Micro/Nanomechanics

Nanomechanics research at Northwestern focuses on the modeling and experimentation of novel materials and devices. Multiscale methods of experimentation ranging from wave propagation experiments to in-situ electron microscopy mechanical testing of thin films and 1-D nanostructures are being pursued. Likewise, next generation of electronics components based on nanoelectromechanical systems and their parallel nanomanufacturing are being developed. The group is also very active in the analytical modeling of size scale plasticity, contact and fracture of materials and structures.

Nanomechanical testing of nanostructures, e.g., MWCNTs, nanowires, and thin films, is quite challenging. At Northwestern University we have developed the first nanomaterial testing systems (N-MTS) capable of the simultaneous measurement of force-displacement signatures and acquisition of high resolution atomic images (Zhu and Espinosa, PNAS, 2005). The system is based on MEMS technology and can fit in tiny volumes, as required by in-situ transmission electron microscopy (TEM) testing. Using this novel testing platform we have correlated force measurements to the failure of a single shell in a multiwalled carbon nanotube. Electron diffraction induced by the carbon atoms is used to determine the CNT diameter and chiral numbers.

Increase of material strength with size reduction, as the nanoscale is approached, is now a well established phenomenon. Much less understood are the mechanisms leading to such behavior. At Northwestern University we are investigating size scale plasticity in single crystals by means of in-situ electron microscopy experiments and discrete dislocation dynamics simulations. By generation of virtual but physically realistic dislocation networks we have identify the origin for the size effects in thin metal films (Espinosa et al., PNAS, 102 (47) 2005). In the case of nanopillars, we have captured the staircase behavior observed experimentally and identified defect structures leading to dislocation source activation and shut-down.

See movie:

http://clifton.mech.northwestern.edu/~espinosa/movies/Sci-movie1.avi