Electrochemical Droplet Cells for Detecting Triboelectric Charge

Abstract

Triboelectrically charged surfaces are important in a range of technologies, but it remains difficult to measure charge density on these surfaces, particularly at intermediate length scales. Here, we use an electrochemical droplet cell to measure local triboelectric charge densities on insulating polymer surfaces, with a spatial resolution of up to 100 μm. A glass capillary filled with electrolyte and containing a Ag/AgCl electrode is approached vertically toward a triboelectrically charged surface until the droplet contacts the surface. The resulting transient current, measured at the Ag/AgCl electrode positioned within the glass capillary more than 1 cm from the surface, reflects charge equilibration between the fixed surface charge and ions in the droplet and can be integrated to give the local surface charge density. For PTFE, we measure charge densities of approximately −2.6 nC/cm2 on charged surfaces and −0.17 nC/cm2 on uncharged surfaces, in good agreement with reported values. The sign of the transient reverses when the sample is changed from negatively charged PTFE to positively charged glass, confirming sensitivity to charge polarity. Repeated contacts at the same position distinguish between the initial mechanically induced surface charge and spontaneous charge separation upon liquid–solid separation within the second droplet contact. Varying the droplet size shows that the measured charge scales linearly with contact area. This simple approach bridges the gap between macroscopic and nanoscale methods for measuring surface charge and provides a simple tool for studying triboelectric charge on technologically relevant materials.