Collisional charging of individual submillimeter particles: Using ultrasonic levitation to initiate and track charge transfer

Abstract

Electrostatic charging of insulating fine particles can be responsible for numerous phenomena ranging from lightning in volcanic plumes to dust explosions. However, even basic aspects of how fine particles become charged are still unclear. Studying particle charging is challenging because it usually involves the complexities associated with many particle collisions. To address these issues we introduce a method based on acoustic levitation, which makes it possible to initiate sequences of repeated collisions of a single sub-millimeter particle with a flat plate, and to precisely measure the particle charge in-situ after each collision. We show that collisional charge transfer between insulators is dependent on the hydrophobicity of the contacting surfaces. We use glass, which we modify by attaching nonpolar molecules to the particle, the plate, or both. We find that hydrophilic surfaces develop significant positive charges after contacting hydrophobic surfaces. Moreover, we demonstrate that charging between a hydrophilic and a hydrophobic surface is suppressed in an acidic environment and enhanced in a basic one. Application of an electric field during each collision is found to modify the charge transfer, again depending on surface hydrophobicity. We discuss these results within the context of contact charging due to ion transfer and show that they lend strong support to OH − ions as the charge carriers.

Precision measurement of tribocharging in acoustically levitated sub-millimeter grains

“…The buildup of charge due to repeated collisions between small particles is thought to be responsible for lightning in volcanic ash clouds 3 , the electrification of grains in sand storms 4 , and potentially also the very early stages of the formation of planetesimals from interstellar dust 5,6 . Several controlling parameters for particle charging have been measured, notably the effects of particle size 7,8 , atmospheric conditions and external electric fields 9,10 , surface hydrophobicity 11 , frequency of contact 12 , and kinetic energy prior to impact 13 . However, the underlying mechanism for charge exchange remains an area of debate, particularly between insulators, which have very low charge mobility.…”

“…Several candidates for the charge carrying species have been suggested, including electrons in trapped surface states [14][15][16] , ions in atomically thin water layers 11,17,18 , and mechanoradicals produced during contact 19,20 . Elucidating the fundamental mechanism of collisional charging between grains thus calls for systematic, quantitative experiments.…”

“…These experiments typically involve a large number of particles, which results in both particle-wall and particle-particle collisions, hindering access to the basic physics of a single collision. Recent experimental techniques have made precise measurements of the impact charging of a single submillimeter particle with a fixed substrate 11,[25][26][27] . There remains, however, a need to track the evolution of the charging process over repeated, highly controlled collisions between a pair of grains.…”

“…There remains, however, a need to track the evolution of the charging process over repeated, highly controlled collisions between a pair of grains. a) jaeger@uchicago.edu Here, we introduce such a method by combining highspeed videography and acoustic levitation, allowing for the contact-free manipulation of a wide variety of constituent materials, particle sizes, and shapes 11,28,29 . We dynamically control the location of stable levitation positions within the acoustic field, generating controlled collisions between a pair of particles.…”

Sticking and restitution in collisions of charged sub-mm dielectric grains

“…Our choice of method is the direct observation of individual low-speed collisions under microgravity; we concentrate on collisions between charged grains and a metal (copper) wall. We do not analyze the charging or the charge transfer during a single collision with the wall itself, although this is certainly an important part of the whole story (Lee et al 2018). Our aim is to quantify the threshold velocities below which grains stick to the wall and to quantify the coefficients of restitution depending on charge.…”