Photogelling fluids based on light-activated growth of zwitterionic wormlike micelles

Abstract

The creation of fluids with light-tunable rheological properties using simple, inexpensive chemicals has been a recent focus of our laboratory. Such fluids could be useful in applications ranging from sensors to microfluidic valves. Towards this end, we report in this paper a class of aqueous fluids that exhibit “photogelling”, i.e., a substantial (10 000-fold) increase in fluid viscosity upon exposure to light. Such an increase in viscosity is triggered by the light-activated growth of wormlike micelles in the sample. The key components in the above fluids are the zwitterionic surfactant, erucyl dimethyl amidopropyl betaine (EDAB) and the photoresponsive molecule, trans-ortho-methoxy-cinnamic acid (OMCA), both of which are commercially available. OMCA binds to EDAB, and when the two are combined at high (>2 : 1) OMCA : EDAB molar ratios, short cylindrical micelles are formed in aqueous solution. Upon irradiation by UV light (<400 nm), OMCA gets photo-isomerized to its cis form, which then desorbs from EDAB micelles. In turn, a transition from short to long, entangled micelles ensues. Support for the above mechanism is provided by ζ potential and small-angle neutron scattering (SANS) studies. The photogelling effect can be reversed by further addition of OMCA, allowing the sample to be cycled through high- and low-viscosity states.