A dual-mode orbital calorimeter-rheometer for dense granular media

Dense granular media under horizontal orbital forcing exhibit rich state transitions and energy dissipation; however, quantitative studies require instruments that impose controlled excitation and measure the resulting mechanical response. Here, we introduce a compact orbital instrument that simultaneously serves as a calorimeter and rheometer for dense granular metafluids. The apparatus enables orbital excitation and calorimetric-rheometric measurements in dense granular beds composed of fine glass beads that are immersed in air. The device integrates a variable-speed brushless DC motor, an orbiting plate supported by elastomeric pins and precision bearings, and a noncylindrical "tulip-shaped" Perspex container mounted eccentrically above the drive shaft, enabling horizontal orbital motion with tunable amplitude and frequency. A triaxial accelerometer mounted on the plate provides in situ motion sensing; its gravity-based calibration and double integration yield the container center-of-mass trajectory and the peak-acceleration parameter Γ, while a digital wattmeter and torque-speed characterization of the motor quantify the mechanical power transferred to the granular sample. In the calorimetric mode, the instrument revealed a transition from a rigid-body-like rotation at low Γ to a convective tilted-bed state at a critical value Γc, accompanied by a distinct increase in the dissipated power. In the rheometric mode, a thin granular metafluid film in the gap between a rotating core and a stationary wall enables shear viscosity measurements, and an inclined film configuration supports elongational viscosity and Trouton ratio studies in turbulent dense granular flows. This article documents the device's design, actuation and sensing architecture, calibration procedures, and operating envelope, providing a single reference for its use in granular calorimetry and rheology.