An octopole electrodynamic balance for three-dimensional microparticle control

A new octopole electrode design improves the particle trapping stability of an electrodynamic balance (EDB) and allows one to make three-dimensional f orce measurements on a trapped particle. A conventional double-ring EDB was modified by splitting each ring into four equal segments that are electric ally independent. Three dc sources were combined such that eight potentials were applied to the eight segments of the electrodes. An additional ac vol tage was superimposed on each ring segment as in a conventional double-ring EDB. The resulting electric field has dc components in the x, y, and z dir ections, which can be controlled independently by the three dc supplies. Th e z component is used to balance and measure vertical forces such as gravit y, radiation pressure, or phoretic forces. The x and y fields can be used t o suppress lateral oscillations of the trapped particles, and lateral force s on the particle can be measured in terms of the x and y dc voltages. The apparatus and the electric fields are described herein, and the operation o f the device is demonstrated for spheres and aggregated particles. (C) 2001 American Institute of Physics.