Experimental measurements of the internal dynamics of 3D granular systems are essential for the development and verification of new models and numerical simulations. However, granular materials are challenging to probe since they are mostly opaque. Thus, optical techniques that commonly provide high temporal and spatial resolution measurements and are routinely used to probe the dynamics of fluids can only image the outer boundary of 3D granular systems. Instead, different tomographic techniques including position emission tomography (PEPT), X-ray computed tomography, electric capacitance tomography (ECT) and magnetic resonance imaging (MRI) have be applied to image the interior of granular systems. MRI comes with the particular advantage that it allows to measure not only particle density, but can directly quantify particle velocity through the implementation of motion-sensitive magnetic gradient pulse sequences [1]. However, MRI of granular materials suffers from inherently low temporal resolutions; the fastest reported acquisition time for a velocity measurement in a 2D slice has been 5 minutes [2, 3]. This lack of speed makes it impossible to study transient dynamic phenomena that are happening at timescales of the order of 10 – 100 ms, e.g. the formation or coalescence of bubbles in fluidized beds. In this work, we overcome speed limits of MRI in granular systems. Our acceleration strategy enables MRI at unprecedented speeds, paving the way for a plethora of dynamic granular studies. We illustrate the capabilities of our method by imaging bubble dynamics in a large cylindrical fluidized bed (diameter 190 mm, height 250 mm).
