Characterisation of the weak interactions between a particle and a plane surface using total internal reflection microscopy and radiation pressure forces

Recent experimental work has shown that total internal reflection microscopy (TIRM) can be employed to monitor the position of a particle undergoing diffusion near an interface and thereby characterise weak potential-energy profiles between a particle and a surface. We have previously demonstrated an extension of this technique, which involves the manipulation of colloid particles by the use of radiation pressure forces.This paper describes how we have extended the technique to allow measurement of the absolute separation of a particle from a flat surface as a function of NaCl concentration and how we can monitor the modification, by O-n-dodecylhexaethylene glycol (C12EO6), of the interactions between a quartz plate and a polystyrene particle.