R. Di Leonardo, F. Ianni, F. Saglimbeni, G. Ruocco, S. Keen, J. Leach and M. Padgett
A tightly focused light beam can stably trap small objects in three dimensions. Using spatial light modulators we can engineer the wavefront of a laser beam in such a way that, once focused by a microscope objective, it produces an almost arbitrary light intensity distribution. Arrays of optical traps can be thus generated in three-dimensional space and dynamically reconfigured. Optical traps allow direct manipulation and sensing on those length and energy scale that are most relevant in many colloidal processes. In the presence of long range interactions optical traps actually provide a unique tool of direct investigation allowing the precise relative positioning of particle pairs, far from boundaries or other particles. We have used optical trapping to directly measure two very long range interactions governing colloidal dynamics in two-dimensional fluid films: hydrodynamic interactions, which are found to decay logarithmically slow with distance, and capillary forces, whose intensity decreases as a power law with an exponent slightly smaller than one.
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