Description
Microfluidic devices are ideally suited for the study of complex fluids undergoing large deformation rates in the absence of inertial complications, providing access to well-defined extensional and shear flows that are difficult to achieve in conventional rheometric geometries. Previous studies employing narrow-width microfluidic channels, however, have faced challenges in accurately quantifying flow-induced deformation in long-chain systems, owing to limited spatial resolution, short residence times, and uncertainties in local flow conditions. Here, we combine advanced microfluidic design with multimodal in-flow characterization—including flow birefringence measurements, synchrotron-based small-angle X-ray scattering (SAXS), particle image velocimetry (PIV), and numerical simulations—to resolve how wormlike micelles and semiflexible polymers respond to extensional and shear deformation in contraction–expansion microfluidic geometries.
Two complementary microfluidic devices were developed for this purpose. A glass–SU8 contraction–expansion chip enables precise control of flow kinematics and quantitative optical measurements under flow, while a multilayer microfluidic chip incorporating thin diamond X-ray windows provides high X-ray transmission and mechanical stability for in-flow SAXS experiments. Together, these platforms allow direct correlation between microfluidic flow fields and molecular-scale structural response under non-equilibrium conditions.
Using these devices, we investigate the flow-induced ordering of three representative wormlike systems with distinct molecular architectures and interaction mechanisms: semiflexible wormlike polymer chains (ctDNA), ionic wormlike micelles formed by CTAB/NaSal, and nonionic wormlike micelles based on C12E6/n-dodecanol. ctDNA behaves as a semiflexible wormlike chain with a large persistence length and relatively slow relaxation dynamics, leading to strong extensional alignment but limited capacity for rapid structural reorganization. In contrast, CTAB/NaSal forms highly dynamic ionic wormlike micelles whose contour length and connectivity are governed by electrostatic interactions and micellar scission–recombination processes, resulting in pronounced viscoelasticity and rapid flow-induced restructuring. The nonionic C12E6/n-dodecanol system exhibits weaker electrostatic interactions and slower breaking dynamics, providing a complementary wormlike micellar system with distinct relaxation pathways and sensitivity to extensional stresses.
In-flow SAXS measurements corroborate these differences by revealing pronounced anisotropic scattering and clear azimuthal intensity modulation downstream of the contraction region, directly reflecting variations in alignment strength and relaxation behavior among the three systems. Velocity fields obtained from PIV were incorporated into numerical simulations using a Giesekus constitutive model. The resulting extensional stress distributions quantitatively reproduce the experimentally observed optical anisotropy, highlighting strong agreement between structural, optical, and hydrodynamic measurements.
Together, these results establish a unified multimodal microfluidic framework for probing flow-induced ordering in wormlike systems and provide mechanistic insight into how extensional stresses govern molecular alignment in confined microfluidic environments, demonstrating the potential of microfluidic platforms as enabling interfaces for high-energy spectroscopic studies of complex fluids under non-equilibrium flow conditions.