Structural Dynamics of Complex Fluids Subjected to Planar Extensional Flow Studied Using In Situ X-ray Scattering


Complex fluids are ubiquitous, from natural materials to manufactured products. Understanding their behavior under flow is vital for engineering these materials. Extensional flow, despite being industrially relevant and often producing dominant impacts upon complex fluids, is an underserved topic compared to shear flow due to a lack of reliable apparatuses to apply well-defined extensional flow. The unusual behavior of complex fluids is manifested in the presence of microstructures at a mesoscopic length scale, frequently tens to hundreds of nanometers, where x-ray scattering is a powerful tool. This thesis reports the design and implementation of a high aspect ratio cross-slot flow instrument to facilitate in situ synchrotron x-ray scattering of low-to-moderate viscosity complex fluids subjected to planar extensional flow. This instrument is applied to two categories of complex fluids: dilute carbon nanotube suspensions and self-assembled surfactant solutions. The structural dynamics of dilute carbon nanotube suspensions subjected to both homogeneous shear and planar extensional flow have been studied, under both transient and steady-state flow conditions. The transient dynamics of CNTs in shear follows the classic behavior expected from Brownian rod-like particles. The relaxation behavior reflects polydispersity of our sample. Under steady-state conditions, planar extension aligns CNTs more effectively than shear flow. The orientation of CNTs can be mostly captured by simulation of orientation in dilute Brownian rods. However, the difference in orientation observed in shear and extension requires that the nanotubes be described with finite aspect ratio due to their bent and crooked shape. Finally, consistency in nanotube orientation measured under shear in the cross- slot flow cell and a dedicated homogeneous shear flow cell validates our design and implementation. Two types of self-assembled surfactant solutions were studied under planar extensional flow: wormlike micelles and lamellar surfactants. The wormlike micelles exhibit increasing orientation as extensional rate grows for modest flow rates. Beyond a critical rate, viscoelastic flow instabilities set in, which disrupt the intended flow field. Two different lamellar surfactants display different inherent ordering: one exhibits negligible relaxation while the other loses most of its flow-induced orientation, though both align along the flow with little change of anisotropy to different flow rates.

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