DROPLET DYNAMICS CHANGES IN ELECTROSTATIC SPRAYS OF METHANOL-WATER MIXTURES

Two-dimensional phase Doppler anemometry measurements have been carrie d out to determine the size and velocity distributions of electrospray ed droplets generated from methanol-water mixtures. We investigated sp raying conditions close to those of electrospray ionization sources. T he droplet size and the axial and radial velocity distributions were m easured 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 similar to 7 to similar to 1.6 mu m as th e conductivity increased as a consequence of a 3 orders of magnitude i ncrease in ionic strength. As the position of the probe volume was mov ed along the spray axis, two different spray dynamics were observed. S olutions of low conductivity (c < 10(-5) M) on the average produced 5. 5 mu m droplets at the capillary that gradually decreased to 4.0 mu m 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 mu m) that increased in size to 4.2 mu m as the particles traveled toward the counter electrode. The droplet size reduction can be expla ined by evaporation and/or Coulomb explosion, whereas the increase in droplet size may be the consequence of droplet segregation or coalesce nce. Axial velocity distributions show compression along the spray axi s (e.g., observed at 10 mm similar to 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 mo ves away from the capillary. These profile changes can be explained by differences in the electric field distribution along the spray center line.