Many people enjoy the scent of magnolia trees and basking in the Adirondack chairs this time of year, but it’s a particularly exciting time for Moneesh Upmanyu, associate professor of mechanical and industrial engineering at Northeastern. That’s because the rhododendrons, he said, have started “to breathe again.”
“The leaves of rhododendrons are very interesting,” Upmanyu explained. “If the temperature is below freezing, they roll up. But that doesn’t mean the plant is unhealthy. As the temperatures warm up, the leaves uncurl and eventually become flat again.”
This natural phenomenon, called thermotropism, is driven by responses to changes in temperature. A leaf’s edges, for example, usually dry faster than its core, leading to an imbalance of hydrostatic stress across the surface. The leaves curl up to relieve the differential contraction.
In an article published recently in the scientific journal Nanoscale, Upmanyu and former postdoctoral associate Hailong Wang show that crystalline nanoribbons — strips of ordered material one or two atoms thick — behave similarly to rhododendron leaves depending on their width as opposed to the air temperature.
The team developed a theoretical framework to predict how nanoribbons will twist and curl depending on their width and the shape in which they are cut. Since surface topology determines how efficiently electrons and atoms will move along the ribbon, Upmanyu said, the information is crucial for technological applications that rely on the electronic and mechanical properties of nanoribbons, including switches, transistors and biochemical sensors.
Upmanyu said that when a nanoribbon is cut out of a larger sheet, such as graphene, which is a one-atom-thick matrix of carbon, dangling bonds lead to increased stress along the edges. To relieve this stress, the edges move out of plane from the ribbon, causing it to twist and bend.
Similarly, a tapered nanoribbon naturally curls up like a leaf tip, and can be employed as a mechanically robust probe to sense and manipulate individual atoms.
“We treat the edges as a different material because they also have different mechanical properties — the missing atoms can render them softer or harder than the core of the nanoribbon” Wang said.
While this fact helped the team develop its theoretical framework, it also has an impact on how the ribbons can be used in the field. “Electrons can go either very slowly or very fast along the edges,” Wang said. “An extreme example: The core of the ribbon could be semiconducting but the edge could be metallic. It’s drastically different because once you’re missing one bond, it changes things very dramatically.”
Upmanyu’s expertise is in the mechanics of crystalline materials, but these new results have prompted a keen interest in the mechanics of similar systems in the natural world, such as rhododendron leaves, which have perfected strategies for reversible shape control, he said.
“Understanding of these principles offers elegant and robust routes to dynamically tune the overall properties of their synthetic counterparts,” said Upmanyu, who is now pursuing follow-up research on the unexplained mechanical properties that enable the carnivorous plant Drosera capensis to capture its prey.