Granular contact-line pinning and the collapse of macroscopic surface-energy barriers

Dense granular media lack a direct liquid tensiometry analog, although macroscopic interfacial barriers strongly influence buoyancy. Using an orbital rheometer, this study demonstrates that a partially submerged intruder can serve as an in situ granular tensiometer. At low peak acceleration and low granular temperature, high-speed tracking reveals a stable circular orbit with zero radial drift, zero intrinsic spin, and negligible localized slip, indicating that conventional sliding friction is structurally locked out. In this regime, the centripetal demand is balanced by an effective interfacial energy barrier that pins the three-phase air–grain–intruder contact line. Because the intruder breaks the bulk symmetry at the free surface, this line pinning establishes a macroscopic granular meniscus that exerts a net restoring force via virtual work without invoking Marangoni-type gradients. A virtual-work force balance then yields an operational granular surface-energy barrier, in agreement with within approximately 1.2% with independent thermomechanical estimates in the same driven steady state, thereby clarifying the apparent Archimedean anomaly observed at Γ = 2.5. Conversely, fluidization at higher orbital acceleration thermalizes the bed, collapses the barrier, mobilizes the friction, and triggers the convective submersion of the intruder.