Session A31: Suspensions: General I

Mucus Comet tails in Marine Snow

Phytoplankton in the upper layers of the ocean agglomerate and sink in the form of aggregates called marine snow. This transport mechanism is a dominant carbon sequestration pathway that regulates atmospheric CO₂ and global climate. Although sedimentation physics dates back to the late nineteenth century, a predictive understanding of marine snow remains a puzzling fluid-structure interaction problem, which can be attributed to the lack of a dynamical underpinning of the various ecophysiological parameters that are central to the sinking behavior. In this work we experimentally study marine snow using vertical tracking microscopy in a field setting, in the Gulf of Maine on R/V Endeavor. We discover hitherto unexplored invisible degrees of freedom coming from mucus, which significantly modifies the geometry and sinking dynamics of marine snow. The microscopically resolved in-situ PIV of marine snow reveals a comet tail like flow morphology that is universal across a range of hydrodynamic fingerprints of marine snow, which renders porosity unimportant in the sedimentation dynamics. We construct a minimal model based on Stokesian sedimentation and viscoelastic distortions of mucus to understand the scaling behavior of both sinking speeds and tail lengths of these mucus comets. Our theoretical framework is in good agreement with our field observations and paves the way towards more detailed in-situ observations and predictive understanding of this crucial transport phenomena.   

Rolling microshuttles: trapping and shipping colloids by pure hydrodynamics

Trapping and transporting materials within nano to micrometer length scales is crucial in living systems. Molecular motors translate along proteins and carry organelles to proper locations. While white blood cells deform and travel in confined channels, trapping and engulfing bacteria. These and many other biological processes consist in the controlled and precise transport of objects within thermalized complex fluids like blood or in the interior of cells. In order to replicate this artificially, possibly for a localized delivery of medicine in the body, a shuttle particle must be employed. This shuttle particle must attract to a targeted cargo to deliver the cargo to a specific location. It is normally the case that the attraction between the shuttle and cargo be mediated by electrostatic interactions, however, an alternative mechanism relies on advective flow created by the motion of the shuttles. When these shuttles translate they generate closed streamlines known as microvortices, it is in these regions where the cargo is trapped and thus orbits around the shuttle. One benefit of this mechanism is that it does not rely on adding a specific interaction between the shuttles and the cargoes. In this talk, we experimentally demonstrate how micron sized rollers can efficiently transport cargo of various sizes and masses by purely hydrodynamic means within a thermalized fluid, effectively acting as microshuttles. These microrollers can pick up particles three times its size due to microvortices, and we are able transport the cargo to desired locations by manipulating the external driving field. In addition, we use Stokesian dynamics simulations to understand the cargo mechanism, and show that it is possible to selectively pick the desired size/density of the particles from a cluster even when the fluid is thermalized.   

Slender body theory for fluid-particle interactions at finite Reynolds numbers

We study the effects of finite fluid inertia on the translational and rotational dynamics of high aspect ratio fibers using slender body theory. We build on the weakly inertial slender body theory of Khayat and Cox (1989) (valid when the Reynolds number Re_D based on the fiber diameter is small) and extend the theory to Re_D = O(1). This is achieved by matching the quasi-two-dimensional solution to the full Navier-Stokes equation in the inner region (fiber diameter scale) to the three-dimensional solution of the linearized Navier-Stokes equation in the outer region (fiber length scale). The results for the orientation-dependent drag, lift, and torque for a broad range of values of fiber aspect ratios and Reynolds numbers are obtained and compared with complementary numerical simulation results. The finite-Re_D slender body theory is then used to explore the unsteady dynamics resulting from the coupled effects of translation and rotation of a freely sedimenting fiber. By incorporating the convection of the transient fiber-induced fluid momentum disturbance by the imposed flow in the fiber-attached reference frame, we obtain a more accurate description of fiber rotation than that given by the commonly invoked quasi-steady approximation.     

Instability of Particle-Laden Flow in a Spatially-Periodic Channel

Laboratory experiments are conducted for suspension flows in a channel with a geometry that is textured with cylindrical obstructions placed periodically in the streamwise direction. In the absence of particles, the pure flow becomes convectively unstable at a critical Reynolds number of approximately 130 and exhibits a supercritical bifurcation to a secondary flow in the form of non-turbulent 2D wave packets.  We examine the effect of adding inertial particles on the critical Re and the onset of instability.  Moreover, we describe novel particle accumulation behaviors that arise in the presence of the secondary flow.  The experimental results are compared to direct numerical simulations. 

Supported by NSF grants 22-30893 and 22-30894.    

Emergence of a hexagonal pattern in shear-thickening suspensions under orbital oscillations

Dense particle suspension under shear may lose its uniform state to large local density and stress fluctuations which challenge the mean-field description of the system. Here, we explore a novel dynamics of a non-Brownian suspension under horizontal circular oscillations, where localized density waves along the flow direction appear beyond an excitation frequency threshold and self-organize into a hexagonal pattern across the system. The spontaneous occurrence of the inhomogeneity pattern results from the competition between kinematic instability and particle migrations and symmetry argument. Our analysis shows that this instability is closely related to the onset of discontinuous shear thickening transition of the suspension. In addition, the density waves degenerate into random fluctuations when replacing the free surface with rigid confinement, however, they will long live under a constrained soft silicone oil layer. It indicates that the shear-thickened state is intrinsically heterogeneous, and the boundary conditions/deformability are crucial for developing local disturbance.   

Self-propulsion of rotating axisymmetric particles at intermediate Reynolds number

In this talk, we discuss the dynamics of an axisymmetric particle rotating in a quiescent fluid. At zero Reynolds number, time-reversal symmetry implies such a particle does not move regardless of its boundary profile. For nonzero Reynolds numbers, however, we find head-tail asymmetry in the particle can cause it to self-propel. Combining laboratory experiments, numerical simulations, and boundary layer analysis, we examine the role of geometry on propulsion, focusing in particular on conical particles. We identify scaling laws in the propulsion speed as a function of the rotation frequency and various geometric factors, and show these laws have their origin in boundary layer flows. Finally, we discuss the collective dynamics of suspensions of rotating particles.   

Colloidal fingering in miscible liquids

We report a new "Colloidal fingering" instability that form spontaneously when a colloidal suspension is placed above a miscible liquid of higher mass density in an isothermal situation. A similar in appearance, though of a different origin, "salt fingers" develop when a warm salty solution is placed above colder fresh water of a higher mass density. Eliminating other possibilities, we argue that the instability is due to double mass-diffusive convection resulting from the large difference be­tween the mass diffusivity of the miscible liquids and the colloidal-mass diffusivity in the mixed medium. The principle is general and this kind of fingering should be observable in a variety of colloidal systems and biofilms, including where it can rise upwards.   

Microstructure appearing in soft microswimmer suspensions

Microstructures in a suspension of microswimmers are of interest in various fields including bioengineering and robotics. Previous studies using rigid swimmers have reported that microstructures are heavily dependent on near-field hydrodynamic interactions. Although the near-field interactions are altered by the deformability of swimmers, its effect on the microstructures are largely unknown. In this study, we numerically investigated the effect of swimmer deformation on the suspension microstructures. The soft microswimmer was modeled as a capsule with a hyperelastic membrane. Thrust is generated by the torque distribution acting slightly above the membrane, mimicking ciliary activity of a ciliate. We solved the fluid mechanics of Stokes flow by a boundary element method and solid mechanics of membrane deformation by a finite element method. By increasing the membrane deformability, the polar order observed for pullers is weakened. On the other hand, the swimming velocity and the straightness in the path are gained as the deformability is increased. The maximum tension on the membrane and its position differ depending on the swimmer type. These results are useful for understanding suspension properties of biological swimmers and soft microrobots.   

Structure-driven percolation enhancement of particle-laden flow in Vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) are promising energy storage solutions with advantages such as scalability, longevity, safety, and suitability for integration with renewables. However, their performance is often limited by high cell resistance and pumping losses linked to electrolytes. The use of slurry electrodes, specifically active carbon in the carrier fluid, is often considered due to its enhanced performance, though it results in higher viscosity and pumping losses. This study introduces a unique approach to alleviate these issues using a microscale experimental model system to visualize and enhance percolation in particulate flow with patterned electrodes. Anthraquinone disulfonic acid (AQDS), an organic compound that fluoresces upon reduction, was used as an indicator to visualize redox reactions in the slurry electrode VRFB. By observing fluid flow and redox reactions, we identified key factors influencing percolation related to carbon particle concentration, flow rate, and pattern size. We found that optimizing the pattern structure of the electrodes was more effective in reducing pumping losses than merely increasing the carbon particle concentration. These findings provide insights for optimizing slurry electrodes and contribute to the development of efficient energy storage systems.