Mechanics at the Nanoscale: From the Breakdown of Continuum Plate Mechanics in
Graphene to the Phase Transition Plasticity of Silicon Nano-spheres
Traian Dumitrica
University of Minnesota
Mechanics at the nanoscale is a cross-disciplinary area where the traditional concepts of mechanics
overlap with the fundamentally different aspects of quantum chemistry and solid-state physics. In our
studies of graphene nanostructures, ZnO nano-wires, and Si nano-particles, we have encountered
situations when this overlap gives rise to new and useful phenomena. This talk will concentrate on
three such examples:
(i) Graphene is intensely researched for electromechanical applications. I will discuss how simulations
carried out with a new microscopic technique called objective molecular dynamics, reveal the
breakdown of classical elasticity concepts in bent and twisted graphene nanostructures. This result has
implications for a broad class of phenomena, where the monolayer bubbles, scrolls its edges, ripples, and
twists in spite of its enormous stiffness.
(iii) Twisted zinc oxide nanowires and nanotubes were recently synthesized by screw-dislocation
growth. We show, based on objective molecular dynamics simulations, that once their diameter
increases above a critical size of the order of a few atomic spacings, the existence of these structures
can be rationalized in terms of the energetics of surfaces and veritable Eshelby's twist linear elasticity
mechanics supplemented by a nonlinear core term. For Burgers
vector larger than the minimum allowed one, a twisted
nanotube, rather than a nanowire, is the most stable
nanostructure.
(i) Recently, our colleagues discovered that although silicon
nanoparticles are superhard, they stick to a substrate when
colliding at 1-2 km/s. Molecular dynamics simulations explain
this puzzling result in a surprising way: Although the contact
force is relatively low by macroscopic standards, the impact Fig. A nanosphere containing some
30,000 silicon atoms and moving at
pressure causes the high speed particle to change its crystalline 900 meters per second will bounce off
a surface (left sequence), but at
structure and soak up so much energy that the particle can't 2,000 meters per second, it sticks
bounce away. This understanding may help develop wear- (right sequence). The higher-speed
resistant coatings created by many such impacts.
impact causes two sequential changes
in the crystalline structure.
About the Speaker
Dr. Traian Dumitrica received a doctorate in physics from Texas A&M University in 2000. Since then
he has worked at Rice University, Freie Universitaet Berlin, and Universitaet Kassel. He joined the
University of Minnesota faculty in 2005. His research focuses on understanding the mechanical
properties of materials using advanced atomistic computational methods and creating links between
atomic level physics and the ordinary structures of mechanical engineering: rods, tubes, plates, etc.
His multidisciplinary research has involved problems arising in nanotechnology, mechanical
engineering, and materials science. He has published over fifty refereed papers. Several of his research
projects have been featured in the press, including the New York Times Science Times, Physical
Review Focus, Nanotechweb.org, Materials Today, MRS Bulletin, and Proceedings of the National
Academy of Sciences. He is a recipient of the 2008 NSF CAREER award.