Session RV: Suspensions III

A falling cloud of particles at small but finite Reynolds number

Through a comparison between experiments and numerical simulations, we have examined the dynamics of a cloud of spheres at small but finite Reynolds number. The cloud is seen to flatten and to transition into a torus which further widens and eventually breaks up into droplets. While this behaviour bears some similarity with that observed at zero-inertia, the underlying physical mechanisms differ. Moreover, the evolution of the cloud deformation is accelerated as inertia is increased. Two inertial regimes where macro-scale inertia and micro-scale inertia become successively dominant are clearly identified.   

Sedimentation and Effective Temperature of Active Colloidal Suspensions

We investigate experimentally the non-equilibrium steady state of an active colloidal suspension under gravity field. The active particles are made of chemically powered colloids, showing self propulsion in the presence of an added fuel, here hydrogen peroxide. The active suspension is studied in a dedicated microfuidic device, made of permeable gel microstructures. Both the microdynamics of individual colloids and the global stationary state of the suspension under gravity are measured with optical microscopy. This yields a direct measurement of the effective temperature of the active system as a function of the particle activity, on the basis of the fluctuation-dissipation relationship. Our work is a first step in the experimental exploration of the out-of-equilibrium properties of active colloidal systems. Working along this line, we also present first signatures of collective properties in active suspensions.   

Probing particle transport in closed-streamline flows with microfluidic devices

We use microfluidic devices to study the flow of neutrally-buoyant suspension around bluff-bodies. We use low-viscosity liquids and monodisperse particles of diameter below 10 $\mu $m at a constant concentration of 8.4 volume {\%}. Several bluff-body geometries are introduced and by using high-speed video imaging we observe a striking segregation of the particles and fluid in the wake region at elevated Reynolds numbers. Based on 2-D and 3-D flow field simulations, we interpret the migration of particles and their trajectories across the streamlines based on the geometry of the bluff-body. Experimental observations reveal that if particles are forced into an initially particle-depleted region, they will eventually leave and will bring the system to its original state.   

Fluid Mechanics of Cellulose Fiber Suspensions Using MRI

Efficient processing of fibrous biomass requires understanding the mechanics of fiber suspensions having large particle sizes of biomass particles, fast settling, and entanglements. Direct imaging of velocity profiles using magnetic resonance imaging provides a way of characterizing flow in the presence of such non-idealities. We found a strong influence of fiber length, concentrations and flow rates on velocity profiles and pressure drops. We map different regions in the concentration-velocity plane that serve as a guide to decide whether or not to use generalized newtonian rheological models. The concentration effects were best described by the use of a crowding number, with large changes in pressure and velocity profiles occurring in a narrow range of crowding numbers. Qualitative differences between the behavior of the long fibers and the short and medium fibers demonstrate a strong effect of fiber aspect ratio on rheology.   

The suspension balance model revisited

This paper addresses a fundamental discrepancy between the suspension balance model and other two-phase flow formulations. The former was proposed to capture the shear-induced migration of particles in Stokesian suspensions, and hinges on the presence of a particle phase stress to drive particle migration. This stress is taken to be the ``particle stress'', defined as the particle contribution to the suspension stress. On the other hand, the two-phase flow equations derived in several studies show only an average force acting on the particle phase, but no stress. We show that the identification of the particle phase stress with the particle stress in the suspension balance model is incorrect, but there exists a well-defined particle phase stress. Following the rigorous method of volume averaging, we show that the average force on the particle phase may be written as the sum of an inter-phase force and the divergence of the particle phase stress. We derive exact relations for these quantities. We also comment on the interpretations and results of previous studies that are based on the identification of the particle phase stress with the particle stress.   

Coupling Between Translational and Orientational Ordering in Fiber Suspensions

Suspensions of non-colloidal particles under slow periodic strain undergo a non-equilibrium dynamical phase transition from an absorbing state to an active fluctuating state. In the case of spherical particles, this critical absorbing-phase transition is observed at a single strain amplitude. In the case of rod-like particles, the transition between fluctuating and absorbing phase is observed over a continuous range of applied strain amplitude: Orientational degrees of freedom couple to translational degrees of freedom, expanding the critical domain from a point to a line. Experiments and calculations show the orientation distribution of the rods with time and its relation with respect to the critical strains. Power-law relaxations are observed close to criticality and the measured exponents are consistent with Manna universality class of directed percolation models.