Electrostatic interaction between two aqueous microdroplets in an apolar medium

In an apolar medium, the electrostatic interactions between two aqueous microdroplets coated with surfactants are investigated by the mean-field theory. The electric field and ion distributions within the aqueous core are described by the Poisson-Boltzmann equation. Under Debye-Hückel approximation, the interaction energy is obtained analytically. Due to the polarization effect, the Coulomb interaction is altered by the induced multipoles in the aqueous droplet. The interaction, however, is insensitive to the ion concentration. In comparison with the Coulomb interaction, the repulsion for a pair of similarly charged droplets is reduced and the attraction for a pair of oppositely charged droplets is enhanced. More importantly, the interaction between a neutral and charged droplets is attractive. The effect of interdroplet interactions on the collision frequency is also discussed.