Ultra-fast Magnetic Resonance Imaging of intruders sinking into a concentrated suspension

Abstract

Surprisingly, liquid media of dispersed dry micron-sized particles can generate tremendous impact resistance, can grow solid protrusions when acoustically-vibrated and can cause a range of other peculiar phenomena to objects sinking in their interior, even though their appearance essentially remains ’liquid-like’. These heterogeneous mixtures of undissolved particles, referred to as suspensions, can be found all around us: e.g. in paints, blood, mudslides, landslides, etc. Moreover, due to some suspensions’ shear-thickening behaviour their use has been investigated for liquid-like body armor applications, new damper designs and many more. Yet, the core physical mechanism of their behaviour in its various regimes is still not completely understood. This thesis work employs the novel ultra-fast Magnetic Resonance Imaging methodology proposed by Alexander Penn, also a member of the ETH Zurich laboratory from which this work originates, to image non-intrusively into a shear-thickening suspension of cornstarch and water while a spherical intruder impacts and settles in its interior. First off, by fitting a rigid steel wire inside the settling object and imaging a marker on top of it with a high-speed camera, behaviours similar to those observed by von Kann et al. are evident: the intruder oscillates around a terminal velocity in the bulk of the suspension and experiences stop-and-go cycles near the container base. This act, however, inherently restricts motion by allowing the intruder to move only in the vertical direction. Therefore, we utilize 1D and 2D MR imaging to obtain information about the intruder’s horizontal and undisturbed vertical motion to draw conclusions that the object exhibits similar oscillations in amplitude and frequency also in the transverse component. In addition to those, we perform 2D velocity-encoded MRI scans to obtain the velocity and shear rate fields with sufficient time resolution to capture the suspension behaviour at the relevant phenomenological regimes. The data suggested that, on comparison with reference rheology experiments, the shear rates during the terminal velocity oscillations, resultant from the fluid inertia around the object as it settles, appear to be in close proximity to the critical shear rate for discontinuous shear thickening (DST, or an orders-of-magnitude jump in stress). To continue, 1D MRI measurements revealed a phenomenon new to us to be visible just after the impact: upon increasing mass, size, and drop height of intruders, the transient oscillations which transition to stable bulk oscillations were found to transform into several distinct stop-and-go cycles very much alike those that occur close to the container bottom. A qualitative physical mechanism was proposed based on the dependencies found in these behaviours. Last but not least, the effects of size while keeping the same buoyancy ratio were studied qualitatively. Direct evidence from all executed experiments confirmed secondary stable bulk oscillation regions for objects of larger size.