Zhu chose to work with materials that move in response to a magnetic field. Others have designed such magnetically actuated materials by infusing polymers with magnetic particles. However, Wang says it's difficult to control the distribution and therefore the movement of particles through a polymer.
Instead, she and Zhu chose to manufacture an array of microscopic pillars that uniformly tilt in response to a magnetic field. To do so, they first created molds, which they electroplated with nickel. They then stripped the molds away, and bonded the nickel pillars to a soft, transparent layer of silicone. The researchers exposed the material to an external magnetic field, placing it between two large magnets, and found they were able to control the angle and direction of the pillars, which tilted toward the angle of the magnetic field.
"We can apply the field in any direction, and the pillars will follow the field, in real time," Zhu says.
Tilting toward a field
In experiments, the team piped a water solution through a syringe and onto the microhair array. Under a magnetic field, the liquid only flowed in the direction in which the pillars tilted, while being highly "pinned," or fixed, in all other directions an effect that was even seen when the researchers stood the array against a wall: Through a combination of surface tension and tilting pillars, water climbed up the array, following the direction of the pillars.
Since the material's underlying silicone layer is transparent, the group also explored the array's effect on light. Zhu shone a laser through the material while tilting the pillars at various angles, and found she could control how much light passed through, based on the angle at which the pillars bent.
In principle, she says, more complex magn
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology