The charge of a falling water drop

Fluid triboelectrification, also known as flow electrification, remains an under-explored yet impactful phenomenon with potential applications across various fields. Building upon previous efforts to position water within the triboelectric series, we investigate the charge on individual, millimetric water drops falling through air. Our experiments utilize careful electrostatic measurements of charge using a Faraday cup and an electrometer mounted on a scale which monitored the mass increase of each drop. First, for pure water in a glass syringe with a grounded metal tip, we find the smallest amount of charge, approximately -3 pC/g (-0.7 V/drop). This was independent of the release height of the drop, needle diameter and length, needle cleaning preparation (such as plasma cleaning), and whether the experiment was shielded with a Faraday cage. This charge is due to the relative voltage between the liquid and the grounded metal, biasing the needle to different voltages allowed linear control of the drop charge. Introducing plastic into the experiment (whether the syringe body or tip) drastically changed the drop charge. For example, using a PTFE tip led to an increasing voltage on each drop that scaled with the tip length, yet the absolute charge strongly depended on the cleaning procedure of the plastic. These results suggest that surface chemistry with non-conducting materials is the dominant factor affecting flow electrification of dripping water drops.