Session J45: Suspensions: General II

Fracture and relaxation in dense cornstarch suspensions

We probe the fracture and relaxation characteristics of a dense cornstarch suspension, a complex fluid that exhibits discontinuous shear-thickening behavior. We inject air at a constant pressure into suspensions of different mass fractions of cornstarch in water placed in an open three-dimensional container. Because the suspension is opaque, fast X-ray radiography is required to image the growth of the air cavity upon air injection. The X-ray images reveal shapes ranging from smooth bubbles that rise upwards under the action of buoyancy to sharp cracks that remain attached to the injection nozzle. Cracks form in suspensions with cornstarch mass fractions close to the jamming transition, while bubbles form in suspensions with lower mass fractions that can still discontinuously shear thicken. We further show that the shape and the relaxation dynamics of the air cavity are linked to the cornstarch rheology: sharp cracks relax into bubbles when the shear rate applied to the suspension by the bubble growth decreases below the critical shear rate for discontinuous shear thickening.   

Suspension plug in an oscillatory pipe flow: simulations

Transport phenomena in neutrally buoyant dense particle suspensions subject to shear depend strongly on the applied strain amplitude (related to the distance travelled by particles in one oscillation divided by the characteristic lengthscale) at vanishingly low Reynolds numbers. However, the evolution of these particle-fluid interactions at low, but finite Reynolds numbers and its dependence on both the strain amplitude and the number of oscillations is currently less understood. We use particle-resolved Direct Numerical Simulations (pr-DNS) to investigate the movement of a particle plug subject to pressure-driven oscillatory Poiseuille flow inside a plane channel. Simulations are carried out at Reynolds numbers O(1-10), and the effects of weak inertia on particle dynamics are explored. Understanding particle suspension behavior at finite Reynolds numbers could inform experimental and industrial applications of such flows.   

Suspension plug in an oscillatory pipe flow: experiments and modeling

Non-colloidal particles under non-uniform shear can migrate across streamlines even at low Reynolds numbers, breaking the well-known reversibility of the Stokes flow. We conduct systematic experiments to study this phenomenon by oscillating a neutrally buoyant non-colloidal suspension plug with finite length inside a cylindrical tube. We visualize the particles on the central axial plane of the tube via refractive index matching and track the motion of individual particles. Our preliminary results qualitatively match the existing literature that shows the initial extension and plateauing of the suspension plug at a critical strain. In addition, we experimentally connect the macroscopic evolution of the plug with the trajectories of individual particles. Finally, we develop a simplified particle collision based model to understand the role of particle contact interactions in the plug evolution.     

Revisiting Batchelor & Green's study of the hydrodynamic interaction of two spheres in a simple shear flow for aerosol applications

In their pioneering study, Batchelor & Green (J. Fluid Mech. 1972) provided a comprehensive picture of pair trajectories for inertialess non-Brownian spheres in a simple shear flow. They showed that the pair trajectories comprise two families, open and closed, separated by a separatrix. However, all their trajectories are non-colliding. Continuum lubrication forces prevent particles from touching, and collisions occur only with the inclusion of attractive non-hydrodynamic forces (for example, the van der Waals force). In a gaseous medium, the lubrication forces are weaker than their continuum counterpart and thus allow for particle contact in finite time. Our study revisits this classical study and explains the collision dynamics of small solid/liquid spherical particles in gaseous media subjected to a simple shear flow, incorporating various physics that enable collision; the non-continuum lubrication forces being the primary one. We also highlight that when non-continuum effects are weak, the collisional dynamics of two spheres in a simple shear flow significantly differ from other linear flows with purely open pair trajectories. Finally, we discuss the modification to the pair trajectory topology and the collision rate when the role of particle inertia is included in a perturbative manner.   

Dynamics of inertialess sedimentation of conically deformed rigid disks.

When thinking of everyday examples of sedimenting objects, we often imagine a periodic motion, be that a fluttering motion of a leaf falling in the air, or a coin falling in water. By contrast, sedimentation in the limit of vanishing inertia, known to be linked to particle shape, is typically more predictable - achiral particles such as flat disk, rods and ellipsoids, follow oblique paths set by their initial orientation, while chiral particles, such propellers and helices, have a screw motion. 

In this talk we study inertialess sedimentation of an achiral particle with one plane of symmetry. It is created by wrapping a circular disk around cones, so that one side the disk is more tightly curved than the other. We find that such particles tend to well-defined equilibrium orientations (with the tightly curved end pointing downwards), but sediment either in a straight line or a spiral depending on how curved they are. They also exhibits transient dynamics of increasing complexity as their asymmetry tends to zero. In the limiting case of two identical curvatures, when the disk has two planes of symmetries, i. e. it is cylindrically deformed, we recover a complex sedimentation motion that involves periodic reorientation of the disk in ever-repeating sequence of pitching and rolling.   

Natural Deposition and Removal Efficiency of Bioaerosols in Confined Spaces

In this study, we measure the decay rates of bioaerosols in a confined space under diverse working conditions (i.e. HEPA filtering, closed/open door, and sealed/open AC vents etc.). The results were collected from various sensors placed at different locations and heights, revealing that aerosol concentrations in confined spaces quickly became uniform and concentration decay occurred concurrently at multiple points. Notably, our investigation involved studying four types of aerosols: bioaerosols originating from procedures on extracted human teeth, aerosols from procedures on synthetic teeth, and aerosolized distilled and tap water, allowing for a direct comparison of respective decay rates. The most efficient particle removal method was the use of a HEPA air purifier, following by an extraoral suction device, AC system, and finally natural deposition. Notably, if used during procedures, the extraoral suction resulted in a significant reduction of particles being suspended in the environment, but for comparable aerosol concentrations, a standard HEPA air purifier was more effective at removing aerosols. This multi-particle approach provides valuable insights into the behavior of diverse aerosol types and allows for substitution scenarios for results from previous experiments which used synthetic teeth or aerosolized sprays to assess decay rates and fallow time. Furthermore, we developed a semi-empirical model capable of predicting the decay rates of various bioaerosols under a multitude of environmental scenarios.   

Clustering due to negative effective diffusion coefficients in a magnetorheological fluid.

A magnetorheological fluid consists of magnetic particles suspended in a viscous fluid. These are used in applications requiring rapid flow control, where the suspension flows like a fluid in the absence of a field, but the particles cluster and jam the conduit in the presence of an applied field. The effect of shear flow and an applied field on the rheology is examined by incorporating the particle-particle hydrodynamic and magnetic interactions in the low Reynolds number limit. When an isolated particle is subjected to a magnetic field, the particles align in the field direction, while they rotate in `Jeffery' orbits when subjected to a shear flow. The effect of interactions in the presence of shear and an applied magnetic field is included in the dilute limit where the particle separation is much larger than the radius. In a uniform suspension, the total force due to interactions is zero. When there is a concentration modulation, there is a force on the particles which leads to a drift velocity. The effect of interactions can be expressed as an anisotropic diffusion tensor. The elements of this tensor are negative in the plane perpendicular to the field, indicating a strong clustering effect, and the are positive in the field direction, resulting in damping of concentration fluctuations. This explains the mechanism for initiation of anisotropic clusters that span the conduit and result in jamming upon application of a magnetic field.