A pinch of salt can steer colloids for improved water purification and drug delivery

The ability to better steer particles suspended in liquids could lead to better water purification processes, new drug delivery systems, and other applications. The key ingredient, say Yale researchers, is a pinch of salt.

The research team, led by Prof. Amir Pahlavan, has published their results in Physical Review Letters.

The phenomenon of diffusiophoresis causes suspended particles known as colloids to move due to differing concentrations of a dissolved substance—the gradient—in the solution. Haoyu Liu, a graduate student in Pahlavan's lab, notes that the phenomenon was discovered more than 50 years ago, yet its applications in microfluidics and implications in environmental flows have just recently been realized.

"Chemical gradient is actually everywhere in our natural systems and also in our industrial processes," said Liu, lead author of the study. "So this phenomenon has drawn very much interest from scientists and engineers."

Scientists have traditionally used electric or magnetic fields to manipulate colloids. But Pahlavan and Liu report that varying concentrations of salt can lead to the spontaneous motion of colloids. These effects can lead to unexpected outcomes, and even create a swirling vortex that reverses the particles' usual paths.

Using gradients in salt, polymer, or other molecular solutes, they say, offers some advantages over the other processes.

"One is the simplicity of using a salt gradient," said Pahlavan, assistant professor of mechanical engineering & materials science. "All you need is just more salty or less salty water to manipulate the colloids, as opposed to a more sophisticated setup."

This process might be most useful in natural systems

"You don't always have an electric field or a magnetic field, but you do always have a salt or chemical gradient, either because of human activities, or because of many other processes that might happen in nature."

As far as applications, Pahlavan and Liu said it could have benefits for environmental cleanups.

"With contaminant remediation, where they inject polymer particles to react with a chemical plume somewhere to prevent its further spreading, we can utilize the solute gradients to make sure that the particles that are being injected end up at the right location," Pahlavan said.

Drug delivery is another potential application.

"Here, you want to deliver particles to certain tumor cells or perhaps a biofilm," he said. "Maybe we can use the solute gradients to guide the particles to go where we want. These are hidden targets whose location we don't know a priori; solute gradients, however, could steer the colloids toward the right destination."