Entropy keeps small particles on edge.

The second law of thermodynamics-which says that entropy, or disorder, always increases-is the bane of physics. Whether limiting an engine's efficiency or scrambling the socks in a drawer, entropy thwarts many a plan. Physicists have now turned the tables on entropy, however, using it to marshal microscopic spheres into an organized pattern.

Experimenters have long known that if they mix two different sizes of spheres in salt water, the larger particles cluster against the container wall (SN: 4/2/94, p. 223). Building on this research, a new method harnesses entropy to line up particles in neat rows.

"The cool idea is making shapes on the surfaces to trap particles," said Arjun G. Yodh of the University of Pennsylvania in Philadelphia. Yodh and his colleagues at Penn and at the University of California, Santa Barbara describe the experiment in the Sept. 19 Nature.

The group packed a mixture of polystyrene spheres into a drop of salt water between two glass microscope slides and adjusted the salinity of the water so that the spheres neither floated nor sank. Yodh and his colleagues used numerous 0.08-micrometer balls and a few 0.46-micrometer ones. The researchers also etched a channel into the lower glass slide.

In the middle of the mixture, the small spheres battered their larger brethren randomly on all sides. When experimenters placed the big particles directly on the lower glass surface, however, the bumping by small spheres sent them staggering until they reached the edge of the channel. Though they teetered there, the large spheres never fell in, because the motion of the small spheres in and near the channel barred their entry.

The obstructive behavior of the small spheres comes from their tendency to maximize entropy, explains Yodh. The tiny spheres spread themselves out in as disorderly a fashion as possible. The large spheres would behave in the same way, but outnumbered by small particles bent on maximizing their own freedom, the big ones remain pinned to the glass surface like wallflowers at a raucous high school dance.

Whenever a big globe threatens to move into the open space over the etched channel, which would further limit the small spheres' movement, the small spheres push back with a cumulative force that maximizes disorder.

The force exerted by all the small spheres on each of the larger ones is only 40 femtonewtons (10-15N)-a little less than the gravitational pull between two people standing a kilometer apart. "This is the first direct measurement of entropic interaction," says physicist David G. Grier of the University of Chicago.

Yoking entropic forces to push molecules into place might pay off in better lasers, paints, and models of cell behavior, according to Grier. "Rather than calculating and then ignoring entropy, we're actually using it."