Manipulation of Contact Angles and Interfacial Lengths of Liquid Drops using Electro-Kinetic Techniques

Abstract:

Traditionally, capillary pressure is determined by increasing or decreasing external fluid pressures to change the immiscible fluid saturation in a porous medium. The resulting saturation and interfacial area are then linked to the capillary pressure through constitutive equations. A key question is whether externally measured pressures are sensitive to changes in distributions that arise from internal changes in contact angles. As a first step in addressing this question, we investigated the effect of electro-kinetic manipulation on interfacial area and contact angles for a fixed saturation.

An EWOD (electro-wetting on dielectric) technique was used to alter the contact angle of single 10 µL droplets of a 1M KCl-H₂O solution. A liquid droplet was placed on a glass cover slip (18 mm x 18 mm) coated with a layer of silver (100 nm in thickness) to act as an electrode and then spin-coated with polyimide (a dielectric). A platinum wire was inserted into the droplet and connected to an AC voltage source. The glass plate electrode was connected to ground. Measurements were made for V_rms voltages between 0 to 300 V at a frequency of 50 Hz. Two CCD cameras were used to image changes in the shape of a droplet. One camera was placed on a microscope to capture a top view of a drop in order to measure changes in areal extent and the perimeter of the drop. The second camera imaged a drop from the side to measure contact angles and side-view areal extent and perimeter.

At low voltages, the cosine of the contact angle, θ, after applying voltage was linearly dependent on V_rms². Several experiments showed that the slope of the low-voltage relationship of cos θ vs V_rms² remained constant for all trials. As the voltage increased, the contact angle saturated. From the side-view images, the contact angle and interfacial length decreased with increasing voltage. From the top-view images, the drop shape changed from circular to elliptical-to irregular as the voltage increased, yielding an increase in interfacial length as the contact angle decreased. These experiments demonstrate that EWOD techniques can be used to alter contact areas and interfacial area without the need for an external pressure source.

Acknowledgment: This research was supported by the National Science Foundation (1314663-EAR).