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Novel surface actively controls fluids, particles

Posted: 05 Aug 2014     Print Version  Bookmark and Share

Keywords:MIT  surface  microfluidic  ferrofluid  magnetic field 

A team of researchers from MIT and in Saudi Arabia has come up with a way to make surfaces that can actively control how fluids or particles move across them. According to them, the discovery might enable novel kinds of biomedical or microfluidic devices, or solar panels that could automatically clean themselves of dust and grit.

"Most surfaces are passive," said Kripa Varanasi, an associate professor of mechanical engineering at MIT, and senior author of the paper. "They rely on gravity, or other forces, to move fluids or particles."

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, which 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.

Thin cloak of ferrofluid

When exposed to a magnetic field (magnet is seen at far right), the droplet is pulled toward the magnet by its thin cloak of ferrofluid, even though the droplet itself is not magnetic.

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, about 10nm 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. Unlike the fixed channels of conventional microfluidics, such surfaces could have "virtual" channels that could be reconfigured at will.

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 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," noted MIT graduate student Karim Khalil, the paper's lead author.

Water droplet sitting on a ferrofluid-impregnated surface

Photo shows a water droplet sitting on a ferrofluid-impregnated surface, which has cloaked the droplet with a very thin layer.

The approach, he added, 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 per centage 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.

"Fouling is a big problem on such mirrors," Varanasi stated. "The data shows a loss of almost one per cent of efficiency per week."

But at present, even in desert locations, the only way to counter this fouling is to hose the arrays down, a labour- and water-intensive method. The approach, the researchers said, could lead to systems that make the cleaning process automatic and water-free.

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