  The molecular rearrangements of most fluids under flow and deformation do not
directly follow the macroscopic strain field. In this work, we describe a
phenomenological method for characterizing such non-affine deformation via the
anisotropic pair distribution function (PDF). We demonstrate now the
microscopic strain can be calculated in both simple shear and uniaxial
extension, by perturbation expansion of anisotropic PDF in terms of real
spherical harmonics. Our results, given in the real as well as the reciprocal
space, can be applied in spectrum analysis of small-angle scattering
experiments and non-equilibrium molecular dynamics simulations of soft matter
under flow.
