Production of Dry Powder Clots Using 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 pure liquid or liquid suspensions of powdered bulk materials, can successfully operate with dry powder. Spherical silver powder with maximum particle diameter of 20 (micro)m (-635 mesh) was loaded into and ejected from a 100 (micro)m orifice glass dropper with flat piezoelectric disk driver. Time of flight experiments were performed to optimize the dropper operation parameters 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 dropper piezoelectric disk, one can produce ejection of powder clots of a stable size, comparable with the dropper orifice diameter. In contrast to the dropper operation with a liquid, in the case of silver powder, a clot is not ejected at each high voltage pulse, but quasi-periodically with an interval corresponding to thousands of pulses. The application of the dry powder clot generation technique for injection of atoms into helium buffer gas at cryogenic temperatures is discussed