 |
Center for Nonlinear Studies |
Soft and biological systems are rarely homogeneous. Even when the composition is uniform, stress can be heterogeneous. One dramatic example of stress heterogeneity is the fracture of a thin film under uniaxial tension. We are developing new approaches to map stresses in soft matter. First, we are imaging the stresses applied by materials at interfaces – including contact line forces of sessile liquid droplets and the delamination and fracture of thin films. We extract these forces by analyzing the deformation of a well-characterized elastomeric substrate in contact with the sample of interest. Our approach is similar to that of ‘traction force microscopy’ used to study motile and adherent cells, but we have generalized the method to include normal forces and to provide meaningful results for small spatial frequencies. Next, we consider heterogeneous forces at much smaller length scales. Here, we analyze fluctuations of small colloidal systems near equilibrium to extra ct distinct forces on each particle. Our approach, based on the fluctuation-dissipation theorem, requires no knowledge of the properties of the particles or the solvent, only the temperature. I will illustrate this technique with a measurement of the many-body contributions to electrostatic interactions deep inside the diffuse layer.
Host: Cristiano Nisoli, T-4/CNLS