A simple route to fluids with photo-switchable viscosities based on a reversible transition between vesicles and wormlike micelles

Recently, there has been much interest in photorheological (PR) fluids, i.e., fluids whose rheological properties can be tuned by light. In particular, there is a need for simple, low-cost PR fluids that can be easily created using inexpensive, commercially available ingredients and that show substantial, reversible changes in rheology upon exposure to different wavelengths of light. Towards this end, we report a class of photoreversible PR fluids prepared by combining the azobenzene derivative 4-azobenzene carboxylic acid (ACA) (in its salt form) with the cationic surfactant erucyl bis(2-hydroxyethyl)methyl ammonium chloride (EHAC). We show that certain aqueous mixtures of EHAC and ACA, which are low-viscosity solutions at the outset, undergo nearly a million-fold increase in viscosity when irradiated with UV light. The same solutions revert to their initial viscosity when subsequently exposed to visible light. Using an array of techniques including UV-vis and NMR spectroscopies, small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM), we have comprehensively characterized these PR fluids at the molecular, nanostructural, and macroscopic scales. Initially, EHAC–ACA are self-assembled into unilamellar vesicles, which are discrete container structures and give the sample a low viscosity. Upon exposure to UV light, ACA undergoes a trans to cis photoisomerization, which alters the geometry of the EHAC–ACA complex. In turn, the molecules self-assemble into a different structure, viz. wormlike micelles, which are long, entangled chains and impart a high viscosity to the sample. The above changes in viscosity are repeatable, and the sample can be reversibly cycled back and forth between low and high viscosity states. Our photoreversible PR fluids can be easily replicated in any industrial or academic lab, and it is hoped that these “smart” fluids will eventually find a host of applications.