Self-generated electrostatic forces of drops rebounding from hydrophobic surfaces

We study the charge separation of drops rebounding from hydrophobic surfaces. Based on high-speed video imaging and the deflection of drops by electric fields, we reproducibly detected the amount of charge. Here, we show that the charge separation of bouncing drops can be 2 orders of magnitude higher on hydrophobic than superhydrophobic surfaces. We observed the existence of self-generated electrostatic forces between the drop and the surface. These forces affect the maximum rebounding height and slow down the retraction motion of drops. We additionally calculated the electrostatic forces using an energy conservation approach. Our results indicate that electrostatic forces on hydrophobic surfaces can be even stronger than gravity, reducing the restitution coefficients up to 50%. This new approach becomes advantageous compared with other methods that require more complicated setups for drop charge detection.