Velocity Compression in Cylindrical Capacitor Electrospray of Methanol−Water Mixtures

Phase Doppler anemometry was utilized to measure the >0.5-μm portion of the size distributions and the corresponding velocity distributions of methanol−water droplets generated by a miniature cylindrical capacitor electrospray (CCES) source. Droplets exceeding 1 μm in diameter were observed with the tail of the size distributions extending up to 5 μm. Droplets produced by this source displayed significantly narrower velocity distributions and smaller mean axial velocities relative to those generated by forced flow electrospray. In terms of flow rate, spraying voltage, droplet size, and axial velocity distributions, the CCES source represents a transition between conventional electrospray and nanospray sources. The CCES is quasi-monodisperse, and the droplets have close to uniform velocities. There is a significant effect of the spraying voltage on the width and mean values of the droplet size and velocity distributions for methanol solutions. Both mean diameters and average axial velocities are shifted toward larger values as the applied voltage is increased. The emission diameter predicted by Wilm and Mann provided a good lower estimate for the measured droplet diameters.