Colloidal probe technique

One can equally study similar biological phenomena, such as deposition of bacteria or the infection of cells by viruses.

Forces are equally most informative to investigate the mechanical properties of interfaces, bubbles, capsules, membranes, or cell walls.

Such measurements permit to make conclusions about the elastic or plastic deformation or eventual rupture in such systems.

Suppose for the moment that the probe and the substrate are hard and non-deformable objects and that no forces are acting between them when they are not in contact.

In the constant compliance region, the lever deformation is given by In this fashion, one can detect deflections of the cantilever with typical resolution of better than 0.1 nm.

Since the deflection is monitored with a precision better 0.1 nm, one typically obtains a force resolution of 1−100 pN.

In the case of a soft repulsive force, the cantilever is repelled from the surface and only slowly approaches the constant compliance region.

From stability considerations one finds that the cantilever will be unstable provided This instability is illustrated in the right panel of the figure on the right.

However, this problem can be avoided by using a stiffer cantilever, albeit at the expense of an inferior force resolution.

This problem can be avoided by attaching the colloidal particles under wet conditions in AFM fluid cell to appropriately functionalized cantilevers.

[6] The latter geometry further requires a lateral centering of the two particles, which can be either achieved with an optical microscope or an AFM scan.

This approach relies on the determination of the mean square amplitude of the cantilever displacement due to spontaneous thermal fluctuations.

In the added mass method one attaches a series of metal beads to the cantilever and each case one determines the resonance frequency.

By exploiting the relation for a harmonic oscillator between the resonance frequency and the mass added one can evaluate the spring constant as well.

The frictional force method relies on measurement of the approach and retract curves of the cantilever through a viscous fluid.

Since the hydrodynamic drag of a sphere close to a planar substrate is known theoretically, the spring constant of the cantilever can be deduced.

From the retraction part of the force curve, one can obtain information about stretching of the polymer or its peeling from the surface.