Fig. 3: Experiments inside a parallel plate capacitor.

Correlations between the charge density of pendant droplets and electric field strengths for water-filled and air-filled hydrophobic and hydrophilic capillaries (based on Supplementary Movies 4–10). Insets depict experimental configuration with the pendant water droplets inside the parallel plate capacitor with the positively charged plate on the left-side. Note: the red arrows present qualitatively the direction and the relative magnitudes of the deflections experienced by water droplets under various scenarios. When the hydrophobic capillaries were a air-filled (or c water-filled), the pendant water droplets repelled away from (or attracted towards) the positively charged plate. In the case of the hydrophilic glass capillaries, the droplets always tilted towards the positive plate of the capacitor, and we detected very low charges in b the air-filled case, and higher charges in d the water-filled case. Note: we found no deflection for droplets of methanol and hexadecane dispensed from air-filled hydrophobic capillaries in the specified range of electric fields. We obtained the charge density along the y-axis by normalizing the charge of the droplet obtained from the force balance [Eqs. (1–5)] and liquid-solid interfacial areas of the dispensed droplets. (Dotted lines have been added to guide the eye.) Error bars represent the standard deviation of duplicates. The hollow symbols represent the air-filled cases, and the solid symbols represent the water-filled cases for each interface (Color scheme: blue: PP-water; red: glass-water; magenta: FDTS-water; black: PP-Methanol; green: PP-hexadecane).