"How Size and Strain Determine the Strength of Soft Materials" by Daniel Blair

Soft and biological materials often exhibit disordered and  heterogeneous microstructure. In most cases, the transmission and distribution of stresses through these complex materials reflects their inherent heterogeneity. We are developing a set of techniques that provide the ability to apply to quantify the connection between microstructure and local stresses.  We subject soft and biological materials to precise deformations while measuring real space information about the distribution and redistribution of stress.

Using our custom confocal rheometer platform we can determine the role of shear stress in a variety of materials. First, I will describe our  
recent results on the nonlinear rheology of in vitro collagen networks.  We apply precisely controlled shear strains to collagen networks that are adhered to a thin elastic polyacrylamide gel substrate embedded with fiduciary markers.  By utilizing a modified version of traction force microscopy we can calculate the distribution of forces as a function of the applied strain. We find that the signatures of yielding in these materials follow a universal form.  Second, I will discuss how the application of a cyclic load can determine the mechanical strength of a biopolymer system.  We observe that when actin networks are cyclicly strained, they either work harden, or soften depending on the specifics of the cross-linking protein.