Combined effect of sedimentation velocity fluctuation and self‐sharpening on interface broadening

Bacterial activity hinders particle sedimentation

Sedimentation in active fluids has come into focus due to the ubiquity of swimming micro-organisms in natural and industrial processes. Here, we investigate sedimentation dynamics of passive particles in a fluid as a function of bacteria E. coli concentration. Results show that the presence of swimming bacteria significantly reduces the speed of the sedimentation front even in the dilute regime, in which the sedimentation speed is expected to be independent of particle concentration. Furthermore, bacteria increase the dispersion of the passive particles, which determines the width of the sedimentation front. For short times, particle sedimentation speed has a linear dependence on bacterial concentration. Mean square displacement data shows, however, that bacterial activity decays over long experimental (sedimentation) times. An advection-diffusion equation coupled to bacteria population dynamics seems to capture concentration profiles relatively well. A single parameter, the ratio of single particle speed to the bacteria flow speed can be used to predict front sedimentation speed.

Settled and unsettled issues in particle settling

Colloid sedimentation has played a seminal role in the development of statistical physics thanks to the celebrated experiments by Perrin, which provided a concrete demonstration of molecular reality and gave strong support to Einstein's theory of Brownian motion. This review, which mostly focuses on settling at low Peclét number, where Brownian fluctuations are dominant, aims to show that a lot more can be learnt both from the sedimentation equilibrium and from the particle settling dynamics of a wide class of systems, ranging from simple colloids to mesogenic suspensions, from soft solids to active particles and living organisms. At the same time, the occurrence of unexpected and surprising effects brings about challenging questions in statistical and fluid mechanics that make sedimentation an exciting field of research.

Particle-like and fluid-like settling of a stratified suspension

The gravitational settling of inhomogeneously suspended particles in a fluid has been investigated. Of particular interest is whether collective or individual motion of particles is dominant during their settlings, i.e., whether the particles settle as a continuous suspension or they settle individually relative to the surrounding fluid. We observed the settling of a stratified suspension which has the lower and upper concentration interfaces in a quasi-two-dimensional vessel. In some cases, the suspension behaves perfectly as a continuous fluid and the motion of the constituent particle is subject to bulk flow caused by the interfacial instability. In other cases, the particle behaves individually relative to the surrounding fluid. The existence of a concentration interface plays a significant role in these extreme behaviors of suspension. The transition from the collective to individual behaviors can be predicted quantitatively by a parameter which expresses the border resolution of the concentration interface.

Particle density stratification in transient sedimentation

Theoretical predictions for the scaling of particle velocity fluctuations with container size in homogeneous Stokes suspensions are not consistent with experimental observations. Several explanations have been advanced, including the formation of stratification in bounded systems, such as those used in experiments. Numerical simulations of transient Stokes sedimentation in bounded cells are presented here for several cell sizes. The simulated cells have top and bottom wall boundaries and periodic boundaries in the horizontal. Throughout the course of the simulations the number and distribution of particles in the cell evolve, with impacts on the bulk mean particle velocity, velocity fluctuations, and particle density gradient. Initially the sedimentation follows the classical description, with a sharp front and uniform particle concentration below, but this is not sustained. A layer of higher particle concentration develops below the front. This is unstable and there is a large-scale overturning of the fluid. As a consequence, there is a redistribution of the particles, leaving behind a mass loading of the particles, which is stably stratified (subject to small density fluctuations between horizontal levels). The mean velocity and fluctuations of the particles initially grow and then decay once stable stratification has developed.

Chemically selective NMR imaging of a 3-component (solid-solid-liquid) sedimenting system

A novel magnetic resonance imaging (MRI) technique which resolves the separate components of the evolving vertical concentration profiles of 3-component non-colloidal suspensions is described. This method exploits the sensitivity of MRI to chemical differences between the three phases to directly image the fluid phase and one of the solid phases, with the third phase obtained by subtraction. 19F spin-echo imaging of a polytetrafluoroethylene (PTFE) oil was interlaced with 1H SPRITE imaging of low-density polyethylene (LDPE) particles. The third phase was comprised of borosilicate glass spheres, which were not visible while imaging the PTFE or LDPE phases. The method is demonstrated by performing measurements on 2-phase materials containing only the floating (LDPE) particles, with the results contrasted to the experimental behaviour of the individual phases in the full 3-phase system. All experiments were performed using nearly monodisperse particles, with initial suspension volume fractions, phi(i), of 0.1.

Nonuniversal velocity fluctuations of sedimenting particles

Velocity fluctuations in sedimentation are studied to investigate the origin of a hypothesized universal scale [P. N. Segre, E. Herbolzheimer, and P. M. Chaikin, Phys. Rev. Lett. 79, 2574 (1997)]. Our experiments show that fluctuations decay continuously in time for sufficiently thick cells, never reaching steady state. Simulations and scaling arguments suggest that the decay arises from increasing vertical stratification of particle concentration due to spreading of the sediment front. The results suggest that the velocity fluctuations in sedimentation depend sensitively on cell geometry.