Transport and Manipulation of Immiscible Fluids using Electro-Kinetic Techniques

Abstract:

Applied electric fields modify the surface tension between a solid and a liquid and hence modify the wettability and contact angle on solid surfaces. Direct control over internal fluid distributions can be achieved by controlling contact angles that subsequently influence capillary pressures and drive motion of droplets across many millimeters. In this study, an Electro-Wetting on a Dielectric technique, EWOD, is used to: alter contact angles, merge and transport droplets on flat surfaces, and control the distribution of fluid phases.

Liquid droplets were supported on flat glass substrates that had been evaporated with a 50 nm thick layer of silver (i.e., ground electrode) and then spin-coated with a ~5-10 µm thick layer of PDMS, a dielectric material. A platinum wire was inserted into 10 µL droplets of 1M KCl-H₂O and connected to a 50 Hz AC voltage source. Measurements were made for a range of voltages (V_rms ~0-425V). CCD cameras were used to measure changes in areal extent, perimeter, and contact angles. For V_rms=0, the contact angle on PMDS was 118^o. For the range of applied voltages, the contact angle of the droplets changed by over 60^o. These experiments demonstrated that contact angle can be controlled over a wide range of values.

Unsealed micro-models were used in experiments to merge and transport drops. In the merging experiments, three 50 nm thick electrodes were formed on the top plate separated by a gap of 0.69 mm, while the bottom plate contained a single large area silver electrode 50 nm thick. A 10 µL 1M KCl-H₂O droplet was placed on the left electrode and another on the right electrode and merged when 424 V was applied to the middle electrode. The contact angle of the drops on the middle electrode decreased by 60 ^o relative to the portions of the drops on left and right electrodes. The resulting pressure difference translated and merged the two drops over a distance of ~1mm in ~13 seconds. These experiments demonstrate that EWOD techniques can be used to alter contact areas and interfacial area as well as merge and transport fluids without the need for an external pressure source. These results are of key importance to determine the effect of internal control of fluid saturations on externally measured pressures porous media.

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