Nonaqueous photorheological fluids based on light-responsive reverse wormlike micelles.

Fluids whose flow properties can be altered by light are an emerging class of functional materials, with potential applications in microscale actuators and valves. While many such photorheological (PR) fluids have been developed over the years, most are based on specialized organized molecules that require synthesis. We have instead sought to develop PR fluids using inexpensive chemical components that are commercially available, and this approach has been successfully applied to aqueous systems. Here, we demonstrate a simple class of nonaqueous PR fluids based on the phospholipid, lecithin, and the organic derivative, para-coumaric acid (PCA). The combination of lecithin with the trans form of PCA results in reverse wormlike micelles, which are long, flexible chains that undergo entanglement, thereby giving rise to viscoelastic fluids. Upon UV irradiation, trans-PCA is photoisomerized to its cis form, which has a lower polarity and hydrogen-bonding tendency. This causes a significant reduction in the length of the micellar chains, and in turn, the fluid viscosity drops more than 1000-fold. We show that photoresponsive reverse micelles can be formed in a range of organic solvents including cyclohexane, n-alkanes, alkenes, and fatty acid esters.