Principal Investigator: Igor Bargatin
Scientists and engineers are engaged in a global race to make new materials that are as thin, light, and strong as possible. These properties can be achieved by designing materials at the atomic level, but they are only useful if they can leave the carefully controlled conditions of a lab.
Researchers in Prof. Bargatin’s group at the University of Pennsylvania have now created the thinnest plates that can be picked up and manipulated by hand. Despite being thousands of times thinner than a sheet of paper and hundreds of times thinner than household cling wrap or aluminum foil, these plates of aluminum oxide spring back to their original shape after being bent and twisted.
Like cling wrap, comparably thin materials immediately curl up on themselves and get stuck in deformed shapes if they are not stretched on a frame or backed by another material. Being able to stay in shape without additional support would allow this new material, and others designed on its principles, to be used in aviation and other structural applications where low weight is at a premium.
A Distinctive Structure
The Bargatin group’s ultrathin plates are between 25 and 100 nanometers thick and are made of aluminum oxide, which is deposited one atomic layer at a time to achieve precise control of their thickness and distinctive honeycomb corrugation. Once finished, the plates’ corrugation provides enhanced stiffness. When held from one end, where comparably thin films would readily bend or sag, the honeycomb plates remain rigid. They avoid the common flaw in un-patterned thin films, which curl up on themselves.
Most ultra-thin films share another trait which makes them hard to use outside controlled conditions: they have the tendency to conform to the shape of any surface and stick to it (due to Van der Waals forces). Once stuck, they are hard to remove without damaging them. Totally flat films are also particularly susceptible to tears or cracks, which can quickly propagate across the entire material. Corrugation used in the new plates effectively inhibits crack propagation.
The corrugated pattern of the plates is an example of a relatively new field of research: mechanical metamaterials. Like their electromagnetic counterparts, mechanical metamaterials achieve otherwise impossible properties from the careful arrangement of nanoscale features. In mechanical metamaterials’ case, these properties include traits like stiffness and strength.
Other existing examples of mechanical metamaterials include “nanotrusses,” which are exceptionally lightweight and robust three-dimensional scaffolds made out of nanoscale tubes. The Penn researchers’ plates take the concept of mechanical metamaterials a step further, using corrugation to achieve similar robustness in a plate form and without the holes found in lattice structures.
This combination of traits could be used to make wings for insect-inspired flying robots, or in other applications where the combination of ultra-low thickness and mechanical robustness is critical. The wings of insects are a few microns thick, and can’t be thinner because they’re made of cells. Existing man-made wings are made by depositing a one-half-micron-thick Mylar film on a frame. The Bargatin group’s plates can be ten or more times thinner than that, and don’t need a frame at all. As a result, they weigh as little as a tenth of a gram per square meter.
- Two-Layer Plate Mechanical Metamaterials
- Ultralight Shape-Recovering Plate Mechanical Metamaterials
- Plate Mechanical Metamaterials Playlist
- Plate Mechanical Metamaterials Slide Deck, Igor Bargatin
- A Nanoscale Object You Can Pick Up, Mechanical Engineering, February 2016
- Small Scale, Big Impact, Penn Engineering Magazine, Spring 2016
About the Lab
The Bargatin group engages in research at the intersections of mechanical engineering, electrical engineering, materials science, and applied physics, with a focus on new metamaterials and microelectromechanical systems (MEMS). We recently invented plate mechanical metamaterials and used this concept to create the thinnest plates that can be picked up by hand. We also design and test new types of energy devices, such as microfabricated thermionic energy converters, which convert heat directly to electricity at very high temperatures by literally boiling electrons off a surface and using them as a “working fluid” in a heat engine.