Mechanism of Silica Charging
Contact charging of sand particles, essentially silica, is responsible for the lightning observed in natural outcomes like sand storms, dust devils, and haboobs. Field studies have found that the updraft of air separates the positively charged lighter particles, which remain levitated in the atmosphere and the negatively charged larger particles which settle on to the ground. The lightning occurs between the two oppositely polarized parts. Although the contact charging has immense importance, the basic physics behind the mechanism of charge transfer is not yet well understood. We have designed experiments to microscopically understand the charge transfer mechanism when two clean and pre-treated silica surfaces come in contact.
Figure 1. Experimental setup
Our goal is to investigate the effects of relative humidity, temperature, pressure and surface defects on the extent of charge separation. Our findings, so far, indicate a systematic effect of relative humidity on the charge separation. As we increase the relative humidity in our contact charging experiment between two silica surfaces, the extent of charge separation decreases. This prompts us to propose a mechanism for charge transfer in which the water adlayer on the surface of silica plays an important role. Adsorbed water hydrolyses the terminal silica groups at the surface to silanol and more water deprotonates those silanol groups to generate hydronium ions in the water adlayer. During the contact of silica surfaces, transfer of ions occur. This is most likely due to temperature gradient generated during the contact process (Soret Effect), and hence the surfaces get a net charge after being separated. More experiments are being done to confirm the role of water adlayer like functionalizing the surface of silica to prevent ion production at the surface and studying the conductivity within the water adlayer.
Figure 2. Two possible mechanisms of charge transfer: a) Transfer of an electron and b) Transfer of an ion
Image revised from: McCarty, L.S.; Whitesides, G.M. Angewandte Chemie-International Edition 2008, 47, 2188-2207