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We report the experimental measurement of the relationship between the size of particles being moved by optically patterned dielectrophoresis in an Optoelectronic Tweezers (OET) device and the force that they experience. The OET device turns an optical pattern into a pattern of electrical fields through the selective illumination of a photoconductive material. In this work we use a data projector to create the structured illumination which gives a relatively flat optical profile with steep optical gradients and hence steep electrical gradients at the edges of the light patterns created. For a small particle in a constant electrical gradient it would be expected that the force due to dielectrophoresis would scale with the cube of the particle’s radius whereas the forces needed to move it against the viscous fluid scale with the radius so that there would be a an increase of the velocity at which we can move particles with a relationship of the radius squared. As the particles in an OET device are often larger than the area over which the electrical gradients are produced it is not obvious how their forces scale with size. In this paper we show that there is a small size regime where the particle size relationship with force is well described by a linear fit and a regime where it is not. We show that the magnitude of the force is dependent on the light pattern used and that with larger particles and optimized light patterns velocities of around 1mms-1 can be achieved.

Size scaling with light patterned dielectrophoresis in an optoelectronic tweezers device