  We image semi-flexible polymer networks under shear at the micrometer scale.
By tracking embedded probe particles, we determine the local strain field, and
directly measure its uniformity, or degree of affineness, on scales of 2-100
micron. The degree of nonaffine strain depends on polymer length and crosslink
density, consistent with theoretical predictions. We also find a direct
correspondence between the uniformity of the microscale strain and the
nonlinear elasticity of the networks in the bulk.
