Droplet Dynamics Changes in Electrostatic Sprays of Methanol−Water Mixtures

Two-dimensional phase Doppler anemometry measurements have been carried out to determine the size and velocity distributions of electrosprayed droplets generated from methanol−water mixtures. We investigated spraying conditions close to those of electrospray ionization sources. The droplet size and the axial and radial velocity distributions were measured as a function of liquid flow rate, needle-to-counter electrode distance, bias voltage, position of the probe volume, and electrical conductivity of the liquid. In 90:10 (v/v) methanol−water mixtures the droplet size decreased from ∼7 to ∼1.6 μm as the conductivity increased as a consequence of a 3 orders of magnitude increase in ionic strength. As the position of the probe volume was moved along the spray axis, two different spray dynamics were observed. Solutions of low conductivity (c < 10-5 M) on the average produced 5.5 μm droplets at the capillary that gradually decreased to 4.0 μm as the drops moved away from the tip. Solutions of higher conductivity (c > 10-3 M), however, resulted in smaller droplets at the needle (1.6 μm) that increased in size to 4.2 μm as the particles traveled toward the counter electrode. The droplet size reduction can be explained by evaporation and/or Coulomb explosion, whereas the increase in droplet size may be the consequence of droplet segregation or coalescence. Axial velocity distributions show compression along the spray axis (e.g., observed at 10 mm ∼50% reduction of the width at 4 mm is seen). Moving downstream, the average velocity of droplets from water−methanol mixtures decreases monotonically. Solutions containing KCl exhibit a maximum in axial droplet velocity as the probe volume moves away from the capillary. These profile changes can be explained by differences in the electric field distribution along the spray centerline.