(371f) Condensation of Immiscible Vapors on Single Microdroplets

We have examined heterogeneous nucleation on single droplets that were suspended in an electrodynamic balance mounted inside a thermal diffusion cloud chamber. The droplets were exposed to vapors of various immiscible compounds, and the vapor concentration around a droplet was increased from unsaturated to supersaturated levels in small steps by altering the temperature difference between the top and bottom plates of the chamber. A resonance based light scattering technique was used to detect formation of a second phase in the droplets. We have examined dioctyl phthalate (DOP) droplets exposed to water vapor, Santovac droplets (i.e., five ring polyphenyl ether) to invoil 90 (i.e., straight chain alkane) vapor, hexadecane vapor and Fomblin (i.e., perfluorinated polyether) vapor. Results show that when a droplet is exposed to an environment that is supersaturated with an immiscible vapor, heterogeneous nucleation occurs through three possible ways: by dropwise condensation, nucleation inside the droplet, and nucleation leading to the formation of a layer. High surface tension compounds (e.g., water vapor on DOP droplets) do not form layers. We also observed that when a Santovac droplet is exposed to invoil 90 vapor, above a certain supersaturation level a second phase forms by nucleation inside the droplet phase, and nuclei grow to form an emulsion in the droplet. Nucleation of low surface tension compounds on a droplet results in the formation of a layer, and transpires in the absence of any significant supersaturation. The results show that the formation an adsorbed layer depends on the surface and interfacial tensions. For example, hexadecane layers form on Santovac droplets at hexadecane saturation ratios in the range S=1.005 to 1.02. Below S=1.005, a droplet absorbs hexadecane vapor, while the droplet remains homogeneous. After the formation of a layer on a droplet, the layer initially grows slowly, but the growth rate increases as the layer thickness increases. Above a certain thickness, the layer behaves like a macroscopic phase whose growth rate can be described by diffusion equations.