No contact-manipulation of drops on a liquid-infused surface using electric fields

We propose a simple way to actuate an aqueous droplet sitting on a hydrophobic grounded substrate using a pin electrode placed above it. The highly non-uniform electric field causes the droplet to move towards the pin electrode or follow its movement relative to the substrate, respectively. Drop velocities in excess of 1 cm/s can be achieved. The main challenge in that context was to provide an appropriate hydrophobic surface with low contact-angle hysteresis. This has been addressed by using silicon-oil based liquid infused surfaces that exhibit a contact angle (CA) of around 100° for water and a CA hysteresis below 5°. Using water-glycerol mixtures for the droplets we can vary the viscosity while retaining an almost constant permittivity and minimizing evaporation. The experiments are supplemented by numerical computations of the electrostatic force on a droplet, assuming a perfectly spherical surface. The numerical results show good agreement with the experimental data, from which we conclude that the underlying mechanism is dielectrophoresis. Furthermore, we propose a simplified (closed-form) model for the electric force on the droplet based on the induced dipole moment that fits the experimental data very well.