The colloidal probe technique is commonly used to measure interaction forces acting between colloidal particles and/or planar surfaces in air or in solution. This technique relies on the use of an atomic force microscope (AFM). However, instead of a cantilever with a sharp AFM tip, one uses the colloidal probe. The colloidal probe consists of a colloidal particle of few micrometers in diameter that is attached to an AFM cantilever. The colloidal probe technique can be used in the sphere-plane or sphere-sphere geometries (see figure). One typically achieves a force resolution between 1 and 100 pN and a distance resolution between 0.5 and 2 nm.
The colloidal probe technique has been developed in 1991 independently by Ducker and Butt. Since its development this tool has gained wide popularity in numerous research laboratories, and numerous reviews are available in the scientific literature.
Alternative techniques to measure force between surfaces involve the surface forces apparatus, total internal reflection microscopy, and optical tweezers techniques to with video microscopy.
Purpose
The possibility to measure forces involving particles and surfaces directly is essential since such forces are relevant in a variety of processes involving colloidal and polymeric systems. Examples include particle aggregation, suspension rheology, particle deposition, and adhesion processes. 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.
The colloidal probe technique provides a versatile tool to measure such forces between a colloidal particle and a planar substrate or between two colloidal particles (see figure above). The particles used in such experiments have typically a diameter between 1–10 μm. Typical applications involve measurements of electrical double layer forces and the corresponding surface potentials or surface charge, van der Waals forces, or forces induced by adsorbed polymers.
Principle
The colloidal probe technique uses a standard AFM for the force measurements. But instead the AFM cantilever with an attached sharp tip one uses the colloidal probe. This colloidal probe is normally obtained by attaching a colloidal particle to a cantilever. By recording the deflection of the cantilever as a function of the vertical displacement of the AFM scanner one can extract the force acting between the probe and the surface as a function of the surface separation. This type of AFM operation is referred to as the force mode. With this probe, one can study interactions between various surfaces and probe particles in the sphere-plane geometry. It is also possible to study forces between colloidal particles by attaching another particle to the substrate and perform the measurement in the sphere-sphere geometry, see figure above.
The force mode used in the colloidal probe technique is illustrated in the figure on the left. The scanner is fabricated from piezoelectric crystals, which enable its positioning with a precision better than 0.1 nm. The scanner is lifted towards the probe and thereby one records the scanner displacement D. At the same time, the deflection of the cantilever ξ is monitored as well, typically with a comparable precision. One measures the deflection by focusing a light beam originating from a non-coherent laser diode to the back of the cantilever and detecting the reflected beam with a split photodiode. The lever signal S represents the difference in the photocurrents originating from the two halves of the diode. The lever signal is therefore proportional to the deflection ξ.