Evolution of the layering structure in dense liquid-particle flows down an inclined rough channel

Dense liquid-particle flows are common in natural and industrial settings, yet their internal dynamics remain poorly understood. Under conditions of high particle concentration and strong particle–fluid coupling, these flows exhibit pronounced layering and temporal evolution. In this study, refractive index matching combined with particle tracking velocimetry (RIM-PTV) is employed in an inclined channel to directly visualize internal structures at two representative inclinations. Particle concentration, velocity, shear rate, and granular temperature are measured and reconstructed, revealing that the flow evolves through four stages: front, quasi-steady, fluctuation, and decline. Inclination strongly influences layering evolution. At low inclination, the flow gradually transitions from a dual-layer structure of ordered friction flow beneath disordered friction flow to a single-layer ordered friction structure during the fluctuation stage, with the upper layer collapsing under near-critical frictional conditions. At high inclination, a dual-layer structure of basal collision flow beneath disordered friction flow persists throughout. A stress-based analysis, using the effective friction coefficient and inertial number, reveals that the evolution is governed by the competition between shear driving and normal compression, which determine the persistence or collapse of layering structures. These results provide experimentally resolved evidence for stage-wise layering evolution in dense liquid-particle flows and establish a benchmark for developing predictive rheological and dynamical models.