Spontaneous Charging from Sliding Water Drops Determines the Interfacial Deposition of Charged Solutes

It has been discovered during the last decade that when water drops slide on hydrophobic surfaces, they spontaneously leave negative charges along the drop path. The drops become positively charged with a potential of 1 kV. This process, called slide electrification, influences drop motion and alters contact angles. Here, a third effect of slide electrification is demonstrated: the preferential deposition of dissolved solutes with positive charges. To illustrate this, water drops containing dissolved charged fluorophore ions are allowed to slide down a tilted hydrophobic surface, and their track is imaged. Two perylene derivatives are applied as fluorophores, one chromophore carrying positive charges, PDI⁺, and one carrying negative charges, PDI^─. PDI⁺ is deposited at a concentration as low as 0.5 µm. In contrast, PDI^─ is only deposited above 5 µm. Experiments using grounded drops or a hydrophobic coating on a conducting substrate indicate that the electric field generated from the negative surface charges behind the drop causes a preferential deposition of the dissolved ions near the interface. This hypothesis also agrees with Kelvin probe measurements. Complex biomolecules deposition e.g. DNA can be also affected by this. These findings contribute to a better understanding of mass transfer processes at interfaces.