Production of dry powder clots using a piezoelectric drop generator

We have demonstrated that piezoelectrically driven, squeeze mode, tubular reservoir liquid drop generation, originally developed as a "drop-on-demand" method for ejection of microdrops of liquids or suspensions, can successfully operate with dry powder. Spherical silver powder with maximum particle diameter of 20 mum was loaded into and ejected from a 100 mum orifice glass dropper with a flat piezoelectric disk driver. Time of flight experiments were performed to optimize the dropper operation and to determine the size and velocity of the ejected particles. It was found that at certain values of the amplitude, duration, and repetition rate of the voltage pulses applied to the piezoelectric disk, one can eject powder clots of a stable size, comparable with the dropper orifice diameter. In contrast to the operation with a liquid, a clot is not ejected at each pulse, but quasiperiodically with an interval corresponding to thousands of pulses. The application for injection of atoms into helium buffer gas at cryogenic temperatures is discussed. (C) 2002 American Institute of Physics.