Networks formed by long cylindrical or wormlike molecules appear in various shapes in nature and technology. They occur on a variety of length scales from nanometers to centimeters and determine especially the mechanical properties of a material, e.g., they control stabilization and transport, and induce interesting phenomena like viscoelasticity. By our nanorheology study on the viscoelastic properties of photorheological liquids, we gained new insight into the complex processes in microscopic networks on the nanometer length scale, in particular, the dynamics mediating the stress relaxation and determining the high-frequency rheology of the system.
Photorheological fluids contain entangled networks of wormlike molecules, where the molecular structure and thereby network morphology can be altered by irradiation with light. Aqueous solutions of highly concentrated cetyl trimethylammonium bromide (CTAB) and ortho-methoxycinnamic acid (OMCA) exhibit enhanced viscoelasticity due to the self-assembly of long cylindrical (wormlike) micelles. The photoresponsivity of OMCA gives rise to the characteristic photorheological properties of those fluids that can be tailored by UV illumination.
Employing passive nanorheology, the network structure and the associated stress relax- ation processes are accessible through the correlation functions of silica tracer particles measured by microsecond X-ray photon correlation spectroscopy (XPCS). The methods we developed to overcome limitations like beam damage and to analyze big datasets in the low intensity limit will be valuable for future XPCS studies on radiation sensitive systems and microsecond dynamics.
We found that the nanorheology results deviate from macroscopic measurements when the confinement of the tracer particles is reduced below a certain threshold indicative of the complex network microstructure.
We could further show that the functional form of the stress relaxation is reflected in the shape of the XPCS correlation functions. Anomalous diffusion, especially subdiffu- sive behavior, originates from the coupling between micelle and tracer dynamics. More precisely, we found a transition from exponential stress relaxation due to random scission of micelles to strongly non-exponential, reptation driven stress relaxation of the network, that can be tuned by UV illumination.
From the short-time caging motion of the tracers, information on the local viscoelas- tic properties could be reduced on the nanometer length scale and microsecond time scales beyond the limits of classical rheology. Our results indicate high-frequency strain- stiffening of the OMCA-CTAB network due to short-range rigidity of the micelles.