title: Porosity Governs Normal Stresses in Polymer Gels.
abstract: 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 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.
authors.affiliation: Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: Henri C G
authors.initials: HC
authors.lastname: de Cagny
authors.affiliation: FOM-Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands.
authors.forename: Bart E
authors.initials: BE
authors.lastname: Vos
authors.affiliation: Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.
authors.forename: Mahsa
authors.initials: M
authors.lastname: Vahabi
authors.affiliation: FOM-Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands.|Department of Biomedical Engineering and Institute for Complex Molecular Systems, Eindhoven University of Technology, 5612 AP Eindhoven, The Netherlands.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Kurniawan
authors.affiliation: Center of Soft Matter Physics and its Applications, Beihang University, 100191 Beijing, China.
authors.forename: Masao
authors.initials: M
authors.lastname: Doi
authors.affiliation: FOM-Institute AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands.
authors.forename: Gijsje H
authors.initials: GH
authors.lastname: Koenderink
authors.affiliation: Department of Physics and Astronomy, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands.|Departments of Chemical & Biomolecular Engineering, Chemistry, and Physics & Astronomy, Rice University, Houston, 77005 Texas, USA.|Center for Theoretical Biophysics, Rice University, Houston, 77030 Texas, USA.
authors.forename: F C
authors.initials: FC
authors.lastname: MacKintosh
authors.affiliation: Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: Daniel
authors.initials: D
authors.lastname: Bonn