Direct observation of particle interactions and clustering in charged granular streams

Abstract

Clustering of fine particles is of crucial importance in settings ranging from the early stages of planet formation [1][2][3] to the coagulation of industrial powders and airborne pollutants [4][5][6][7] . Models of such clustering typically focus on inelastic deformation and cohesion 1,4,6,8 . However, even in charge-neutral particle systems comprising grains of the same dielectric material, tribocharging can generate large amounts of net positive or negative charge on individual particles, resulting in long-range electrostatic forces [9][10][11] . The e ects of such forces on cluster formation are not well understood and have so far not been studied in situ. Here we report the first observations of individual collide-and-capture events between charged submillimetre particles, including Kepler-like orbits. Charged particles can become trapped in their mutual electrostatic energy well and aggregate via multiple bounces. This enables the initiation of clustering at relative velocities much larger than the upper limit for sticking after a head-on collision, a long-standing issue known from pre-planetary dust aggregation 1,12 . Moreover, Coulomb interactions together with dielectric polarization are found to stabilize characteristic molecule-like configurations, providing new insights for the modelling of clustering dynamics in a wide range of microscopic dielectric systems, such as charged polarizable ions, biomolecules and colloids [13][14][15][16] . One of the key difficulties in studying the interplay between repulsive contact forces, short-range cohesion and long-range electrostatic forces during cluster formation has been to obtain sufficiently detailed experimental data. Seeing how this process unfolds demands in situ observation of the collision trajectories among charged grains to extract quantitative information about their interactions. This requires the grains to be freed from gravity and tracked with high spatial and temporal resolution to capture individual collision events 17,18 . We overcome these obstacles with the set-up shown in Fig. 1a (refs 19,20). The granular material, in our experiments fused zirconium dioxide-silicate grains a few hundred micrometres in diameter, is contained in a vessel mounted inside a 3.0-m-tall cylindrical chamber. We evacuate this chamber to mitigate air drag. When a shutter covering a small orifice at the bottom of the vessel is opened, particles fall out freely, forming a dilute stream. Outside the chamber, a high-speed video camera falls alongside the grains, guided by low-friction rails. In the co-moving frame seen by the camera, the effect of gravity is eliminated, making it possible to track particle interactions in detail for about 0.2 s until the camera is decelerated by a foam pad. The same apparatus also allows determination of the net charge on individual grains: during free fall a horizontal electric field can be applied and the resulting horizontal acceleration observed by the camera gives the charge to mass ratio, q/m.Using particles with a narrow size d...

Small-Scale Particle Interactions Are Having Significant Effects on Global Fluidized Bed Behavior

“…In addition, the surface morphology is likely to affect the efficacy of charge transfer during contact, which in turn affects long-ranged electrostatic interactions. Lee et al (2015) used high-speed video of 300 micron, free-falling zirconium dioxide silicate particles to explore how electrostatic charging increases the efficiency of particle capture and aggregation processes. They were able to track in detail how particles attract each other over distances of 100's of microns, undergo multiple collisions, and finally stick to each other.…”

“…Asperities from a rough surface could have a similar effect (Wilhelm and Kwauk, 1948). Lee et al (2015) also noted that cluster formation often requires multiple bounces to dissipate the kinetic energy such that cohesion can dominate. Clustering was at its lowest probability for the collision of two particles.…”

Early-stage aggregation in three-dimensional charged granular gas

“…In fact, a collide-and-capture mechanism has been observed experimentally in [14], for collisional charging in a falling dilute granular stream. In the experiments [14], the particles collide, bounce multiple times and then tend to stick together giving rise to local aggregates. One particular observation made in [14] is that when a single particle hits a cluster, it can either get trapped in the electrostatic field or can cause other particles to leave the cluster leading to fragmentation.…”

“…Understandably, a vast amount of attention has been put to explore what mechanisms stimulate the charge buildup, separation, transport and its effect on the dynamics of granular flows [1,3,8,17,24,38]. Additionally, the collective behavior is are unclear in spite of great experimental [5,14,16] FIG. 2.…”

“…The origin of the above intriguing processes inside a granular gas is due to the charging of particles during collisions. This phenomenon, however, is rather stochastic as indicated by experimental measurements [5,14,15]. On the other hand, the consequences of this local exchange on a collection of particles have been experimentally observed to be quite complex as it shows highly fluctuating characteristics [16].…”