Photorheological (PR) fluids, i.e., those with light-tunable rheological properties, may be useful in a variety of applications, such as in sensors and microfluidic devices. Currently, the need to synthesize complex photosensitive molecules hampers the applicability of these fluids. Here, we report a simple class of PR fluids that require no special synthesis and can be easily replicated in any lab from inexpensive chemicals. The fluids consist of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), and the photoresponsive organic derivative, trans-ortho-methoxycinnamic acid (OMCA). Aqueous mixtures of CTAB and OMCA in basic solution self-assemble into long, wormlike micelles. Upon irradiation by UV light (<400 nm), OMCA undergoes a photoisomerization from its trans to its cis form, which alters the molecular packing at the micellar interface. The result is to transform the long micelles into much shorter entities and, in turn, the solution viscosity decreases by more than 4 orders of magnitude. Small-angle neutron scattering (SANS) is used to confirm the dramatic reduction in micellar length. The extent of viscosity reduction in these PR fluids can be tuned based on the composition of the mixture as well as the duration of the irradiation.

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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.