Attaching micrometer-sized particles to the end of atomic force microscope (AFM) cantilevers allows to measure the interaction of these particles with an air-water interface of e.g. a bubble or a droplet. Such colloid probe experiments are an interesting model system for floation (see Particle Bubble Interaction). In many practical cases, the properties of the air-water interface may be dominated by the presence of surface active molecules (surfactants). Colloid probe experiments on air bubbles in the presence of surfactants have shown the strong dependence of the interaction forces on the surfactant concentration. However, a detailed interpretation of these experiments was difficult due to the unknown amount of surfactant at the air-water interface.

To overcome this limitation, we have developed a new type of instrument, that allow the measurement of surface forces at the air-water interface in the presence of a molecular monolayer of surfactant with well known density (or more precisely, surface area per molecule). Basically, the novel custom-made instrument is an (AFM), integrated into a Langmuir film balance from below. A colloid particle with diameters of some micrometers is attached to the AFM cantilever. This allows studying the forces and adhesion between the particle and a monomolecular film (monolayer) at the air-water interface, as well as the monolayer stiffness and the wettability of the particle. The interactions between lipids, block copolymers, proteins, polyelectrolytes and silicon/polymer/crystal particles are investigated.

• Gillies, G., K. Büscher, M. Preuss, M. Kappl, H.J. Butt, K. Graf: Contact angles and wetting behaviour of single micron-sized particles.
  J. Phys.-Condes. Matter 2005, 17, S445-S464.