Porosity governs normals stresses in polymer gels

Abstract

When sheared, most elastic solids such as metals, rubbers and polymer hydrogels dilate in the direction perpendicular to the shear plane. This well-known behaviour known as the Poynting effect is characterized by a positive normal stress. Surprisingly, biopolymer gels made of fibrous proteins such as fibrin and collagen, as well as many biological tissues exhibit the opposite effect, contracting under shear and displaying a negative normal stress. Here we show that this anomalous behaviour originates from the open network structure of biopolymer gels, which facilitates interstitial fluid flow during shear. Using fibrin networks with a controllable pore size as a model system, we show that the normal stress response to an applied shear is positive at short times, but decreases to negative values with a characteristic time scale set by pore size. Using a two-fluid model, we develop a quantitative theory that unifies the opposite behaviours encountered in synthetic and biopolymer gels. Synthetic polymer gels are impermeable to solvent flow and thus effectively incompressible at typical experimental time scales, whereas biopolymer gels are effectively compressible. Our findings suggest a new route to tailor elastic instabilities such as the die-swell effect that often hamper processing of polymer materials and furthermore show that poroelastic effects play a much more important role in the mechanical properties of cells and tissues than previously anticipated.

arXiv commentMain text 6 pages and 3 figures SI 7 pages and 5 figures

References in corpus (7)

• Nonlinear Elasticity in Biological Gels
• Elasticity of Semiflexible Biopolymer Networks
• Microscopic Viscoelasticity: Shear Moduli of Soft Materials Determined from Thermal Fluctuations
• One- and two-particle microrheology
• Nonaffine rubber elasticity for stiff polymer networks
• Crosslinked networks of stiff filaments exhibit negative normal stress
• Shear dilatancy in marginal solids

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• Normal stresses, contraction, and stiffening in sheared elastic networks
• Normal stress anisotropy and marginal stability in athermal elastic networks
• Role of normal stress in the creep dynamics and failure of a biopolymer gel
• Creep and drainage in the fast destabilization of emulsions Creep and drainage in the fast destabilization of emulsions
• Non-affinity and fluid-coupled viscoelastic plateau for immersed fiber networks
• Tuning local microstructure of colloidal gels by ultrasound-activated deformable inclusions
• Nonlinear Poisson effect in affine semiflexible polymer networks
• Inverted and programmable Poynting effects in metamaterials