Cover of Scientific American, October 2003

Source: Actuator driven by thermal expansion of water and air

V.H. Ebron et al., Science (2006) 311, 1580 A chemical reaction with methanol creates heat in the NiTi alloy wire, which causes it to shrink - thus lifting a weight. Example of negative thermal expansivity!

Weak Nano-scale Forces Can be Measured The AFM probe is exceedingly sharp so that only a few atoms are at its tip! Sensitive to forces on the order of nano-Newtons. Atomic force microscope (AFM)

AFM tips from NT-MDT. See Tips for Scanning Probe Microscopy Radius of curvature ~ 10 nm Ideally, one of the atoms at the tip is slightly above the others. The tip is on a cantilever, which typically has a spring constant on the order of k = 10 N/m. Modelled as a simple spring: F = kz where z is the deflection in the vertical direction. F

Measuring Attractive Forces at the Nano-Scale A = approach B = “jump” to contact C = contact D = adhesion E = pull-off Tip deflection  Force Vertical position A B C DE C

Measuring Force of Attraction to a Polymer Surface Pulling on the AFM probe tip Pushing on AFM probe tip