The treated surface is covered with a mixture of micro-islands and nanospikes that render the substrate superhydrophilic. Water that comes in contact with the sharp features spreads into a thin, uniform film with a contact angle of less than 5° independent of the wettability of the native surface. So far, the team based at the University of Arkansas has used its process to treat glass, stainless steel and silicon wafers and believes that the technique will suit a wide range of surfaces.

Longer lasting
It's possible to generate a superhydrophilic surface using nanostructures alone, but the team prefers its dual-scaled approach.

"Once the nanoscale layer is worn out, treated surfaces typically lose their superhydrophilicity," Min Zou, director of the university's nanomechanics and tribology laboratory told nanotechweb.org. "In our case, the micro-bumps help to protect some of the nanostructures and allow us to provide a much more durable treatment."

The group uses a technique known as aluminum-induced crystallization to texture the surface. The process begins with amorphous silicon and aluminum deposition steps followed by annealing and then etching. Tests show that the effect is long lasting with superhydrophilic properties remaining unchanged throughout the duration of the 192 hour evaluation period.

Controlling surface wettability
The researchers can make selected regions superhydrophobic – in other words, a no go area for water – by treating portions of the sample with C4F8 in a DRIE system. This gives the group the ability to direct liquid along pre-patterned paths and could be ideal for use in various microfluidic devices.

Zou's team is now subjecting the samples to a longer-term study and is keen to commercialize the technology.

The researchers presented their work in Nanotechnology.