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.