Friction is resistance. In this case, it tells you ways onerous it’s for one thing to get throughout the membrane. If you engineer a membrane that has much less resistance to water, and extra resistance to salt or no matter else you wish to take away, you get a cleaner product with doubtlessly much less work.

But that mannequin acquired shelved in 1965, when one other group launched an easier model. This one assumed that the plastic polymer of the membrane was dense and had no pores by which water may run. It additionally didn’t maintain that friction performed a task. Instead, it presumed that water molecules in a saltwater resolution would dissolve into the plastic and diffuse out of the opposite facet. For that purpose, that is known as the “solution-diffusion” mannequin.

Diffusion is the movement of a chemical from the place it is extra concentrated to the place it is much less concentrated. Think of a drop of dye spreading all through a glass of water, or the odor of garlic wafting out of a kitchen. It retains shifting towards equilibrium till its focus is similar in all places, and it doesn’t depend on a stress distinction, just like the suction that pulls water by a straw.

The mannequin caught, however Elimelech all the time suspected it was mistaken. To him, accepting that water diffuses by the membrane implied one thing unusual: that the water scattered into particular person molecules because it handed by. “How can it be?” Elimelech asks. Breaking up clusters of water molecules requires a ton of vitality. “You almost need to evaporate the water to get it into the membrane.”

Still, Hoek says, “20 years ago it was anathema to suggest that it was incorrect.” Hoek didn’t even dare to make use of the phrase “pores” when speaking about reverse osmosis membranes, for the reason that dominant mannequin didn’t acknowledge them. “For many, many years,” he says wryly, “I’ve been calling them ‘interconnected free volume elements.’”

Over the previous 20 years, pictures taken utilizing superior microscopes have bolstered Hoek and Elimelech’s doubts. Researchers discovered that the plastic polymers utilized in desalination membranes aren’t so dense and poreless in spite of everything. They really comprise interconnected tunnels—though they’re completely minuscule, peaking at round 5 angstroms in diameter, or half a nanometer. Still, one water molecule is about 1.5 angstroms lengthy, in order that’s sufficient room for small clusters of water molecules to squeeze by these cavities, as an alternative of getting to go one after the other.

About two years in the past, Elimelech felt the time was proper to take down the solution-diffusion mannequin. He labored with a crew: Li Wang, a postdoc in Elimelech’s lab, examined fluid movement by small membranes to take actual measurements. Jinlong He, on the University of Wisconsin-Madison, tinkered with a pc mannequin simulating what occurs on the molecular scale as stress pushes salt water by a membrane.

Predictions based mostly on a solution-diffusion mannequin would say that water stress needs to be the identical on either side of the membrane. But on this experiment, the crew discovered that the stress on the entrance and exit of the membrane differed. This steered that stress drives water movement by the membrane, fairly than easy diffusion.