Electrokinetic Phenomena across Length Scales – from Nanofluidics to Sliding Drops

Solid surfaces in contact with a liquid are usually charged and attract a diffuse layer of countercharges in the liquid. Together, surface charge and diffuse countercharge form the electric double layer. The electric double layer generally has a thickness of 1 − 1000 nm. It mediates electrostatic interactions on the nanoscale and for example enables selective membranes, as in desalination or in biological cells. Gate electrodes can be used to manipulate the layer of diffuse charge. Here, we use gate electrodes in two different nanofluidic contexts. We apply an unbiased ac voltage to a gate electrode on the inside wall of a conical nanopore. Even in the absence of pressure, concentration or electric potential gradients across the pore, this yields a substantial directional flow through the pore. This resonant nanopumping is strongest at a characteristic resonance frequency. Additionally, we look at electrostatic fluidic particle traps. They trap individual nanoparticles, subject to Brownian motion, through electrostatic interactions between the electric double layers of the particles and the surrounding walls. We embed a patterned gate electrode into one of the walls. Applying a dc voltage can control the interaction strength and modulate the trapping. Electric double layers also form underneath drops when they sit on solid surfaces. As drops slide, they spontaneously charge and deposit an opposite surface charge. We show how this phenomenon called slide electrification can be explained through transport processes in the electrical double layer at the receding contact line of a drop and we develop a predictive theory for the charge separation. We further show that sliding drops spontaneously acquire voltages of the order of kilovolts. Finally, we show that electrostatic interactions between the drop and the deposited surface charge cause a previously overlooked contribution to contact angle hysteresis.