'Active' surfaces control what's on them

Researchers, including one of Indian-origin, have developed a new way of making surfaces that can actively control how fluids or particles move across them.
The work may allow for new kinds of biomedical or microfluidic devices, or solar panels that could automatically clean themselves of dust and grit.
"Most surfaces are passive. They rely on gravity, or other forces, to move fluids or particles," said Kripa Varanasi, an associate professor of mechanical engineering at Massachusetts Institute of Technology, and senior author of the study.
Varanasi's team decided to use external fields, such as magnetic fields, to make surfaces active, exerting precise control over the behaviour of particles or droplets moving over them.

The system makes use of a microtextured surface, with bumps or ridges just a few micrometres across, that is then impregnated with a fluid that can be manipulated - for example, an oil infused with tiny magnetic particles, or ferrofluid, which can be pushed and pulled by applying a magnetic field to the surface.
When droplets of water or tiny particles are placed on the surface, a thin coating of the fluid covers them, forming a magnetic cloak.

The thin magnetised cloak can then actually pull the droplet or particle along as the layer itself is drawn magnetically across the surface.

Tiny ferromagnetic particles, approximately 10 nanometres in diameter, in the ferrofluid could allow precision control when it's needed - such as in a microfluidic device used to test biological or chemical samples by mixing them with a variety of reagents.
While other researchers have developed systems that use magnetism to move particles or fluids, these require the material being moved to be magnetic, and very strong magnetic fields to move them around.
The new system, which produces a superslippery surface that lets fluids and particles slide around with virtually no friction, needs much less force to move these materials.
"This allows us to attain high velocities with small applied forces," said MIT graduate student Karim Khalil, the study's lead author.

The new approach, he said, could be useful for a range of applications. For example, solar panels and the mirrors used in solar-concentrating systems can quickly lose a significant percentage of their efficiency when dust, moisture, or other materials accumulate on their surfaces.
But if coated with such an active surface material, a brief magnetic pulse could be used to sweep the material away, Khalil said.
The study, co-authored by Numan Abu-Dheir of the King Fahd University of Petroleum and Minerals (KFUPM) in Saudi Arabia, is published in the journal Applied Physics Letters.