Local probing of medium properties using optical tweezers and a confocal setup

Summary form only given. In the study of colloidal suspensions, polymer solutions and in cell and micro-biology it is often desirable to have a direct observation in real time of the changes in the local viscosity, which affect the dynamics of objects like colloidal particles, cells and organelles. For example, the cell´s transport and diffusion of macro-molecules both depend on the local environment. It is especially important to have high spatial resolution in complex environments in which local inhomogeneities are present. We describe a method to probe the environment of such objects in a fairly simple way that is both sensitive and fast. We use the probe beam of an inverted confocal microscope as optical tweezers so that only light scattered from the focal point of the tweezers is confocally detected. In our experiment we nudge a particle periodically by moving the laser tweezers back and forth across it, with an oscillation amplitude that is slightly larger than the particle diameter.