In Situ Visualization of the Structural Evolution and Alignment of Lyotropic Liquid Crystals in Confined Flow

Abstract

Self‐assembled materials such as lyotropic liquid crystals offer a wide variety of structures and applications by tuning the composition. Understanding materials behavior under flow and the induced alignment is wanted in order to tailor structure related properties. A method to visualize the structure and anisotropy of ordered systems in situ under dynamic conditions is presented where flow‐induced nanostructural alignment in microfluidic channels is observed by scanning small angle X‐ray scattering in hexagonal and lamellar self‐assembled phases. In the hexagonal phase, the material in regions with high extensional flow exhibits orientation perpendicular to the flow and is oriented in the flow direction only in regions with a high enough shear rate. For the lamellar phase, a flow‐induced morphological transition occurs from aligned lamellae toward multilamellar vesicles. However, the vesicles do not withstand the mechanical forces and break in extended lamellae in regions with high shear rates. This evolution of nanostructure with different shear rates can be correlated with a shear thinning viscosity curve with different slopes. The results demonstrate new fundamental knowledge about the structuring of liquid crystals under flow. The methodology widens the quantitative investigation of complex structures and identifies important mechanisms of reorientation and structural changes.

A Review of Microfluidic Devices for Rheological Characterisation

“…Buscema and coworkers [115] employed a contraction device to explore the flow-induced structural changes of liposomes. Very recently, Rodriguez-Palomo et al [116] employed SAXS to study the structure of lyotropic liquid crystals and their behaviour in contraction microfluidic devices. Few other studies were also reported to explore the flow behaviour of complex fluids in geometries substantially different from the contraction one, including serpentine channels and microfluidic pillars [111,117].…”

Microfluidic Nanomaterial Synthesis and In Situ SAXS, WAXS, or SANS Characterization: Manipulation of Size Characteristics and Online Elucidation of Dynamic Structural Transitions

“…It is worth also noting that the flow strains and the employed shear stresses in microfluidics may lead to the alignment of vesicles and affect the orientation of lamellar and hexagonal phases under shear flow, resulting in the appearance of anisotropic SAXS (or SANS) patterns [ 31 , 71 , 74 , 79 ]. For instance, a slight alignment was detected during the microfluidic synthesis of MLVs on increasing TFR from 5 to 15 μL/min ( Figure 4 A,B).…”

“…A shear-induced deformation of MLVs was also detected under shear flow during their formation from a lamellar phase [ 79 ]. Through a microfluidic SAXS scanning study, Liebi and co-workers [ 74 ] reported on the flow-induced transformation of the lamellar (L α ) phase to an aligned lamellae region via MLVs to an extended lamellae region by increasing the shear rates ( Figure 4 D). A significant effect of the microfluidic constriction, leading to an orientation of the lamellar phase under flow, was also reported ( Figure 4 E,F) in another study [ 71 ].…”

“…This microfluidic method led to a significant reduction in materials used in lyotropic liquid crystalline preparations, and the obtained phases (including lamellar and inverse bicontinuous cubic phases) and their structural features agreed well with those produced chip-off [ 48 ]. In another study, it was reported on online scanning SAXS of lamellar (L α ) and normal hexagonal (H 1 ) phases in microfluidic channels [ 74 ]. It was found that these phases are aligned under flow, but the flow-induced changes are not identical.…”

“…Right: Corresponding radial SAXS profiles in the three areas (A–C). Adapted with permission from [ 74 ]. 2021, Wiley.…”