Measurements of the compressibility of colloidal suspensions by radiation pressure

This paper reports an experimental study of the interparticle interactions present in a model colloid system composed of fluorescently labeled 100 nm diameter polystyrene particles in aqueous suspension. By independently measuring the fluorescence intensity as a function of particle number density, we were able to determine the relationship between the radiation pressure generated by the optical trap and the resulting number density increase, yielding the calculation of the isothermal compressibility of the colloid system. Optical trapping was made by a tightly focused and periodically blinking IR laser beam. A green laser beam, aligned co-linearly with the IR laser, was used as the fluorescence excitation light. The fluorescence signals from particles trapped by the blinking IR laser were measured by a lock-in amplifier to improve the signal to noise ratio required to detect the changes in local particle density induced by optical trapping. The use of confocal detection ensured that the fluorescent signals measured were only from the diffraction-limited focal region of the two laser beams.