Like-charge attraction may explain mysteries of colloids

Colloids -- materials in which the particles of one substance are stably dispersed in another -- include such substances as paint, milk, ink and foam.

"The question of what keeps colloidal particles from clumping together and falling out of solution -- what keeps colloids stable -- is critical to many industries, including the paint and paper industries," Grier said. "It's bad if paint aggregates in the can, yet the paper industry wants their colloid to aggregate -- though they also want to be able to control the process."

Short-range repulsion between like-charged particles is thought to be what keeps colloids from clumping. Pigment particles in ink, for example, repel each other and can't bind together to separate from the liquid base. But Grier and Larsen, using a model system, discovered that tiny, long-range attractive forces sometimes arise between particles that otherwise repel each other. The colloid they studied formed crystals under conditions for which the conventional theory predicts a uniform suspension. The attractive forces they measured may be responsible for a variety of unexplained phenomena in bulk colloids, such as voids.

"The attractive forces that we are measuring are some of the smallest forces in nature -- smaller than the forces that hold atoms together -- but they are responsible for the bulk properties of these materials, the viscosity of butter, for example," said Grier. "The forces are so small -- roughly equivalent to the gravitational attraction between two people standing a kilometer apart -- that no one has been able to measure them before."

Grier and Larsen began using colloidal suspensions of tiny plastic balls in water to study how solids melt. Squeezed together between glass(plates, the plastic balls form ordered crystals analogous to a solid, like ice. When the density of these crystals is reduced, they "melt" into a disordered liquid, just as ice melts into water.

Grier and Larsen discovered that rather than undergoing the usual transition from the "solid" to "liquid" states, ultra-pure colloidal suspensions actually showed an in-between stage. Small clusters of spheres remained aggregated together -- like slush in a frozen margarita -- long after the melting point had been passed.

The icy slush in water can be explained by weak bonds formed between water molecules, which carry both positive and negative charges. But the colloidal particles are all the same charge, so the formation of these metastable crystals was very puzzling. "In this system, there aren't supposed to be any long-range attractions," said Grier.

To further study the phenomenon, Grier and Larsen used a novel technique, called "optical tweezers," which uses two lasers as "tractor beams" to capture and hold two of the tiny spheres next to each other. Using a video camera hooked up to a light microscope, they studied how the particles behaved when they were released.

"For a pair of particles on their own, the old theory of repulsive interactions works perfectly well," said Grier. "But if the spheres are confined between two glass walls, then the spheres attract each other. The metastable crystals we see arise from a many-body effect -- the spheres themselves are doing the confinement and producing the attraction, just as the glass walls do.

"Now that we are aware of these attractions, we may be able to apply them in protein crystallization, nanofabrication and self-assembly of advanced optical materials," all of which have implications for scientific and industrial processes, Grier said.

Protein crystallization, for example, is an extremely important step in the design of many pharmaceuticals. But proteins -- which are colloidal suspensions in water -- are notoriously difficult to crystallize. Knowing more about factors that affect the crystallization of colloids may enable scientists to alter suspensions of proteins so that they crystallize more easily.

-- Diana Steele