  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.
