David S. Corti, Professor of Chemical Engineering, Elias Franses, Professor Emeritus of Chemical Engineering, and graduate student, An-Hsuan (Andy) Hsieh have unexpectedly discovered liquid-like dispersions with iridescent colors which were previously only observed in solid systems, such as gemstones, or in feathers and butterfly wings. The paper titled, “Formation of gem-like dispersions of soft crystallites in water by vesicles of a cationic surfactant” concluded that when aqueous dispersions were formed from a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB) in varying weight fractions, the iridescent colors were a result of the formation of “soft” crystallites formed by self-assembled vesicles.

Corti explains the significance of the research team’s discovery: “Interestingly, the volume fractions of the vesicles are lower than 0.3, which is lower than what would be expected theoretically for the onset of a fluid-solid transition. By contrast, the effective volume fractions of the vesicles, which account for long-range electrostatic interactions between the charged vesicles, are higher, and may drive the formation of “soft” crystallites. These crystallites remain suspended, and are easily broken up by shear or mixing, but are nevertheless quick to reform. For these reasons, the vesicle dispersions remain liquid-like, as they still flow at high shear stresses as bulk dispersions with high viscosities.”

Despite their pleasant visual appearances, these dispersions may also yield other practical benefits; they can act as good stabilizers against sedimentation of aqueous silica suspensions, which otherwise would settle quickly. Suspensions of titania and other inorganic particles in water containing such crystallites can be used as white inks in ink-jet printers because of their favorable optical properties and their flow properties that make them usable in high-speed flows.

Figure 1 (Left): Photographs of DDAB dispersions at various weight fractions w, 14 days after their preparation. The photographs were obtained with either fluorescent light or white LED light illumination at several scattering angles θ. The little squares in the top row of photographs indicate the location of the incident light beam in the ST instrument used. Figure 2 (Right): Photographs of DDAB dispersions obtained at various weight fractions at various times after their preparation, with white LED light illumination at a scattering angle θ = 160