H. Skulason et Cd. Frisbie, Rupture of hydrophobic microcontacts in water: Correlation of pull-off force with AFM tip radius, LANGMUIR, 16(15), 2000, pp. 6294-6297
The purpose of this work was to investigate the functional dependence of th
e adhesion (pull-off) force on probe radius in AFM force measurements. Pull
-off forces for hydrophobic AFM probes in contact with hydrophobic substrat
es were measured in water. Chemical functionalization of probes and substra
tes was accomplished by self-assembly of octadecanethiol on Au. Average pul
l-off forces for 10 SAM/Au-coated probes were measured and found to depend
linearly on the probe tip radii, which were determined by scanning electron
microscopy (SEM). Using the JKR contact mechanics model, the thermodynamic
work of adhesion (W-ad) determined from the pull-off forces was 110 +/- 10
mJ/m(2) which compares favorably with estimates of Wad based on reported i
nterfacial energies. These results verify the linear dependence of the adhe
sive force on probe radius, as expected from contact mechanics models. They
also verify that SEM can provide self-consistent and reproducible estimate
s of the probe radius.