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Biopolymer gels show unusual negative normal stress
 When
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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