Triboelectric Charging and Electrostatic Effects in Granular and Particle-Laden Flows

It is well known that materials acquire static charges upon contact with other materials [1], a phenomenon commonly referred to as tribocharging. In fluidized beds, granular particles often undergo tribocharging due to particle-particle and particle-wall collisions [1]. Particles of different sizes made of the same material have been known to charge differently [2]. Some studies found that bigger particles tend to charge positively while smaller particles charge negatively [2,3], but the opposite trend has also been reported [4]; this has been attributed to non-equilibrium dynamics leading to asymmetric distributions of charge carrier on different particle sizes [5]. These models do not consider the effect of electric field that could develop due to many body effects. This consideration led to the present study where we address the following two questions: (1) Would tribocharging of a vibrated bed of particles at the walls of the container (which is made of different material) show any systematic effect of particle size? (2) Could a bidisperse mixture of particles of the same substance in a vibrated or fluidized bed whose walls are made of a different material acquire bipolar charging as a result of the interplay of tribocharging and flow?

To address the first question, we conducted vibrated bed experiments in polycarbonate and acrylic containers of different sizes, using soda lime glass particles of different size ranges. In each experiment, we chose one combination of particle size range and mass loading and shook the particles for a long time in a dry nitrogen environment. The total charge of the particles was then measured using a Faraday cup. A Discrete-element-method (DEM) simulation [6] complemented each experiment in order to determine the effective work function difference, a quantity that characterizes particle charging [7], between the particles and container wall that would yield the same level of charge. The relation between total charge and total surface area was analyzed across all particle sizes and both containers. The analysis reveals little systematic effect of particle size on the effective work function difference.

To address the second question, we performed CFD-DEM simulations of fluidized beds and vibrated beds containing a bidisperse assembly of particles of the same material (and so the particle-wall effective work function difference is the same for both particles sizes), where we also considered tribocharging. Surprisingly, bipolar charging developed in these simulations, exposing a new pathway for the emergence of particle size effect.

The details of these two studies will be described in this poster presentation.