A Particle Tracking Velocimetry method to measure size and charge distributions in tribocharged powder particles
Triboelectric charging of dispersed powders is not a fully understood phenomenon that can nonetheless be of use in new applications like dry food separation. The separation process happens at the particle scale and in dispersed particulate materials, thus it is crucial to have data about charge distribution of the particles. For experiments to provide more significant results it is important to obtain both particle electrical charge and size. Toward this end, we have refined a Particle Tracking Velocimetry method (PTV). Image acquisition has been improved to get better image resolution and recording speed, what allows to determine particle size. From the theoretical perspective, we have accounted for the drag and electric forces in the trajectory analysis of charged particles since they are the main actors influencing the particle's movement. A dispersion unit, analysis cell and high speed camera are used to disperse powders in a gas stream and to register the trajectories under the influence of a homogeneous electric field. By considering the acting forces over the particle's movement, its electric charge can be calculated. The system operated dispersions with 10% of solid to gas mass ratio, intending to make particle-particle collisions negligible with respect to particle-wall collisions. We could check that gas regime inside the analysis cell is laminar, since flow turbulences can alter the particle's path thus invalidating the measurement. Single component granular materials and binary mixtures are studied to evaluate the effect of particle size, material and composition on the charge distribution. All cases show bipolar distributions and dependence on particle size. Mixture composition affects the overall charge polarity in the dispersed powder and this effect can not solely be attributed to the weight proportion of the components. As opposed to previous works, we have not found any preference for smaller/larger particles to acquire negative/positive charge, respectively. A PTV technique can be of great help to assess single particle charges in setups and equipment used for triboelectric dry separation of organic and inorganic products, because the efficiency of the separation process relies much on the charge to mass ratio of the particles, and these can be evaluated with PTV for a set of changing process and design parameters like construction materials, flow conditions or geometric design of the equipment.