Spray Application and Release of Microalgae from Water-in-Oil Emulsions

Background: This study was completed as part of a larger effort to develop emulsions that have high stability for long-term algal cell storage and that release cells rapidly when applied to a pond surface as inoculum. Past research has shown that including silica nanoparticles in the oil phase yields stable emulsions but that cell release upon application takes place on the order of days. Methods: Water-in-oil (W/O) emulsions were prepared with Chlorella sorokiniana and brilliant sulfoflavine (BSF) in the internal phase to investigate the effects of oil phase thickeners on BSF and cell release upon spray application of the emulsion to a water surface. Three oil phase thickeners: Aerosil R974, Bentone 38, and Bentone 150, were tested at concentrations ranging from 0.25-1% (w/w) in the oil phase. Results: The oil phase thickeners had a modest impact on the release of BSF but no significant effect on cell release. In particular, emulsions with Bentone 150 had significantly faster release of BSF than other thickeners. Spray application of the emulsion onto a water surface, however, had a dramatic effect on cell release rate without negatively affecting cell viability. Average spray droplet size was found to be ~45 μm and nearly 100% release of both cells and BSF could be achieved within ~30 minutes. Conclusion: Spray application resulted in rapid release of algae and dye even for emulsions with a high concentration of silica nanoparticles. The fact that thickeners affected the release rate of soluble BSF but not cells suggests the potential to design emulsions with differential release rates of cells and compounds upon spray application.