A Simple Class of Photorheological Fluids:  Surfactant Solutions with Viscosity Tunable by Light

Abstract

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.

Practical applications of small-angle neutron scattering

“…35,36 For self-assembling organic species in solution, SANS is particularly useful because of the afforded high contrast between a 1 H-rich material and a 2 D-rich solvent. Consequently, one of the more common uses of SANS is for the characterisation of aggregate structures of surfactants, [37][38][39][40][41] 33,52,53 are also investigated, for a variety of purposes including mustard gas decontamination, 50 art restoration, 51 and drug delivery. 33 In this case, selective deuteration of the different components is frequently used to obtain a more detailed structural picture, as highlighted in Fig.…”

Smart Self‐Assemblies Based on a Surfactant‐Encapsulated Photoresponsive Polyoxometalate Complex

“…By grafting the azobenzene (Azo) group [12] covalently onto the Mn-Anderson-type cluster through a tris(hydroxymethyl)aminomethane connector ( Figure S1 in the Supporting Information), [10] we prepared a novel photoresponsive surfactant-encapsulated organically grafted polyoxometalate (SEOP), as shown in Scheme 1 a.…”

pH-Responsive wormlike micelles based on microstructural transition in a C22-tailed cationic surfactant–aromatic dibasic acid system

“…These stimuliresponsive viscoelastic uids produced by WLMs can nd applications in transmission clutches, 35 shock absorbers, 36 vibration control, 37 human muscle stimulators, and clean fracturing uids. 11,38 Compared to other external stimuli, pH is a simple and economical trigger for controlling viscoelasticity of WLMs solutions.…”