This article presents a multimodal rectangular metal cavity resonator-based subterahertz microfluidic sensor design for extracting the complex dielectric dispersion profile of liquids placed in a microcapillary tube. A variety of fluids with different permittivity, loss and dispersive characteristics were investigated in the system which was calibrated using a standard fluid. The metal-cavity resonator operates in the J-band from 220-330 GHz with an almost equispaced distribution of resonances across the band. Electromagnetic perturbations caused by the test fluid were measured to determine the dispersion profile, and results were compared to established physical and analytical models with good agreement. This work also demonstrated the ability to distinguish between dielectric loss and conduction loss for dielectric profiling of biomatter.