Collisional Charging of Individual Sub-Millimeter Particles: Using Ultrasonic Levitation to Initiate and Track Charge Transfer

TL;DR

This study introduces an ultrasonic levitation method to precisely initiate and measure charge transfer in individual sub-millimeter particles during collisions, revealing the influence of surface hydrophobicity and environment on electrostatic charging.

Contribution

The paper presents a novel acoustic levitation technique enabling controlled collision experiments and in-situ charge measurement of single particles, advancing understanding of particle charging mechanisms.

Findings

Charge transfer depends on surface hydrophobicity.

Hydrophilic surfaces develop positive charges after contact with hydrophobic ones.

Charging is affected by environmental pH and electric fields.

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.