Fig. 4: Quantification of the excess electrical charges carried by water droplets during pipetting.

a Removing droplet: we placed 1 mL of water (reservoir) inside a Faraday cup connected to an electrometer and extracted 15–50 μL aliquots at a time. Dispensing droplet: next, we dispensed the exctracted droplets into an electrometer to quantify their excess charges. We found that the charges incurred by the water reservoir and the withdrawn aliquots were equal and opposite. b Charge balance: charging of the water reservoir and the withdrawn aliquot using capillaries made of borosilicate glass, polypropylene, FDTS-coated glass, and PTFE. The charges on the water reservoir after withdrawing water aliquots using different capillaries are presented in red; commensurate (opposite) charges on the aliquots are presented in blue. (Note: the y-axis presents charge density, obtained by normalizing the charges by the liquid-solid interfacial area). Error bars represent the standard deviation of ten measurements. c Charge density of over 300 water droplets (50 μL each) loaded and dispensed from the same polypropylene capillary into an electrometer. The red dotted lines represent the \(\pm 1\) standard deviations about the mean value of the surface charge density of polypropylene: \(\sigma = - 0.7 \pm 0.1\) nC cm⁻² or 43 \(\pm 7\) # μm⁻².