The sinking dynamics of a solid intruder in concentrated cornstarch suspensions studied using ultra-fast magnetic...

The sinking of intruders in concentrated cornstarch suspensions is governed by localized, transient dynamics that are difficult to access experimentally because of the opacity of the material. Here, we use real-time magnetic resonance imaging (MRI) to investigate the sinking of spherical intruders in cornstarch suspensions at solid fractions ϕ₀ = 0.41 and 0.44. Ultra-fast 1D MRI measurements show that, consistent with earlier reports, the intruders pass through an impact transient, an oscillatory sinking regime, and late stop and go cycles near the bottom boundary. By varying the intruder diameter at fixed suspension composition, we find that larger intruders sink more slowly, while the oscillations in the intermediate sinking regime exhibit similar characteristic frequencies and amplitudes for all three sizes. A reduced drag-memory model further shows that these oscillatory sinking velocities can be described reasonably well by a common phenomenological history-dependent response for the presented conditions. The 1D MRI signal maps reveal synchronous signal modulations around the intruder, indicating that the oscillatory motion is coupled to repeated growth and partial relaxation of a perturbed suspension region. Complementary 2D MR velocimetry shows that the motion is not purely vertical, but also includes oscillations in the horizontal direction. We show that the surrounding flow and strain rate fields are strongly heterogeneous, with deformation becoming progressively concentrated beneath the intruder prior to arrest.