We explore the influence of particle softness and internal structure on both
the bulk and interfacial rheological properties of colloidal suspensions. We
probe bulk stresses by conventional rheology, by measuring the flow curves,
shear stress vs strain rate, for suspensions of soft, deformable microgel
particles and suspensions of near hard-sphere-like silica particles. A similar
behavior is seen for both kind of particles in suspensions at concentrations up
to the random close packing volume fraction, in agreement with recent
theoretical predictions for sub-micron colloids. Transient interfacial stresses
are measured by analyzing the patterns formed by the interface between the
suspensions and their own solvent, due to a generalized Saffman-Taylor
hydrodynamic instability. At odd with the bulk behavior, we find that microgels
and hard particle suspensions exhibit vastly different interfacial stress
properties. We propose that this surprising behavior results mainly from the
difference in particle internal structure (polymeric network for microgels vs
compact solid for the silica particles), rather than softness alone.Comment: 20 pages, 8 figure