Charge Limits on Droplets during Evaporation

We have examined charge stability limits of single evaporating microdroplets that were suspended in an electrodynamic balance. A high precision light scattering technique based on optical resonances was used to determine the size and the size change of a droplet at a charge instability induced breakup. The charge level and the charge loss at the breakup were obtained from the dc voltages required to balance the droplet prior to and following the breakup. The results on droplets of diethyl phthalate (DEP), diethylene glycol (DEG), triethylene glycol (TEG), and hexadecane show that breakups due to the charge instability occur at the Rayleigh charge limit. The observed charge losses during breakups range from about 15.3% for hexadecane droplets to about 41.1% for TEG droplets. Hexadecane droplets lose about 1.5% of their mass, while DEP droplets, about 2.3%. Within the detectable limit of 0.03%, no mass losses were observed during breakups of DEG and TEG droplets. The observation of extremely low mass losses that accompany high charge losses from DEG and TEG droplets suggests that the process of breakups of DEG and TEG droplets is distinct from that of DEP and hexadecane droplets. An analysis of the results indicates that breakups of DEP and hexadecane droplets result in the formation of a few large progeny droplets, while TEG and DEG droplets produce thousands of fine progeny droplets.