Shearmetry in Fluids by Using Size‐Controlled Anisotropic LaPO₄:Eu³⁺ Nanorods as Polarized Luminescent Probes

Abstract Orientation analysis of colloidally dispersed nanorods (NRs) provides a powerful means to measure local shear stress in fluids. Lanthanide‐doped crystalline NRs, with their polarized luminescence properties, offer a promising platform for such shearmetry applications, as their emission spectra reflect the degree of collective orientation induced by shear. However, precise control over the morphology of the NRs is essential for ensuring reliable measurements, particularly in complex and dynamic flow environments. Herein, size‐controlled synthesis of Eu‐doped LaPO 4 NRs is presented, and their application in shearmetry within a microfluidic channel is designed to generate curved streamlines. By analyzing 2D shear maps obtained using NRs of varied sizes, the impact of NR size on the shearmetry performance is quantitatively evaluated. Size control of the NRs is achieved by adding a surfactant during hydrothermal synthesis. The collective orientation of NRs, induced by flow shear, is determined by analyzing the polarized Eu 3+ emission spectra. NRs of smaller aspect ratio (AR) and length respond more rapidly to shear variations, yielding symmetric and accurate shear stress profiles even in complex, dynamic flows. Additionally, smaller NRs extend the measurable shear stress range up to 0–15000 mPa, covering values typical of biofluidic systems and microfluidic devices. These results provide a novel framework for advanced fluidic analysis, highlighting the critical role of the nanoprobe morphology.