1. Simon J. Haward Stylianos Varchanis, Cameron C. Hopkins; Tsamopoulos, John

Transition between solid and liquid state of yield-stress fluids under purely extensional deformations Journal Article

In: PNAS (Proceedings of the National Academy of Sciences of the United States of America), 117 (23) , pp. 12611-12617, 2020.

Abstract | Links | BibTeX | Tags: elastoviscoplastic materials, extensional flow, viscoplastic materials, yield strain, Yield stress

@article{Varchanis2020b,

title = {Transition between solid and liquid state of yield-stress fluids under purely extensional deformations},

author = { Stylianos Varchanis, Simon J. Haward, Cameron C. Hopkins, Alexandros Syrakos, Amy Q. Shen, Yannis Dimakopoulos, and John Tsamopoulos},

editor = {David A. Weitz},

doi = {10.1073/pnas.1922242117},

year = {2020},

date = {2020-05-20},

journal = {PNAS (Proceedings of the National Academy of Sciences of the United States of America)},

volume = { 117 (23)},

pages = {12611-12617},

abstract = {We report experimental microfluidic measurements and theoretical modeling of elastoviscoplastic materials under steady, planar elongation. Employing a theory that allows the solid state to deform, we predict the yielding and flow dynamics of such complex materials in pure extensional flows. We find a significant deviation of the ratio of the elongational to the shear yield stress from the standard value predicted by ideal viscoplastic theory, which is attributed to the normal stresses that develop in the solid state prior to yielding. Our results show that the yield strain of the material governs the transition dynamics from the solid state to the liquid state. Finally, given the difficulties of quantifying the stress field in such materials under elongational flow conditions, we identify a simple scaling law that enables the determination of the elongational yield stress from experimentally measured velocity fields.},

keywords = {elastoviscoplastic materials, extensional flow, viscoplastic materials, yield strain, Yield stress},

pubstate = {published},

tppubtype = {article}

}

We report experimental microfluidic measurements and theoretical modeling of elastoviscoplastic materials under steady, planar elongation. Employing a theory that allows the solid state to deform, we predict the yielding and flow dynamics of such complex materials in pure extensional flows. We find a significant deviation of the ratio of the elongational to the shear yield stress from the standard value predicted by ideal viscoplastic theory, which is attributed to the normal stresses that develop in the solid state prior to yielding. Our results show that the yield strain of the material governs the transition dynamics from the solid state to the liquid state. Finally, given the difficulties of quantifying the stress field in such materials under elongational flow conditions, we identify a simple scaling law that enables the determination of the elongational yield stress from experimentally measured velocity fields.2. Mitsoulis, E; Tsamopoulos, J

Numerical simulations of complex yield-stress fluid flows Journal Article

In: Rheologica Acta, 56 (3), pp. 231-258, 2017, ISSN: 00354511, (cited By 36).

Abstract | Links | BibTeX | Tags: Bingham plastics, Elastoviscoplastic fluids, Herschel-Bulkley fluids, Simulations, unyielded regions, Viscoplastic fluids, Viscoplastic models, Yield stress, Yielded

@article{Mitsoulis2017231,

title = {Numerical simulations of complex yield-stress fluid flows},

author = {E Mitsoulis and J Tsamopoulos},

url = {https://www.scopus.com/inward/record.uri?eid=2-s2.0-85001022076&doi=10.1007%2fs00397-016-0981-0&partnerID=40&md5=78c8b6500b006f7b3ca82c4182414f3f},

doi = {10.1007/s00397-016-0981-0},

issn = {00354511},

year = {2017},

date = {2017-01-01},

journal = {Rheologica Acta},

volume = {56},

number = {3},

pages = {231-258},

abstract = {Viscoplasticity is characterized by a yield stress, below which the materials will not deform and above which they will deform and flow according to different constitutive relations. Viscoplastic models include the Bingham plastic, the Herschel-Bulkley model and the Casson model. All of these ideal models are discontinuous. Analytical solutions exist for such models in simple flows. For general flow fields, it is necessary to develop numerical techniques to track down yielded/unyielded regions. This can be avoided by introducing into the models a regularization parameter, which facilitates the solution process and produces virtually the same results as the ideal models by the right choice of its value. This work reviews several benchmark problems of viscoplastic flows, such as entry and exit flows from dies, flows around a sphere and a bubble and squeeze flows. Examples are also given for typical processing flows of viscoplastic materials, where the extent and shape of the yielded/unyielded regions are clearly shown. The above-mentioned viscoplastic models leave undetermined the stress and elastic deformation in the solid region. Moreover, deviations have been reported between predictions with these models and experiments for flows around particles using Carbopol, one of the very often used and heretofore widely accepted as a simple “viscoplastic” fluid. These have been partially remedied in very recent studies using the elastoviscoplastic models proposed by Saramito. © 2016, Springer-Verlag Berlin Heidelberg.},

note = {cited By 36},

keywords = {Bingham plastics, Elastoviscoplastic fluids, Herschel-Bulkley fluids, Simulations, unyielded regions, Viscoplastic fluids, Viscoplastic models, Yield stress, Yielded},

pubstate = {published},

tppubtype = {article}

}

Viscoplasticity is characterized by a yield stress, below which the materials will not deform and above which they will deform and flow according to different constitutive relations. Viscoplastic models include the Bingham plastic, the Herschel-Bulkley model and the Casson model. All of these ideal models are discontinuous. Analytical solutions exist for such models in simple flows. For general flow fields, it is necessary to develop numerical techniques to track down yielded/unyielded regions. This can be avoided by introducing into the models a regularization parameter, which facilitates the solution process and produces virtually the same results as the ideal models by the right choice of its value. This work reviews several benchmark problems of viscoplastic flows, such as entry and exit flows from dies, flows around a sphere and a bubble and squeeze flows. Examples are also given for typical processing flows of viscoplastic materials, where the extent and shape of the yielded/unyielded regions are clearly shown. The above-mentioned viscoplastic models leave undetermined the stress and elastic deformation in the solid region. Moreover, deviations have been reported between predictions with these models and experiments for flows around particles using Carbopol, one of the very often used and heretofore widely accepted as a simple “viscoplastic” fluid. These have been partially remedied in very recent studies using the elastoviscoplastic models proposed by Saramito. © 2016, Springer-Verlag Berlin Heidelberg.