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Trinity College Dublin

A microscopic interpretation of 2D foam rheology

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Dr Stefan Hutzler
We probe the relationship between dynamics on the bubble scale and bulk foam rheology, using Soft Disk Model simulations. We observe two distinct regimes, classified by the nature of the bubble motion. The first regime is well-described by the Herschel-Bulkley equation, finding a value for the power law exponent, linking excess-stress to strain rate in line with previous results using the Soft Disk Model. We observe the bubbles moving in strongly non-affine, swirl-like motions. Examining the non-affine mean square displacements of the bubbles, we seek to probe the clear transition to shear-induced diffusion over a sufficiently large time interval. We aim to fit diffusion constants as functions of strain rate. We extract a characteristic relaxation time and use it to fit a relaxation model to the shear stresses measured, comparing it with the measured stresses and the Herschel-Bulkley fit.In the second regime, we observe the bubbles to begin to move in lanes. The measured stresses can no longer be fit by the Herschel-Bulkley equation, instead appearing to tend towards a linear relationship with strain rate at very high strain rate, consistent with proposed theories. We seek to further investigate the causes of this transition to lamellar flow.
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