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.

B.M. Mulder (Bela)
Phys. Rev. Lett.
Biological Soft Matter-Former Group

de Cagny, H., Vos, B., Vahabi, M., Kurniawan, N., Doi, M., Koenderink, G., … Bonn, D. (2016). Porosity governs normal stresses in polymer gels. Phys.Rev.Lett., 117(21, Article number: 217802), 1–5. doi:10.1103/PhysRevLett.117.217802