Glancing at sedimenting invisible particles: a Ghost Particle Velocimetry setup (poster)
Abstract
Sedimentation is the widespread natural phenomenon of the deposition of particulate driven by gravity. While our knowledge on the process at the equilibrium condition has reached a rather mature stage, this is not for the kinetics of sedimentation, in which the solvent mediated, long ranged, hydrodynamic interactions between the particles must be taken into account. Despite its illusory conceptual simplicity, sedimentation still needs fundamental understanding, a rather challenging task both from the theoretical and the experimental point of view. In particular, measurements of the fluctuations of the settling velocity are still limited to the case of non-Brownian suspensions, where the presence of big particles (~10µm) makes it possible to employ Particle Image Velocimetry techniques. Here we show that using a low angle scattering setup, conceptually similar to the one proposed in, it is possible to obtain statistical information on the settling velocity (including spatial and temporal correlations of the fluctuations) from the speckle field produced by dilute suspensions of sedimenting subresolution Brownian particles. Moreover, the feasibility to ‘look inside’ a falling suspension gives us the chance to obtain useful insights on other curious effects, such that reported in for sedimenting mixtures of particles with different size and settling velocity, where the settling front rapidly becomes unstable, gets tilted and progressively develops a striped texture. In order to grasp the basic features of these kind of instabilities, it is necessary to measure the velocity field inside the suspension. We believe this setup could provide novel, interesting information on the behavior of settling suspensions.
