Optoelectric Assembly and Manipulation of Beads in a Vertical Tower Configuration

A number of optical and optoelectrical techniques have been proposed for the trapping and manipulation of micro- to nanoscale colloidal particles. Rapid Electrokinetic Patterning (REP) is one such technique. It uses laser induced electrothermal flow and particle-electrode interactions to reversibly capture particles in a monolayer. Particle trapping in a monolayer is mainly governed by AC Electroosmosis, electrothermal flow, van der Waals attraction and electrostatic forces. Particles can be translated on the substrate by optically steering the laser or by moving the translation stage. In this work we show that by a careful selection of parameters, particles can be rapidly trapped in a vertical tower configuration instead of a monolayer. Moreover, this vertical tower can be translated and positioned at any desired place on the chip. We intend to discuss underlying physical mechanism and potential applications in biology.