
Biopolymer gels show unusual negative normal stressWhen subject to stress or external loads, most materials resist deformation. Any stable material, for instance, resists compression—even liquids. Solids also resist simple shear deformations that conserve volume. Under shear, however, most materials also have a tendency to expand in the direction perpendicular to the applied shear stress, a response that is known as positive normal stress. Here, we show that networks of semiflexible biopolymers such as those that make up both the cytoskeleton of cells and the extracellular matrix exhibit the opposite tendency: when sheared between two plates, they tend to pull the plates together. We show that these negative normal stresses can be as large as the shear stress and that this property is directly related to the nonlinear strain stiffening behavior of biopolymer gels. 

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