Physics Colloquium - Friday, February 10th, 2006, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

Elisa Riedo
Georgia Institute of Technology

Nanomechanics: from nanotechnology to biology

The development of new materials with size of few nanometers has opened a new field of scientific and technological research. The goal is to develop faster and better communication systems and transports, as well as smarter and smaller nanodevices for biomedical applications. To reach these objectives it is crucial to have knowledge of and the ability to control the mechanical behavior of the materials and liquids at the nanoscale. Here, I will describe our recent results on the mechanical properties of Carbon nanotubes and oxide nanorods. We study for the first time experimentally and theoretically the radial elasticity of multiwalled carbon nanotubes as a function of external radius. We use atomic force microscopy (AFM) and applied small indentation amplitudes in order to stay in the linear elasticity regime. The number of layers for a given tube radius is inferred from transmission electron microscopy revealing constant ratios of external to internal radii. This enables comparison with molecular dynamics results, which also shed some light onto the applicability of Hertz theory in this context. Using this theory we find a radial Young modulus strongly decreasing with increasing radius and reaching an asymptotic value of 30 GPa. In the same context, we have demonstrated a new, non-destructive method to study in situ the bending modulus of vertically aligned nano-rods. The measurement is based on quantifying the lateral force required to induce the maximal deflection of the nanowire when the AFM tip was scanning over the surface in contact mode. The comprehension of the mechanical properties of nanomaterials and nano-confined liquids is critical also in biology. The knowledge of the mechanical behavior of nanotubulus such as acquaporine and DNA strands is decisive to understand the structural dynamics of cellular processes. Water filled pores of nano-dimensions, for instance aquaporines and ion channels, are present in many membrane-spanning transport protein. In order to understand the fundamental physical mechanisms of ion channel processes, including permeation, selectivity and gating, it is crucial to know what is the behavior of water in sub-nanometer gaps. Here, I will report a study of the forces encountered by a nano-sized tip when it approaches a solid surface in water. We investigate, with sub-Angstrom resolution, tip-surface distances from 2 nm down to 0.4 nm. We find that, for hydrophilic and hydrophobic surfaces, oscillatory solvation forces are present in the last three/four adjacent water layers. Finally, the experiments show that, for hydrophilic surfaces, the dynamic viscosity can grow up to five orders of magnitude in respect to bulk water. In contrast to this behavior, the viscosity and the diffusion constant in the non-wetting system remain almost constant throughout most of the gap-width range.