Optical tweezers as a tool for microrheology of simplex and complex fluids

Optical tweezers have become a widely used tool for manipulate microscopic objects. Combined with fast and high sensitive position detection techniques, they are suitable for microrheological measurements of viscous and viscoelastic media. Such measurements require the knowledge of absolute displacements of the trapped particles. Unfortunately position detection device need to be calibrated and several methods have been used and reported in literature. We report an accurate study of the behavior of the conversion factor in the case of the forward light scattering technique, based on an oil-immersion objective lens, which is commonly used in optical tweezers systems. We show how this fundamental parameter is strongly dependent on the distance of the trapped object from the coverslip surface.