New copper-based material switches from hydrophobic to hydrophilic and back

Most materials are either hydrophobic (repelling water) or hydrophilic (absorbing water like a sponge). For years, engineers have tried to create the ultimate material: one that is both water-repellent and water-absorbent. Now, it seems researchers at the University of British Colombia have succeeded as this. They created a copper-based surface that can change from super hydrophobic to super hydrophilic and back with the flick of a switch!

Other research has modified the wetting behaviour (the ability to repel or absorb water) of copper surfaces using stimuli like heat, UV radiation and X-rays. But in order to achieve this, the required temperatures need to be high — up to 300 degrees Celsius — and the required exposure times are long — from tens of minutes to days. This makes them impractical for a number of consumer and industrial purposes.

In contrast, the electrical stimulus used by the UBC team modifies wetting behaviour rapidly and reversibly, at voltages found in everyday batteries (less than 1.5 V). It does so by changing the oxidation state of the copper surface, which contains a mixture of hydrophilic copper (II) oxide (CuO) and hydrophobic copper (I) oxide (Cu2O).

When tiny voltages are applied to the surface, water droplets that initially roll off stick to the material more tightly. By changing the magnitude of the voltage and how long it is applied, the team can control the angle that each droplet forms with the surface and how quickly this happens. When the electricity is removed, the droplet retains its shape and remains pinned in place.

The researchers used copper because it is a cheap and abundant material, but the researchers believe that the electrochemical manipulation of other metals might yield the same results.

Photos: UBC / Pxhere