Preparation and characterization of multilayer coated microdroplets: droplet deformation simultaneously probed by atomic force spectroscopy and optical detection.

We report the preparation and characterization of multilayer coated droplets in an emulsion. Stability and control of mass transport across the interface are the key issues for such coated microdroplets. Shelf life of cosmetic, pharmaceutical, and food formulations can be improved by increasing stability. Moreover, such emulsions have potential applications in drug delivery and storage. A primary oil-in-water emulsion with caseinate as an emulsifier was prepared. On the basis of attractive electrostatic interactions, polyelectrolytes with opposite charges were added layer by layer. The oil droplets (particle size around 10 microm) were successively coated with casein, pectin, whey proteins, pectin, whey proteins, and pectin. Laser diffraction spectroscopy, particle charge measurements, and confocal laser scanning microscopy were applied to characterize the multilayer droplets. The complementary results indicate that the inner layers merge and the packing density of the interface increases. AFM-induced mechanical compression of single oil droplets coated with casein and pectin is monitored by an inverted optical microscope, and simultaneously AFM force curves are recorded. Thus, the deformation of the droplet is reflected by its lateral expansion and the force curve. Force volume imaging is applied to probe the lateral distribution of mechanical properties of the droplet.