This article reviews recent progress in stress visualization techniques using photoelastic nanocrystal suspensions and soft elastic gels. These materials exhibit stress-induced birefringence, enabling full-field and non-invasive measurement of flow-induced and contact-induced stresses. First, flow-induced birefringence in rectangular channel flows is introduced, highlighting how phase retardation and azimuth reveal shear-stress concentration near walls and its dependence on flow rate. Second, photoelastic tomography using soft gels is presented for both static and dynamic Hertzian contact, demonstrating reconstruction of all stress components and impact-force scaling. Third, stress fields around a settling solid sphere are visualized experimentally for the first time, providing quantitative datasets for particle–fluid interactions. A machine-learning-based reconstruction (PICED) is also discussed as an emerging approach for recovering three-dimensional stress tensors. These techniques offer new opportunities for analyzing powder-handling processes involving slurry transport, particle impact, and particle–wall interactions.
Yoshiyuki Tagawa (Tue,) studied this question.