"Smart" Fluids: Self-Assembled Systems with Viscosity Tunable by Light
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A "smart" fluid is one that undergoes a change in some macroscopic property in response to an external stimulus, such as light or magnetic fields. One class of "smart" fluids is photorheological (PR) fluids, which exhibit changes in their rheological or flow properties (such as viscosity) upon irradiation with light at a given wavelength. These PR fluids 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 widespread use of these fluids. In this dissertation, we are working toward simple classes of PR fluids that require no special synthesis and can thereby be easily replicated in any laboratory from inexpensive chemicals.
In the first part of this study, we report a new aqueous PR fluid that exhibits a 10,000-fold reduction in viscosity upon UV irradiation. The fluid consists of the cationic surfactant, cetyl trimethylammonium bromide (CTAB), and the photoresponsive organic derivative, trans-ortho-methoxycinnamic acid (OMCA). Aqueous mixtures of CTAB and OMCA self-assemble into long, chainlike structures called "wormlike micelles", and the solution thereby has a very high viscosity. 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 wormlike micelles into much shorter entities and, in turn, the solution viscosity decreases by more than 4 orders of magnitude. We use small-angle neutron scattering (SANS) to confirm the dramatic reduction in micellar length. Our studies also show how one can tune the magnitude of viscosity reduction in these PR fluids based on the composition of the mixture as well as the duration of the irradiation.
In the second part of this study, we turn our attention to non-aqueous solvents and demonstrate how to make PR fluids in such solvents. The PR effect in these fluids relies on transformations of "reverse" micellar structures formed by a common lipid (lecithin) in conjunction with a stilbene-based photoresponsive additive, 4-hydroxy-4'-nitrostilbene (HNS). Certain mixtures of lecithin/HNS/water in cyclohexane undergo an increase in viscosity (photogelling) upon irradiation with UV light. Interestingly, other compositions of the same mixtures undergo a decrease (photothinning) in viscosity upon irradiation.
Both PR fluids described above provide a one-way (high to low, or low to high) viscosity switch. In the third and final part of this study, we report an aqueous system that provides a true, reversible PR fluid, where the viscosity can be switched from high to low and back using different wavelengths of light. These fluids are based on mixtures of a 22 carbon-tailed cationic surfactant with an azobenzene-based photosensitive molecule, 4-azobenzene carboxylic acid (ACA). The conceptual basis for these fluids is similar to that in our first study, and moreover, these molecules are also inexpensive and available from commercial sources. This opens the door to future investigations on PR fluids from both academic and industrial laboratories and should eventually lead to new applications for this interesting class of responsive materials.