EAGER: A New Class of Self-Assembled Photorheological Fluids

University Of Maryland, College Park, College Park MD

Investigators

Abstract

1062123 Raghavan Photo-rheological (PR) fluids are those whose rheological or flow properties such as their viscosity can be dramatically altered by illumination with light. Previous formulations of such fluids have necessitated the use of sophisticated organic molecules such as photo-responsive surfactants or polymers. However, the difficulty and cost involved in synthesizing these complex molecules has hampered research in this field. There is a need for low-cost PR fluids that can be prepared using only simple, commercially available nano-scale amphiphilic molecules. If such fluids were available, it is likely to have a transformative effect on a variety of scientific and engineering disciplines. Building on a successful CAREER project, the PI proposes to explore a new class of photo-reversible PR fluids that can be reversibly transformed from low to high viscosity by irradiation with different wavelengths of light. Most importantly, the proposed fluids will only use two commercially available molecules: a cationic surfactant and an azobenzene derivative. The results of this EAGER project are likely to represent a conceptual breakthrough in the field of stimuli-responsive fluids. The preliminary data will provide a framework for a larger systematic study on photo-reversible PR fluids, both aqueous and non-aqueous. Intellectual Merit: The proposed aqueous PR fluids are expected to be based on self-assembled nano-structures called "wormlike micelles", the properties of which will be impacted by the extent of binding of the azobenzene photoisomer. Irradiation with one wavelength of light is expected to elongate these micelles and thereby produce an increase in fluid viscosity. Conversely, irradiation at a different wavelength of light is expected to shorten the micelles and thereby induce a drop in the fluid viscosity. The PI hypothesizes that deviations from planarity of the photoisomer dictate its binding efficacy. This proposal will explore the rheological response of the PR fluids in steady and dynamic (oscillatory) shear before, during, and after irradiation with light at different wavelengths. The results will be correlated with microstructural studies using small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). Altogether, the study is expected to yield a coherent scientific picture for the behavior of these novel fluids. Broader Impact: Compared to other stimuli-responsive systems, PR fluids represent a fresh and exciting technology. This project will help to bring PR fluids "into the mainstream" by offering a range of simple, reversible systems that can be prepared in any laboratory using low-cost commercially available molecules. Potential applications for PR fluids include microfluidic valves, microscale robots, and drag-reducing fluids; additional applications are likely to arise once more scientists begin to explore these systems. Also, fundamental insight on light-responsive self-assembly from this work could be extended to structures other than micelles. This project will also support the graduate training and education of a student in the PI's department at University of Maryland.