Abstract

Magneto-optic tweezers are extension of well known laser tweezers. They combine optical gradient force of tightly focused laser beam(s) and magnetic forces between super-paramagnetic particles induced by modulated magnetic field. Depending on the modulation type, both attractive and repulsive magnetic interactions can be achieved. Magneto-optic tweezers offer great flexibility in particle manipulation and enable experiments that can not be performed with laser tweezers only. This talk will present the working principle of magneto-optic tweezers and results of two experiments in two different soft matter systems. In the first experiment the interaction between colloidal particles suspended in liquid crystal (LC) was investigated. LC colloids exhibit a rich variety of phenomena that arise from long-range anisotropic interactions between them. Optical tweezers are usually used for noncontact interparticle potential measurements. However, a strong electric field of a highly focused laser beam used in optical tweezers can distort the local ordering of LC molecules making experimental results unreliable. A combination of optical and magnetic tweezers is used to improve the measurement quality and to extend the measurement range to smaller particle separations. In the second experiment a water based colloid was used as a model system to experimentally study the phase diagram of colloidal particles interacting with a hard core and repulsive soft shoulder potential. In this experiment magnetic interaction was used to generate the soft shoulder potential which is very difficult to generate by other methods. The results confirmed the existence of several different phases that were predicted by numerical simulations.