How does a soft glassy material flow: finite size effects, non local rheology, and flow cooperativity

Abstract

International audienceIn this paper, we investigate the rheological behavior of jammed emulsions in microchannels on the basis of microvelocimetry techniques. We demonstrate that velocity profiles in this confined geometry cannot be accounted for by the bulk - Herschel-Bulkley - rheological flow curve measured independently in a rheometer. A strong dependence of the flow behavior on the confinement, pressure drop and surface roughness is evidenced, which cannot be described by classical rheological descriptions. We show that these behaviors can be rationalized on the basis of a non local rheological model, introducing the notion of local fluidity as a key rheological quantity. The model reproduces the experimental velocity profiles for any confinements and any surface nature. The non-locality is quantified by a length, z, characterizing the flow cooperativity of jammed emulsions, and typically of the order of several emulsion droplet diameters. We study the influence of volume fraction, droplet diameter, and emulsions polydispersity on this length

Direct evidence of plastic events and dynamic heterogeneities in soft-glasses

“…This is however not the case: the normalized profiles collapse on the same master curve independent of the applied shear, and emphasize that the shear rate is greater at the wall than in the center of the channel. This non-local effect has been discussed in terms of plastic rearrangements of the flow [10,11,13,14]. It is therefore of great interest to provide direct dynamic evidence of such plastic events.…”

“…A step towards this goal has been taken in recent works [8,11,14], where the rate of plastic events is connected to the "fluidity" field, defined as the ratio between the shear rate and the stress, f =γ σ . By using a kinetic model for the elasto-plastic dynamics of the stress distribution function, the local fluidity is shown to obey (in the steady state) the following equation…”

“…without the perturbing effects of the boundaries. The fluidity model has been tested with considerable success both in experiments [11,14,18] and in molecular dynamics simulations [10]. Under the hypothesis of low cooperativity, the model predicts proportionality between the fluidity and the rate of plastic events [10,18].…”

“…We first consider the fluidity averaged in time and in the stream-flow direction. For such a 1d case, the fluidity is predicted to obey a non-local equation of the form [11,14] …”

“…As for the wall fluidity, f w , we directly measure it at the distance evidenced by the vertical dots in the left panel of figure 5 and compare it with the bulk fluidity, f b , in the middle panel of figure 5. The existence of a specific wall rheology is clear: the wall fluidity is significantly larger than the bulk fluidity [14]. To doublecheck the quantitative consistency with equation (8), we rescaled all profiles with respect to the wall fluidity, f w , and studied the quantity ( f (z) − f b (σ ))/( f w − f b (σ )).…”