Emulsifier crystal formation and its role in periodic deformation-relaxation of emulsion droplets upon cooling

When emulsion droplets stabilized by monoglyceride stearate are cooled down to a temperature below the crystallization temperature of the emulsifier, they deform, followed by relaxation back to a spherical shape. This deformation-relaxation process repeats itself multiple times. We monitored the crystallization of monoglyceride stearate at oil-water interfaces with (multiphoton excitation) microscopy and tensiometry, and evaluated the effects of emulsifier type, emulsifier concentration, and cooling rate on this phenomenon. The results prove that the phenomenon is attributable to the formation of brittle crystalline interfacial layers by the emulsifier, and stress build-up in these layers due to shrinkage of the oil phase upon cooling. Because of the high stiffness of the interfacial crystal layers, shrinkage of the oil phase leads to wrinkling of the interface, resulting in shape deformations. This continues until the interfacial structure ruptures.