Oiling the wheels of nanomachinery

Researchers at Ohio State University believe they may have found an effective method of lubricating the high-tech microdevices of the future.

Until now, scientists couldn’t accurately measure the friction that works against miniature motors, pumps, and gears in microelectromechanical systems (MEMS).

Bharat Bhushan and Professor Howard D Winbigler, along with their team of researchers, are said to have pioneered the first direct method for measuring the friction of these tiny parts with results twice as accurate as any previous indirect method could provide.

They are also said to have found a way to bake lubricant onto the surface of microdevices at temperatures as high as 150°C to oil the tiny moving parts.

Normally, Bhushan’s laboratory studies the texture of micro-thin coatings on computer hard disks and data tapes – items for which friction, wear, and lubrication dominate performance.

In 1999, scientists at the Laboratory for Analysis and Architecture of Systems in Toulouse, France, asked Bhushan to help lubricate a new silicon micromotor they were developing for biomedical applications. Friction was preventing the motor’s tiny eight-arm rotor, a type of miniature propeller, from spinning around its central hub.

For this work, Bhushan and his colleagues found a new application for their atomic force microscope (AFM), which records the shape of an object by dragging a tiny needle over its surface.

A typical AFM needle is said to have a radius of 50 to 100 nanometers. To the needle’s sensitive touch, bumpy landscapes only a few atoms thick feel as expansive as mountains and valleys.

The AFM needle detected bumps on the surface of the rotor and the surrounding casing. The bumps, which ranged in size from 11 nanometers to 100 nanometers, were a normal result of the chemical process that shapes the tiny parts out of silicon, said Bhushan. Bumps on the rotor were rubbing against bumps on the casing, causing friction.

These measurements were twice as accurate as any produced by other means, Bhushan said.

The researchers were also able to gauge the amount of friction inside the motor by measuring the force required for the needle to nudge the central rotor into motion.

To grease the motor, Bhushan first tried flooding the device with Z-DOL, a commercially available synthetic lubricant that costs nearly $1,000 per pound.

Liquid Z-DOL only clogged up the motor so the researchers baked a one-nanometer-thick coating of lubricant onto the surfaces of all the moving parts. At 150°C, the Z-DOL solidified into a smooth layer that allowed the components to move more freely.

When the researchers again nudged the rotor into motion, they found that the solid Z-DOL coating reduced friction by half.