  Soft and biological materials are often composed of elementary constituents
exhibiting an incessant roto-translational motion at the microscopic scale.
Tracking this motion with a bright-field microscope becomes increasingly
challenging when the particle size becomes smaller than the microscope
resolution, a case which is frequently encountered. Here we demonstrate
Squared-Gradient Differential Dynamic Microscopy (SG-DDM) as a tool to
successfully use bright-field microscopy to extract the roto-translational
dynamics of small anisotropic colloidal particles, whose rotational motion
cannot be tracked accurately in direct space. We provide analytical
justification and experimental demonstration of the method by successful
application to an aqueous suspension of peanut-shaped particles.
