Nonaqueous Photorheological Fluids Based on Light-Responsive Reverse Wormlike Micelles

Abstract

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.

Applications of Smart Wormlike Micelles

“…Conventional PR fluids are typically a two-phase system that aggregates or settles with time. A new class of WLMs-based PR fluids have been proposed by the group of Raghavan [50][51][52], based on widely available photo-sensitive organic acids or salts mixed with conventional WLMs-forming surfactants: for instance CTAB [50] or EDAB [51] in the presence of OMCA, or lecithin with PCA [52]. PR fluids promise to be true, single-phase solutions, structured at the nanoscale.…”

“…Advantageously, these fluids are likely to tolerate the addition of electrolytes, macromolecules, or nanoparticles. Applications in bioseparation are anticipated, as well as capillary electrophoresis, where the photo-gelling carrier fluid can be loaded into the capillary while it is thin and later transformed into a gel-like state by UV irradiation [52]. Other groups have reported photoswitchable fluids [55,56] based on similar principles.…”

Other Types of Smart Wormlike Micelles

“…Specifically, [34]. Copyright (2012) Elsevier the non-polar tails are directed outwards into the organic solvent, while the polar heads point inwards [37].…”

“…Raghavan and co-workers [37] pioneered the work on stimuli-responsive RWLMs, where lecithin was used as an amphiphile, para-coumaric acid (PCA) as a photo-responder, and cyclohexane as a solvent. The combination of lecithin with the trans form of PCA induces the formation of reverse worms and thereby imparts viscoelasticity to the bulk solution.…”

UV-triggered shape-controllable PP fabric

“…Moreover, its wavelength and irradiation time can be easily controlled. Hence, plenty of light-responsive polymers have been investigated to develop smart materials, such as artificial muscles, 9 photorheological fluids, 10 actuators, 11 and switching devices. 12 It is noteworthy that an azobenzene moiety or its derivatives are the most common compounds to build various light-responsive polymers due to their known photo-isomerization, which can undergo reversible switching between an extended trans form and a shortened cis form with alternating exposure to UV and visible light, respectively.…”