Complex Fluid Dynamics
Butler Research Group
We have systematically evaluated the irreversible dynamics for particles suspended at high concentration in sheared, viscous fluids. The equations of motion for individual particles are reversible, so collective effects between particles must be the source of the irreversibilities when many particles are present. Our experiments and numerical analysis indicates that particle collisions, rather than chaotic hydrodynamic interactions between particles, are the sole cause of irreversibilities. This finding for shearing flows of neutrally buoyant particles directly contrasts with the well-known result that long-range hydrodynamic interactions are a source of chaos in the sedimentation of particles.
Evidence includes experiments (play video, left) demonstrating that particles, initially in contact, do not return to their original positions upon reversing the flow. However, the particles positions were reversible after the second and subsequent cycles of shear since they did not come into contact.
In concentrated suspensions, a transition from reversible to irreversible particle motion is observed as the strain amplitude γo of an oscillatory shear flow increases. The video (left) shows spatio-temporal plots of images from experiments with particles suspended at a volume fraction of 0.25 with three different strain amplitudes. At the higher strain amplitudes, particles collide and the positions are irreversible. At low strain amplitudes, particles do not collide and the positions are reversible.
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