Field-induced formation and manipulation of a microscale liquid ball on an outside wall of the pulled pipette

We report a phenomenological observation of electric-field-induced formation and manipulation of liquid ball on an outer wall of the pulled pipette by using the quartz tuning fork-based atomic force microscope (QTF-AFM). The dye molecule solution with excitation wavelength of 488 nm and detection efficiency of 95% is used to investigate the movement characteristics of liquid droplets when the electric field is applied. The ejected liquid solution forms a microscale liquid droplet at the apex of the pipette by the application of electric field, containing dye molecules, which climbs up along the negatively charged outer surface of the pipette due to the electro-osmosis effect. With positive or negative bias voltages, we manipulate a liquid ball to slide upward or downward, respectively. This field-induced transport of a liquid droplet may be useful to nano-biotechnology or droplet-based microfluidic technology, for example, noncontact delivery and manipulation of liquid solution in the form of separated droplets.