Shearmetry\nof Fluids with Tunable Rheology by Polarized\nLuminescence of Rare Earth-Doped Nanorods

Shear\nstress plays a critical role in regulating physiological\nprocesses within microcirculatory systems. While particle imaging\nvelocimetry is a standard technique for quantifying shear flow, uncertainty\nnear boundaries and low resolution remain severe restrictions. Additionally,\nshear stress determination is particularly challenging in biofluids\ndue to their significant non-Newtonian behaviors. The present study\ndevelops a shearmetry technique in physiological settings using a\nbiomimetic fluid containing rare earth-doped luminescent nanorods\nacting in two roles. First, they are used as colloidal additives adjusting\nrheological properties in physiological media. Their anisotropic morphology\nand interparticle interaction synergistically induce a non-Newtonian\nshear-thinning effect emulating real biofluids. Second, they can probe\nshear stress due to the shear-induced alignment. The polarized luminescence\nof the nanorods allows for quantifying their orientational order parameter\nand thus correlated shear stress. Using scanning confocal microscopy,\nwe demonstrate the tomographic mapping of the shear stress distribution\nin microfluidics. High shear stress is evident near the constriction\nand the cellular periphery, in which non-Newtonian effects can have\na significant impact. This emerging shearmetry technique is promising\nfor implementation in physiological and rheological environments of\nbiofluids.