PERIODIC DRAG FORCE AND PARTICLE-SIZE MEASUREMENT IN A DOUBLE-RING ELECTRODYNAMIC TRAP

A simple method to determine the size of a spherical particle in an el ectrodynamic trap from its dynamic behavior is introduced. Contrary to common usage of electrodynamic traps, gravity is not compensated comp letely. The resulting oscillatory trajectory is phase shifted with res pect to the driving ac field of the trap. A light barrier setup is suf ficient to determine the phase lag, An analytical solution is availabl e in closed form to yield the particle diameter from the measured phas e lag. Numerical trajectory simulations support this solution. Our met hod is independent of the optical properties of the particle. Only the viscosity of the surrounding gas and the particle mass density have t o be known for data evaluation. (C) 1997 American Institute of Physics .