W. Richard Bowen and Nidal Hilal 4

32 2. MEASUREMENT OF PARTICLE ANd SURFACE INTERACTIONS
2.1 IntRoduCtIon
In Chapter 1, discussion was made of the application of the AFM to
the measurement of forces. In this chapter, we will describe the use of the
AFM to quantitatively measure surface forces arising from the interactions
between particles and between particles and surfaces. These forces
are of particular interest in the study of colloidal dispersions, where the
strength of interactions between particles governs the properties of the
dispersion overall.
Most traditional methods used to study colloidal dispersions are
ensemble techniques, where the interactions of a large number of particles
are measured simultaneously. The advantage with the AFM is the ability
to make measurements on the single-particle level and to measure forces,
and hence interaction energies, with respect to intersurface distances.
Depending upon the experimental set-up being employed, a number of
different interactions may be measured, either separately or simultaneously,
including long-range forces such as van der Waals and electrical
double layer forces, hydrophobic interactions, solvation forces, steric
interactions, hydrodynamic drag forces as well as adhesion. In this chapter,
the forces that are likely to be encountered when measuring interactions
between particles and between particles and surfaces whilst using
the AFM are described, with examples of the measurements extant in the
literature being included. It is hoped that this chapter will serve as a useful
and practical guide to anyone who is undertaking such measurements.
2.2 ColloId PRobES
By replacing the pyramidal or conical probe usually present on an
AFM cantilever with a microsphere, the geometry of interactions between
the probe and surface can be greatly simplified, allowing the AFM to be
used to probe surface forces, much akin to the surface force apparatus
(SFA). Microparticles can also be used as probes, which will allow
particle-to-particle adhesion forces to be measured. An example of a colloid
probe is shown in Figure 2.1. Here, a scanning electron microscope (SEM)
image shows a 5 �m diameter silica bead attached with glue close to the
apex of a standard AFM microcantilever.
As the size of a spherical colloid probe is increased, the potential area
of contact will also increase in proportion. For two spheres in close proximity,
the force as a function of distance D is:
R R
F( D)
W( D)
R R �
1 2
2π
�
⎛ ⎞
⎜
⎝⎜
⎠⎟⎟
1 2
(2.1)