Poynting effect of brain matter in torsion
Annaidh, Aisling Ni
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Balbi, Valentina, Trotta, Antonia, Destrade, Michel, & Ní Annaidh, Aisling. (2019). Poynting effect of brain matter in torsion. Soft Matter, 15(25), 5147-5153. doi: 10.1039/C9SM00131J
We investigate experimentally and model theoretically the mechanical behaviour of brain matter in torsion. Using a strain-controlled rheometer, we perform torsion tests on fresh porcine brain samples. We quantify the torque and the normal force required to twist a cylindrical sample at constant twist rate. Data fitting gives a mean value for the shear modulus of mu = 900 +/- 312 Pa and for the second Mooney-Rivlin parameter of c(2) = 297 +/- 189 Pa, indicative of extreme softness. Our results show that brain always displays a positive Poynting effect; in other words, it expands in the direction perpendicular to the plane of twisting. We validate the experiments with finite element simulations and show that when a human head experiences a twisting motion in the horizontal plane, the brain can experience large forces in the axial direction.