Analysis of time-dependent scattering spectra for studying processes associated with microdroplets

On progeny droplets emitted during Coulombic fission of charged microdrops

The charge level at which a drop undergoes a Coulombic fission is given by the Rayleigh limit. The charge and mass losses from the drop during the fission and the characteristics of resulting progeny droplets, however, remain unpredictable. The charge of a dielectric drop arises from ions, and we have examined the effects of ions on fission-related charge and mass emissions from single levitated microdrops. The ion concentration in a drop was varied through addition of appropriate amounts of an ionophore or ionic liquid. The results show that the mass loss from a drop decreases, while the charge-to-mass ratio of progeny droplets increases as the ion concentration in the drop increases. From these observations we establish that nonuniform ion distributions that exist in a charged dielectric drop play a dominant role in the fission process by influencing the electrical conductivity at the drop surface. We show that the charge-to-mass ratio of progeny droplets is proportional to the conductivity at the surface of the mother drop, which has been calculated from the mobilities of ions and ion concentrations at the surface of the mother drop. By minimizing the Gibbs free energy change associated with a Coulombic fission we deduce that a progeny droplet carries 50% of the Rayleigh limit charge, and from the experimental data we establish that the progeny droplet size varies inversely to the surface conductivity raised to the power of 2/3.

Investigation of concentrically and eccentrically layered droplets by light scattering

Two pairs of immiscible liquid compounds are chosen to prepare levitated layered droplets with and without density difference between core and layer phases. The droplets are examined by light scattering along two orthogonal directions. A layered droplet without phase density difference is unambiguously identified as a concentric sphere by matching the observed scattering spectra with those calculated from the Aden-Kerker extension of Mie theory. For layered droplets with phase density difference, only the scattering spectrum from one of the scattering directions can be matched theoretically. These observations suggest that a static layered droplet is predominantly eccentric even though the embedded core is large by volume, as predicted from fluid mechanics. The consistency of the light-scattering characterization with the diffusion theory governing the evaporation of concentrically and eccentrically layered droplets is also established.