(167ah) Relaxation Dynamics of Flow-Assisted Chiral Assembly

Mimicking complex multi-scale hierarchy in biological structures can bring notable functionalities from nature to artificial materials. One of these outstanding functionalities is the remarkable mechanical properties of chiral nematic structures in the complex concentric cylindrical organization. To resemble the axially aligned concentric chiral nematic structure, we attempt to direct the chiral self-assembly of aqueous suspensions of cellulose nanocrystals (CNCs) confined within the cylindrical capillary tubes upon flow cessation. The chiral nematic structure of CNC suspensions, however, depends on the CNC concentration at stationary conditions. As CNC concentration increases, the isotropic phase transitions into biphasic, fully chiral nematic, and eventually nematic structures. The chiral nematic structure can be identified by fingerprint patterns in polarized optical microscopy (POM) images, as the double distance between two consecutive bright/dark stripes in fingerprint patterns is the characteristic pitch length and represents the complete twist of the chiral structure. Using POM, we investigate the dynamics of chiral nematic relaxation in biphasic and fully chiral nematic suspensions. We have found that the shear flow through the capillary tubes aligns CNC particles along the flow direction, leading to a para-nematic structure. After flow cessation, however, the para-nematic structure tends to decrease the net free energy by a smooth transition towards the stationary structure. Within the first minute after flow cessation, fingerprint patterns start to form parallel to the flow direction, indicating axial alignment of concentric chiral structure. Such a fast relaxation rate at the early stages of time is caused by relieving excess elastic energy stored in the para-nematic structure. The rate of pitch length increment also represents the relaxation dynamics of the chiral structure. The pitch length of the meta-stable chiral nematic structure, however, decreases as the CNC content increases, showing a similar trend observed in the stationary condition. Understanding the chiral relaxation dynamics can able us to properly manage the arresting of meta-stable chiral nematic to program the fabrication of the axially aligned concentric chiral nematic architectures.