Ian Wark Research Institute, University of South Australia, The Levels, SA 5095, Australia
School of Chemistry F11, University of Sydney, NSW 2006 Australia.
Langmuir 15 , 553-563 (1998)
Abstract
The mechanism and geometry of force measurement
with the atomic force microscope is analysed in detail.
The effective spring constant to be used in force measurement
is given in terms of the cantilever spring constant.
Particular attention is paid to possible dynamic effects.
Theoretical calculations show
that inertial effects may be neglected in most regimes,
the exception being when relatively large colloidal probes are used.
Model calculations of the effects of friction show that it can cause
hysteresis in the constant compliance region
and a shift in the zero of separation.
Most surprising, friction can cause
a significant diminution of the measured pre-contact force,
and, if it actually pins the surfaces,
it can change the sign of the calibration factor for the cantilever deflection,
which would cause a pre-contact attraction to appear as a repulsion.
Measurements are made of the van der Waals force
between a silicon tip and a glass substrate in air.
The evidence for friction and other dynamic effects is discussed.
Interferometry is used to characterise the performance of the piezo-electric
drive motor and position detector used in the atomic force microscope.
It is shown that hysteresis in the former, and back-lash in the latter,
preclude a quantitative measurement of friction effects.
The experimental data appear to underestimate the van der Waals
attraction at high driving velocities,
in qualitative agreement with the model friction calculations.
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