Session P07: Suspensions: Confined Flows (3:10pm - 3:55pm CST)

Spreading of granular suspensions on a solid surface

The investigation of the spreading of granular suspensions on a solid surface indicates that the apparent viscosity of these suspensions in the vicinity of the triple-phase contact line is dependent on particle size in addition to particle volume fraction. This observation results from the existence of a particle-depleted region at the contact line with beads expelled due to confinement. Our experimental observation leads to interesting outcomes for the spreading of suspension mixture having different particle sizes and further opens a new possible way to control fluid flow.   

Miscible fingering in two-dimensional suspension flows

We study the emergence of miscible fingering in quasi-two-dimensional suspension flows. We experimentally demonstrate the existence of fingering with two distinct wavelengths, by injecting silicone oil into the mixture of the same oil and non-colloidal particles inside a highly confined channel. Here, the channel gap thickness is comparable to the diameter of the particles, so that suspended particles form a monolayer inside the cell. Our experiments demonstrate that the transition in the fingering regimes depends on the local particle area fraction. To qualitatively describe our experimental observations, we seek a kinetic theory that implements long-range hydrodynamic interactions.   

Filtration by porous media: a microfluidics approach

The transport of colloids in porous media is governed by deposition on solid surfaces and pore-scale flow variability. Classical approaches, like colloid filtration theory (CFT), do not capture behaviours observed experimentally, such as non-exponential steady state deposition profiles and heavy tailed BreakThrough Curves (BTC). In the framework of CFT, a key assumption is that the colloid attachment rate $k$ is constant and empirically estimated via a posteriori macroscopic data fitting. We design a novel experimental set-up based on time-lapse microscopy and continuous injection of fluorescent monodisperse colloids into a folded microfluidics device (1mt total length) designed with a controlled level of 2D spatial disorder. This set-up allows us to i) measure both BTC and deposition profile over several orders of magnitude and ii) to perform particle tracking and Lagrangian analysis of single colloid's trajectories. Based on this analysis, we propose a stochastic model that takes into account pore scale heterogeneities in terms of correlation length, velocity and attachment rate distribution, that captures the anomalous behaviour shown by the experimental data.   

Numerical study of a semi-dilute, non-colloidal suspension in a Taylor-Couette flow

We conduct direct numerical simulations for the Taylor-Couette flow of semi-dilute, neutrally buoyant, and non-colloidal suspensions using suspension balance model (SBM) and rheological constitutive laws. In this study, we considered the Taylor-Couett with rotating inner cylinder and stationary outer one. In additon, the radius ratio of the Couette apparatus is constant $\eta =$0.877, the bulk particle volume fraction is fixed $\phi_{b}=$0.1, but the particle size ratios varies, i.e., $\epsilon (=d$/$a)=$60, 200, where $d$ is the gap with and $a$ is the radius of particles. The flow instability and transitions affected by the suspended particles are examined by varying the Reynolds number of suspensions based on the rotating angular velocity and the effective viscosity of suspensions. The critical Reynolds number in which counter-rotating vortices arise in the annulus is predicted, and the nature of the instability is investigated. Flow and particle concentration fields are shown in detail to characterize the flow structure and particle distribution. The sequence of transitions is determined by the flow pattern. Furthermore, friction and torque coefficient of the suspension flow are computed and compared with the values obtained for pure fluid flows.   

Wave propagation, diffusive relaxation, and fragmentation instabilities in flow-driven drop chains under quasi-2D confinement

Drop chains can spontaneously emerge in confined emulsion flows and are frequently present in microfluidic systems. Here we discuss the dependence of the drop-chain dynamics on flow symmetries. For an antisymmetric incident flow (Poiseuille flow) the leading-order interparticle hydrodynamic interactions occur via Hele--Shaw flow dipoles. The vector-like symmetry of these interactions results in density wave propagation in the direction of the dipole orientation. For a symmetric incident flow (Couette flow), the leading-order interparticle interactions are quadrupolar. Due to the fore--aft symmetry of Hele--Shaw quadrupoles, the macroscopic chain dynamics is diffusive, because there in no first-order spatial derivative in the evolution equation. The sign of the diffusion constant depends on the balance between the quadrupolar interactions and near-field hydrodynamic repulsion (the repulsion stems from the swapping-trajectory effect and drop deformation). Density perturbations decay for a positive diffusion constant; for a negative value they grow, eventually leading to the decomposition of a chain into stable chain fragments with a uniform drop spacing.