Beneath a microscope in Radha Boya’s lab, a thin sheet of carbon has an almost imperceptible channel cutting through its center, the depth of a single molecule of water. “I wanted to create the most ultimately small fluidic channels possible,” explains Boya. Her solution: identify the best building blocks to reliably and repeatedly build a structure containing unimaginably narrow capillaries. She settled on graphene, a form of carbon that is a single atom thick.
She positions two sheets of graphene (a single sheet is just 0.3 nanometers thick) next to each other with a small lateral gap between them. That is sandwiched on both sides with slabs of graphite, a material made of many layers of graphene stacked on top of each other. The result is a channel 0.3 nanometers deep and 100 nanometers wide, cutting through a block of graphite. By adding extra layers of graphene, she can tune the size of the channel in 0.3-nanometer increments.
But what fits through something so narrow? A water molecule—which itself measures around 0.3 nanometers across—can’t pass through the channel without application of pressure. But with two layers of graphene, and a 0.6-nanometer gap, water passes through at one meter per second. “The surface of graphene is slightly hydrophobic, so the water molecules stick to themselves rather than the walls,” says Boya. That helps the liquid slide through easily.
Because the gaps are so consistently sized, they could be used to build precisely tuned filtration systems. Boya has performed experiments that show her channels could filter salt ions from water, or separate large volatile organic compounds from smaller gas molecules. Because of the size consistency, her technology can filter more efficiently than others.
Boya currently works at the University of Manchester’s Graphene Research Institute in the U.K.—a monolithic black slab of a building that opened in 2015 to industrialize basic research on the material. It brands itself as the “home of graphene,” which seems appropriate given that Boya’s office is on the same corridor as those of Andre Geim and Kostya Novoselov, who won a Nobel Prize for discovering the material.