Liquid microdroplet sliding on hydrophobic surfaces in the presence of an electric field.

Liquid droplet sliding is an approach for the transportation and manipulation of fluids in micro/nanofluidics related biosensor applications. Liquid droplet sliding is also instructive to evaluate the degree of boundary slip. In this study, liquid microdroplet sliding is studied with and without the presence of an electric field using atomic force microscopy on a polystyrene surface spin coated on a doped silicon wafer with a silicon oxide coating. Droplets with different diameters are slid with an atomic force microscope tip, and the sliding forces or lateral friction forces are quantitatively measured. Experimental results show that lateral friction force linearly increases with increasing droplet diameters, which can be explained by a droplet sliding model. When the electric field is applied, in addition to the decreased contact angle, diameters of the liquid droplets will increase. As a result, the electric field increases lateral friction force. To further investigate the impact of applied potential on lateral friction force, voltages from 0 to 80 V are applied to a droplet. Lateral friction force is found to increase with applied potential.