Controlling water repellency and permeation with electricity

New research paves the way for controlling water permeation through a graphene-covered metal mesh by utilizing electricity

Article  |  Spring 2017

There is a strong need for the development of flow control devices that can be applied to various water treatments such as surface cleaning, hydrophobic surfaces, water condensation, anti-oxidation coatings, and drag reduction. However, as conventional methods for the tuning of wetting properties rely on either surface roughness or chemical coating, active control of wettability is not possible, which limits the applicability of these methods.

Profs. Il-Kwon Oh and Seunghwa Ryu (both at the Mechanical Engineering Department, KAIST) have succeeded in developing active water flow control devices that permeate water when an electric field is applied and repel water in the absence of the electric field. The device accesses two functionally antagonistic functions of repellency and permeability on-demand through adjustments of the magnitude and location of the applied electric field.

The technology utilizes the electro-wetting phenomena, which refers to the modification of the wetting properties of a surface with an applied electric field. Due to the hydrophobicity (property of repelling water) of graphene (atomically thin sheet of carbon atoms on a hexagonal lattice), water cannot permeate the graphene-covered metal mesh under normal conditions. However, in the presence of a sufficient electric field, the attractive force on the water surface induced by the electric field becomes larger than the repulsive capillary force between the mesh openings, which enables water permeation.

Based on this principle, Profs. Oh and Ryu built electroactive flow control devices that release the water confined by the graphene-coated mesh, or allow water droplet locomotion through multiple layers of the graphene-coated mesh. These devices can function for a very long time because the graphene coating prevents oxidation of the metal mesh. These durable and active flow control methods can be applied to various microfluidic devices, water-proof devices, oil-water separation, etc. Such technology can also facilitate new opportunities in electroactive flow control systems that operate in a manner analogous to electrical switches.

An article on this research entitled “Graphene-coated meshes for electroactive flow control devices utilizing two antagonistic functions of repellency and permeability” was published on Oct 31^st, 2016 in Nature Communications (http://dx.doi.org/10.1038/ncomms13345).