. The experiments were conducted within a closed rectangular cuvette, with a wire positioned between the poles of an electromagnet. Two types of nanoparticles, paramagnetic manganese oxide and diamagnetic bismuth oxide, were studied across a broad range of concentrations (10-100 mgL), magnetic field strengths (0.25-1 T), and wire diameters (0.8-3.17 mm). Our experimental findings reveal that upon the application of a magnetic field, paramagnetic nanoparticles experience a strong, attractive force toward the wire periphery. This force generates vortices and secondary flows around the wire, depleting particles from the bulk of the cuvette and concentrating them near the wire surface. The magnetophoresis dynamics of paramagnetic nanoparticles are shown to scale with their initial concentration, wire diameter, and the strength of the external magnetic field. In contrast, diamagnetic nanoparticles exhibit markedly different behavior, with their magnetophoresis dynamics showing minimal dependence on initial concentration and magnetic field strength, while being inversely proportional to the wire diameter. Multiphysics numerical simulations complement the experimental observations, revealing the formation of field-induced particle clusters in weakly paramagnetic nanoparticles, which enhance magnetophoresis. Additionally, the critical magnetic field threshold for the onset of cluster formation is found to be lower than those predicted by theoretical models for clustering in uniform magnetic fields. Under specific conditions, including high magnetic field strengths and elevated nanoparticle concentrations, diamagnetic nanoparticles appear to undergo field-induced clustering.

cond-mat.mtrl-sci↗

arXiv2025

Magnetophoresis of paramagnetic nanoparticles in suspensions under magnetic field gradients

We systematically investigate the magnetophoresis of weakly paramagnetic manganese oxide nanoparticles under nonuniform magnetic fields using a combination of experiments and multiphysics numerical simulations. Experiments were conducted in a closed cuvette exposed to a nonuniform magnetic field generated by an electromagnet, covering a wide range of particle concentrations 25-200 mgL and magnetic field gradients 0-110 T2m. The experimental results reveal that paramagnetic manganese oxide nanoparticles exhibit significant magnetophoretic behavior, leading to particle depletion within the cuvette. The depletion rate is independent of the initial particle concentration but strongly depends on the magnetic field gradient. At low magnetic field gradients, magnetophoresis progresses slowly, while at higher gradients, the particle depletion rate increases significantly before stabilizing. Transient concentration gradients emerge within the cuvette during magnetophoresis, which we hypothesize are driven by magnetic Grashof numbers near unity. When magnetic Grashof is beyond 1, the formation of concentration gradients induces bulk fluid flows that accelerate particle capture at regions of maximum magnetic field strength. In systems where magnetophoresis opposes sedimentation, particle depleted regions form when the ratio of magnetic to gravitational Peclet numbers exceeds 1. The numerical simulations suggest formation field induced aggregation for manganese oxide nanoparticles with radii of 130 nm or larger. These insights highlight the potential of magnetic separation for sustainable metal recovery, offering a scalable and environmental friendly solution for recycling critical materials from spent electronics.

cond-mat.mtrl-sci↗

arXiv2025

Flow of a two-dimensional liquid foam: Impact of surfactant type and boundary conditions

In this study, we experimentally investigate the rheological and flow behavior of two-dimensional (2D) monodisperse aqueous foams, sheared between parallel plates using a custom-made rheo-optical apparatus with smooth and roughened walls. The foams were prepared using two commercially available detergents, Foam 1 and Foam2, while maintaining similar bubble sizes and liquid fractions. The linear viscoelastic results reveal that the Foam 1 consistently exhibits higher elastic and loss moduli than the Foam 2, regardless of boundary conditions, with roughened walls further enhancing these moduli in both foams. Additionally, the Foam 1 shows a lower viscoelastic relaxation frequency compared to the Foam 2, indicating a less mobile interface for the Foam 1. In the non-linear regime, significant differences were observed. Under smooth boundary conditions, Foam 2 exhibits yield stress behavior, whereas Foam 1 does not, despite having higher viscous stresses. The viscous stress in the Foam 1 scales with the capillary number, while for the Foam 2, the scaling depends on the boundary conditions differently for smooth walls and for roughened walls. These variations in rheological behavior are attributed to differences in surfactant chemistry, leading to different interface mobilities, with the Foam1 having a less mobile interface compared to the Foam 2.

cond-mat.soft↗

arXiv2025

Dynamics of Transition Metal Ion Transport in High-Gradient Magnetic Fields

Magnetic separation has emerged as an eco-friendly and sustainable technique with applications in water purification, chemical separation, biochemical, medical, and mining. In this study, we present, a combined experimental and theoretical investigation of the transport of transition metal ions using high-gradient magnetic fields. Experiments were conducted on aqueous solutions containing either paramagnetic manganese chloride (MnCl$_$2) or diamagnetic zinc chloride (ZnCl$_$2) ions, with concentrations ranging from 1 mM to 100 mM under a non-uniform magnetic field of an electromagnet. Our results demonstrate that while paramagnetic MnCl$_$2 is captured by the mesh wool in the magnetic field, diamagnetic ZnCl$_$2 remains unaffected by the presence of magnetic field. The capture efficiency of paramagnetic MnCl$_$2 increases with both the initial ion concentration and the applied magnetic field strength. Furthermore, in binary mixtures, the capture rate of MnCl2 is reduced compared to single-ion solutions, highlighting the role of ion interactions in magnetic separation. Our theoretical modeling indicates that magnetic capture is governed by a balance between magnetic forces and viscous forces. Additionally, the magnetic separation process is enhanced by the field-induced cluster formation of paramagnetic metal ions, which are predicted to be two orders of magnitude larger than individual hydrated ion units. These findings provide insights into the mechanisms of magnetic transport of metal ions and offer potential pathways for improving separation efficiency in complex ion mixtures that contain critical materials.

cond-mat.mtrl-sci↗

arXiv2025

Nuclear Magnetic Resonance Study of Monoclonal Antibodies Near an Oil-Water Interface

Monoclonal antibodies (mAb) represent an important class of biologic therapeutics that can treat a variety of diseases including cancer, autoimmune disorders or respiratory conditions (e.g. COVID-19). However, throughout their development, mAb are exposed to air-water or oil-water interfaces that may trigger mAb partial unfolding that can lead to the formation of proteinaceous aggregates. Using a combination of dynamic surface tensiometry and spatially resolved 1D 1H NMR spectroscopy, this study investigates if adsorption of a model IgG2a-\k{appa} mAb to the oil-water interface affects its structure. Localized NMR spectroscopy was performed using voxels of 375 um, incrementally approaching the oil-water interface. Dynamic interfacial tension progressively decreases at the oil-water interface over time, confirming mAb adsorption to the interface. Localized NMR spectroscopy results indicate that, while the number of mAb-related chemical resonances and chemical shift frequencies remain unaffected, spectral line broadening is observed as voxels incrementally approach the oil-water interface. Moreover, the spin-spin (T2) relaxation of the mAb molecule was measured for a voxel centered at the interface and shown to be affected differentially across the mAb resonances, indicating a rotational restriction for mAb molecules due to presence of the interface. Finally, the apparent diffusion coefficient of the mAb for the voxel centered at the interface is lower than the bulk mAb. These results suggest that this specific mAb interacts with and may be in exchange with bulk mAb phase in the vicinity of the interface. As such, these localized NMR techniques offer the potential to probe and quantify alterations of mAb properties near interfacial layers.

physics.bio-ph↗

arXiv2023

Cavitation Rheology of Model Yield Stress Fluids Based on Carbopol

Measuring 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, and is independent of the rheology of yield stress fluid over a wide range of yield stress values. In addition, we demonstrate that a Young modulus smaller than 1 kPa can be successfully measured for Carbopol gels with NIC method. Finally, we present a time-resolved flow structure around the cavity in a host of yield stress fluids, and assess the impact of fluid rheology on the detailed form of flow around the cavity. Interestingly, prior to the critical point associated with cavitation, the yield stress fluid is weakly deformed suggesting that the measured surface tension data reflect the near equilibrium values. Beyond the critical point, the yield stress fluid experiences a strong flow that is controlled by both the critical pressure and the non-Newtonian rheology of the yield stress fluid.

physics.flu-dyn↗

arXiv2022

Role of Micellar Entanglement Density on Kinetics of Shear Banding Flow Formation

We investigate the effects of micellar entanglement number on the kinetics of shear banding flow formation in a Taylor-Couette flow. Three sets of wormlike micellar solutions, each set with a similar fluid elasticity and zero-shear-rate viscosity, but with varying entanglement densities, are studied under start-up of steady shear. Our experiments indicate that in the set with the low fluid elasticity, the transient shear banding flow is characterized by the formation of a transient flow reversal in a range of entanglement densities. Outside of this range, the transient flow reversal is not observed. For the sets of medium and high elasticities, the transient flow reversals exist for relatively small entanglement densities, and disappear for large entanglement densities. Our analysis shows that wall slip and elastic instabilities do not affect the transient flow feature. We identify a correlation between micellar entanglement number, the width of the stress plateau, and the extent of the transient flow reversal. As the micellar entanglement number increases, the width of the stress plateau first increases, then, at a higher micellar entanglement number, plateau width decreases. Therefore, we hypothesize that the transient flow reversal is connected to the micellar entanglement number through the width of the stress plateau.

cond-mat.soft↗

arXiv2022

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. However, while the flow of the linear solution evolves to a banded structure at longer times, the flow of the branched solution transitions to a curved velocity profile with no evidence of shear banding. Flow-induced birefringence measurements indicate transient birefringence banding with strong micellar alignment in the high shear band for the linear solution. The transient flow-induced birefringence is stronger for the branched system at an otherwise identical Wi. At longer times, the birefringence bands are replaced by a chaotic flow reminiscent of elastic instabilities. Visualization of the flow-induced turbidity in the velocity gradient-vorticity plane reveals quasi-steady banding with a turbidity contrast between high and low shear bands in the linear solution. However, the turbidity evolves uniformly within the gap of the TC cell for the branched solution, corroborating the non-banded quasi-steady velocimetry results. Finally, we show that while elastic instabilities in the linear solution emerge in the high shear band, the flow of branched solution at high Wi becomes unstable due to end effects, with growing end regions that ultimately span the entire axial length of the TC cell.

physics.flu-dyn↗

arXiv2022

Squirmer locomotion in a yield stress fluid

An axisymmetric squirmer in a Bingham viscoplastic fluid is studied numerically to determine the effect of a yield stress environment on locomotion. The nonlinearity of the governing equations necessitates numerical methods, which is accomplished by solving a variable-viscosity Stokes equation with a Finite Element approach. The effects of stroke modes, both pure and combined, are investigated and it is found that for the treadmill or "neutral" mode, the swimmer in a yield stress fluid has a lower swimming velocity and uses more power. However, the efficiency of swimming reaches its maximum at a finite yield limit. In addition, for higher yield limits, higher stroke modes can increase the swimming velocity and hydrodynamic efficiency of the treadmill swimmer. The higher-order odd-numbered squirming modes, particularly the third stroke mode, can generate propulsion by themselves that increases in strength as the viscoplastic nonlinearity increases till a specific limit. These results are closely correlated with the confinement effects induced by the viscoplastic rigid surface surrounding the swimming body, showing that swimmers in viscoplastic environments, both biological and artificial, could potentially employ other non-standard swimming strategies to optimize their locomotion.

physics.flu-dyn↗

arXiv2021

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. At low salt-to-surfactant concentration ratios, for which wormlike micelles are linear, the surfactant self-diffusion is best described by a mean squared displacement, Z2, that varies as Z2 ~ Tdiff 0.5, where Tdiff is the diffusion time. As the salt concentration increases to establish branched micelles, Z2 ~ Tdiff, indicating a Brownian-like self-diffusion of surfactant molecules in branched micelles. This result indicates that NMR diffusometry is capable of differentiating various types of micellar microstructure. In addition, the self-diffusion coefficient of the surfactant molecules in linear and branched micelles are determined, for the first time, by comparing to the existing restricted diffusion models, and shown to be much slower than the diffusion of proton molecules in the bulk. Moreover, in linear and moderately branched wormlike micelles, the dominant mechanism of surfactant self-diffusion is through the curvilinear diffusion of the surfactant molecules along the contour length of the micelles, whereas in the branched micelles, before the second viscosity maxima, the surfactant self-diffusion could arise from a combination of micellar breakage, exchange between micelles and/or the bulk.

physics.chem-ph↗

arXiv2017

Inertio-elastic instability in Taylor-Couette °ow 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 (TC) 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, non-shear 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 observed. The dimensionless wave- length at the onset of this shear banding transition for CTAB/NaSal system turns out to be much larger than those reported for other shear banding wormlike micelle systems. For the same fluid at a temperature where it shear-thins but does not display shear banding, El~1, and for slow ramp speeds, the primary transition is to distinct structures that are not stationary but rather travel in the axial direction. At low elasticity (El<< 1), where the fluid behaves as a nearly Newtonian fluid, several transitions from purely azimuthal Couette flow to modified Taylor vortex flows and finally chaotic regimes are documented. The behavior in the shear-banding and non-shear-banding regimes are discussed and compared with results in related systems.

cond-mat.soft↗