Dependence of suspension complex viscosity on frequency: Strain-controlled vs. stress-controlled tests

The apparent complex viscosity of a non-Brownian Newtonian concentrated suspension is known to be a non- monotonic function of the applied strain, attaining a minimum at a relative strain of about 100%. This behaviour can be nicely described in the framework of the Stokesian dynamics, which also predicts the independence of the complex viscosity on the applied frequency. Despite this, recently it was experimentally shown that the apparent viscosity of a concentrated non-Brownian suspension is function of the applied frequency in experiments run imposing a constant stress (Carotenuto et al. AIP Conference Proceedings 1599, 258, 2014). This behaviour is quite unexpected and we here confirm it in more canonical experiments run imposing a constant strain.