E33 Emulsification

Topic

An emulsion is a dispersion of droplets of one liquid—such as this oil—within another liquid—such as this water—or vice versa; this emulsion is thermodynamically unstable because creating the interfacial area separating the droplets of the dispersed liquid from the surrounding liquid always incurs an energy cost. Consequently, the emulsion requires an emulsifier—such as lecithin or protein from an egg yolk, or mustard—which adsorbs onto the interface between the two liquids, reduces the interfacial tension, and thereby stabilizes the dispersed droplets. The more effectively the emulsifier reduces the interfacial tension between the two liquids, the more stable the resulting emulsion becomes.

Laplace pressure dictates that a small droplet of the dispersed liquid sustains a higher internal pressure than a large droplet of the same liquid; this internal pressure difference drives coalescence—a form of Ostwald ripening—whereby small droplets of the dispersed liquid fuse with larger ones, eventually destroying the emulsion. The emulsifier that stabilizes the dispersed droplets also resists this coalescence, thereby delaying the emulsion's breakdown. Because a smaller droplet of the dispersed liquid has a higher internal pressure (according to Laplace pressure), the process of coalescing into a larger droplet proceeds more rapidly. The size of the droplets of the dispersed liquid is determined by the competition between turbulent disruptive forces—capable of breaking up a large droplet—and the interfacial tension between the two liquids, which resists such deformation. The Weber number compares these turbulent disruptive forces to the interfacial tension; when the Weber number for a droplet exceeds a critical value, the droplet breaks down into even smaller droplets, whereas below that critical value, it remains stable against the turbulent disruptive forces. If the turbulent disruptive forces are more intense—meaning greater turbulent energy is dissipated within the smallest eddies of the continuous liquid phase—the Weber number for the droplet increases, causing it to break into even smaller droplets.

Thus, this model—incorporating the concepts of coalescence and Weber-number-dependent droplet size—describes the process of making mayonnaise: egg yolk provides lecithin, which drastically reduces the interfacial tension between the oil and water, while vigorous whisking supplies the turbulent energy needed to break the oil into tiny droplets within the water. However, the oil must be added slowly; adding it too quickly overwhelms the lecithin's ability to timely coat the new interfaces created by each new oil droplet, resulting in the mayonnaise separating rather than emulsifying. As the droplets of dispersed oil within the already-formed mayonnaise approach the maximum dispersed-phase fraction corresponding to the densest possible packing of spheres, they must deform—ceasing to be spherical and becoming polyhedral—in order to pack even more tightly; this process gives the mayonnaise its characteristic semi-solid texture.

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