Dynamics of concentrated suspensions under vibration in a Hele-Shaw cell

ROSEN M,; M.A.AGUIRRE; A. BOSCHAN; BARBA MAGGI; MARTINO R,; PIVA M

Puerto Varas

Workshop; Southern Workshop on Granular Materials 2018; 2018

Universidad de Chile

Particulate suspensions appear in a wide variety of industrial processes (pharmaceutical, cosmetics, paints, coatings, oil: drilling mud and holding agents). For this reason, the study of its dynamics is currently receiving a lot of attention, both from the basic point of view and from the applications. In the present work, experimental results are presented on the motion initiation and resuspension of micrometric particles deposited in a thin layer of a free surface fluid that is vertically vibrated. It is well known that from a certain value of the excitation frequency a type of subharmonic instability known as Faraday instability is triggered. The instability manifests itself as a smooth, periodic and stationary modulation of the free surface of the fluid. The secondary flow induced by Faraday waves is the mechanism responsible for the resuspension of particles. The problem is complex since the instability depends on both the thickness of the fluid layer and its viscosity, among other factors. However, the effective thickness of the layer changes when the particles are resuspended and the rheological properties of the fluid are modified by the presence of particles in suspension.For the experiments, a narrow container (Hele-Shaw cell) was used, formed by two parallel acrylic plates located 1.87mm apart, mounted on a rigid frame. The registration of the phenomenology was carried out by means of a fast camera. The container was mounted on the shaft of a shaker controlled from a function generator. The working particles, initially sedimented in the bottom of the container, are micrometric spheres of high monodispersivity of 40 $\mu$m and 1.05 g/c$m^3$ of density. As a carrier fluid, a controlled mixture of ethanol and glycerin was used. The excitation frequency was set for all tests at the supercritical value of 24 Hz. In experiments, the acceleration $\Gamma$ = A$\omega^2$ was varied, where A and $\omega$ are the amplitude and the angular frequency of the excitation, respectively, and $\Delta$h, the height of the free surface of the fluid with respect to the bottom of sedimented particles. Initially, the interface between particles and fluid is flat. The study focused on the modulation of the sediment bottom interface, as an indicator of particle movement. From this, the initiation of movement thresholds for a wide range of parameters were obtained. The results are analized in terms of the Shield model for the incipient motion of small particles.