Charge Transfer Altered by Particle Deposition as a Contact Line Moves over a Hydrophobic Surface

An aqueous three-phase contact line moving over a hydrophobic surface is known to give rise to electrical charge transfer in a process that is sometimes referred to as slide electrification. Here it is shown that the charge transfer is significantly altered if the liquid contains small particles that adhere to the solid surface. For TiO₂ nanoparticles in water, it is found that the charge transfer decays nearly exponentially to a very small value with a time constant that depends on the particle concentration. The increase in particle area coverage on the solid surface is correlated to the reduction in charge transfer, and a simple theory is developed to explain this behavior. Further studies of two different types of carbon particles in water reveal that the charge transfer initially increases before decaying even though the particle area coverage monotonously increases with time. It is suggested that this behavior is due to the release of ions, which increase the charge available to be transferred from the electrical double layer.