Cover of Scientific American , October 2003

Cover of Scientific American, October 2003

Source: www.ritsumei.ac.jp

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 www.ntmdt.ru

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

AB

C

D

EC

Measuring Force of Attraction to a Polymer Surface

Pulling on the AFM probe tip

Pushing on AFM probe tip