Porosity governs normal stresses in polymer gels

When sheared, most elastic solids including metals, rubbers and polymer gels dilate perpendicularly to the shear plane. This behavior, known as the Poynting effect, is characterized by a positive normal stress. Surprisingly, fibrous biopolymer gels exhibit a emph{negative} normal stress under shear. Here we show that this anomalous behavior originates from the open network structure of biopolymer gels. 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 behaviors encountered in synthetic and biopolymer gels.