Size and composition of foam droplets with applications to the marine atmosphere and nanoparticle synthesis.

摘要

Foams are used in chemical engineering to separate constituents from the bulk liquid. The industrial foam separation method, known as bubble fractionation, ends with an enriched foam being skimmed from the liquid. Foam bubble bursting, which produces aerosol droplets (i.e. foam droplets), can also result in separation. Like bubble fractionation, foam droplet separation relies on interfacial mass transport to separate material from solution. In this process, the separation ends with the formation of aerosol droplets. The objectives of this work were to (1) study foam droplet separation for naturally occurring oceanic whitecap foams and (2) to generate nanoparticles using foam droplet separation.; Sea salt aerosol (SSA) particles are routinely observed in the remote marine boundary layer (MBL); these aerosols include cloud condensation nuclei and so affect the earth's radiative balance. Here foams designed to mimic oceanic whitecaps were generated in the laboratory using a range of bubbling flow rates and aqueous media: unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and artificial seawater. The number and sizes of dried foam droplets in the particle diameter range 0.015--0.67 mum were measured; an impactor was also used to collect droplets in the size range 0.056--18 mum. Collectively, the results indicate that foam droplet size distributions are bimodal with mass modes in the aerodynamic diameter ranges at 80% relative humidity of 0.56--1 mum and 1.8--2.5 mum. The submicrometer foam droplet mode, which corresponds to a number size distribution mode at a dry diameter of 100 nm, falls within the range of reported mean diameters (dry diameter = 40--200 nm) for submicrometer SSA particles observed in the remote MBL.; A novel approach to nanoscale separation under ambient conditions was developed whereby foam bubble bursting produces aerosol droplets. The ability of the foam aerosol cycle to produce useful nanoparticles was demonstrated by synthesizing nanoparticles composed of sodium chloride, phosphotungstic acid (H3PW12O40), and bovine insulin. Following droplet production, aerosol droplets were dried; the geometric mean diameters of the sodium chloride, H3PW12O40, and insulin particles were in the ranges 60--100 nm, 45--50 nm, and 5--250 nm.