(281g) Characteristics of Progeny Droplets Produced by Charge Instability Induced Breakups of Droplets

During evaporation of a charged droplet, the charge density at the droplet surface increases. Eventually, the charge density reaches the instability limit, where the repulsive electrostatic force overcomes the cohesive force due to the surface tension. The instability causes breakup of the droplet, leading to the formation of a number of progeny droplets. This phenomenon reduces the charge of the parent droplet below the instability limit, but the continuous shrinkage of the droplet through evaporation causes further breakups at regular intervals. The Rayleigh limit predicts the onset of instability, but there is no theory to predict the fractional charge and mass losses or on the characteristics of progeny droplets generated by a breakup. In this study, we have investigated the fractional charge and mass losses at an instability induced breakup, and the factors that control the characteristics of the progeny droplets.

Experiments were conducted on single charged droplets suspended in an electrodynamic balance, and 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 a breakup were obtained from the dc voltages required to gravitationally balance the droplet prior to and following the breakup. The results from pure droplets of diethyl phthalate (DEP), hexadecane, diethylene glycol (DEG) and triethylene glycol (TEG) show that breakups due to the charge instability occur at the Rayleigh charge limit. The observed charge losses during breakups range from about 21% for DEP droplets to about 41% for TEG droplets. While a DEP droplet loses about 2.3% of its mass, no mass losses, within the detectable limit of 0.03%, are 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 shows 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 nanometer size progeny droplets. Since the electrical properties of DEG and TEG differ considerably from those of DEP and hexadecane, the results on pure droplets indicate that the charge and mass loss amounts are related to the electrical properties, such as the electrical conductivity.

To examine the effect of electrical conductivity on the fractional charge and mass losses at a breakup we have experimented on DEP and hexadecane droplets, whose electrical conductivities were increased by dissolving an ionofore (e.g., tridodecyl methyl ammonium chloride, TDMAC). The experimental results show that the electrical conductivity of a droplet has no effect on the charge limit at which an instability-induced breakup occurs, but the fractional charge and mass losses are affected by the conductivity. The fractional charge loss increases, while the mass loss decreases as the electrical conductivity increases. For example, for a DEP droplet containing about 10.0 mg TDMAC/ml, the fractional mass loss decrease to 0.40% from about 2.3% for a pure DEP droplet, while the fractional charge loss increases slightly. At higher concentrations, no mass losses can be observed within the detectable limit of 0.03%. The results indicate that the electrical conductivity dictates the fractional charge and mass losses, while the Rayleigh charge limit for instability remains unaffected.