What is fluidity? Designing an experimental system to probe stress and velocity fluctuations in flowing suspensions

Abstract

Abstract. We develop a method to investigate the microscopic origin of granular fluidity. We design a Couette cell in which we can probe the flow of soft hydrogel suspensions. As we drive the suspension with a rheometer, we have access to global flow characteristics. In addition, the Couette cell has been modified to have a transparent bottom and lid, allowing for imaging of suspension characteristics in transmission, for example flow fields. We can also use transmission imaging to probe local stresses in the suspension: we use hydrogel particles composed of gelatin, which through its photoelastic properties gives access to local stress fluctuations. We thus have access to all local microscopic variables that are relevant in the understanding of granular suspensions. We show here that our setup can indeed visualize stress fields inside the suspensions and perform flow field measurements in transmission mode. We compare the profiles of the gelatin suspension to that of a polyacrylamide hydrogel suspension, to assess robustness of observed phenomenology. We find that the flow profiles for both types of hydrogels are different; gelatin suspensions feature narrower and less rate-dependent flow profiles. We speculate on the origin of the observed difference by considering the frictional properties of the suspended particles.

Enlightening force chains: a review of photoelasticimetry in granular matter

“…As shown in Fig. 3e, gelatin has excellent photoelastic properties [26,52,53], as does agar or konjac [54] (with the later being less transparent). The stiffness of these materials is easy to tune by varying the ratio of gelling agent, and it is possible to achieve arbitrarily low elastic moduli.…”

Contact tribology also affects the slow flow behavior of granular emulsions

“…1b and Ref. 30 . The inner cylinder has radius r i = 25 mm, while the outer cylinder has radius r o = 45 mm, such that the gap r o − r i = 20 mm ≈ 10d with d the particle diameter.…”

“…The Herschel-Bulkley behavior observed for the PAAm suspension, and the flow instability observed for gelatin suggest that our granular emulsions will show shear banding 56,57 . To determine the flow profiles in the gap of our Couette cell, we perform particle image velocimetry (PIV), using a method described in more detail elsewhere 30 . In short: imaging the flowing suspension in transmission provides sufficient contrast to elucidate local velocities using standard PIV methods.…”

“…A more extensive dataset can be found in Ref. 30 . For the PAAm suspensions, the shear bands are relatively wide and insensitive to the driving rate, whereas for gelatin the shear bands are narrow and slightly rate-dependent.…”

Photoporomechanics: An Experimental Method to Visualize the Effective Stress Field in Fluid-Filled Granular Media

“…Under stress, the photoelastic spheres do not exhibit photoelastic fringes [37,44], because the light refracts through the angled surface near the contact points. The well-established fringe inversion algorithm for vector measurements of forces based on fringes on 2D disks [10] cannot be used for spheres.…”