Direct measurement of electrospray droplets in submicron diameter using a freezing method and a TEM image processing technique

The size distributions of electrospray droplets from the Taylor cone in cone-jet mode are directly measured by using a freezing method and a transmission electron microscope (TEM) image processing technique. These results are compared with the data obtained by an aerodynamic size spectrometer (TSI Aerosizer DSP). The use of glycerol seeded with NaI and a freezing method make it possible to sample droplets with their original sizes preserved. Since pictures of droplets are taken with TEM with very low vapor pressure of the solution, evaporation is suppressed by freezing. For liquid flow rates below 1 nl/s, the measured droplet diameters by the TEM image processing technique and the aerosizer are in the range of 0.25¯0.32 and 0.30¯0.40 mu m, respectively. Both results are to some extent qualitatively in agreement with previous scaling law, however, they are two or three factors greater than those of the scaling law because of the much higher viscosity of the solution. Comparing the TEM data with the aerosizer measurements, it has been revealed that the TEM image processing technique can afford more accurate values of droplet size distributions in the submicron range of 0.1_0.4 mu m.