Droplet microfluidics on a planar surface

This work reports on the modelling of, and experiments on, a method in which liquid is transported as droplets on a planar hydrophobic surface with no moving parts, merely through electrostatic forces generated by the underlying electrodes. Two-directional transportation along a straight electrode path and across a junction, fusion of two droplets and methods for importing, exporting and filtering of water droplets were demonstrated, and can be used as basic functions of a lab-on-a-chip type microfluidic system. In this work, the electrostatic droplet actuation is for the first time demonstrated on superhydrophobic surfaces. Such surfaces are composed of air-filled pores and exhibit a very low droplet sliding resistance due to reduced contact angle hysteresis and a high water contact angle (usually > 150°). This work shows that superhydrophobic surfaces can be used to reduce the minimum voltage and to increase the maximum speed under certain conditions, but there are some harmful side-effects. First of all, the electrostatic pressure can push water into the surface pores, which hinders actuation. The phenomenon can also be treated as a vertical electrowetting effect. Another drawback is that the use of superhydrophobic surfaces makes actuation more critical to the properties of the liquid. For example, actuation of biological buffer solutions was not successful. For these reasons, it is concluded that it is more beneficial to use a smooth surface with low hysteresis than a superhydrophobic surface in droplet actuation. Electrostatic droplet actuation is a potential method for manipulating liquid on a microscopic scale, but there is still work to do. This work contains a detailed examination of the droplet actuation mechanism, and trapping of charges in the solid-liquid interface is found to be the most severe problem that needs to be solved.