Avalanche-like fluidization of a non-Brownian particle gel

Abstract

We report on the fluidization dynamics of an attractive gel composed of non-Brownian particles made of fused silica colloids. Extensive rheology coupled to ultrasonic velocimetry allows us to characterize the global stress response together with the local dynamics of the gel during shear startup experiments.In practice, after being rejuvenated by a preshear, the gel is left to age for a time t w before being subjected to a constant shear rate _ g. We investigate in detail the effects of both t w and _ g on the fluidization dynamics and build a detailed state diagram of the gel response to shear startup flows. The gel may display either transient shear banding towards complete fluidization or steady-state shear banding. In the former case, we unravel that the progressive fluidization occurs by successive steps that appear as peaks on the global stress relaxation signal. Flow imaging reveals that the shear band grows until complete fluidization of the material by sudden avalanche-like events which are distributed heterogeneously along the vorticity direction and correlated to large peaks in the slip velocity at the moving wall. These features are robust over a wide range of t w and _ g values, although the very details of the fluidization scenario vary with _ g. Finally, the critical shear rate _ g* that separates steady-state shear-banding from steady-state homogeneous flow depends on the width of the shear cell and exhibits a nonlinear dependence with t w . Our work brings about valuable experimental data on transient flows of attractive dispersions, highlighting the subtle interplay between shear, wall slip and aging whose modeling constitutes a major challenge that has not been met yet.

Triggers and signatures of shear banding in steady and time-dependent flows

“…It has been seen experimentally in polymeric fluids including wormlike micelles [25,26] and linear polymers [13,[27][28][29]; and in soft glassy materials including carbopol gel [7,30], Laponite clay [31,71], a non-Brownian fused silica suspension [34], and waxy crude oil [69]. Molecular simulations have captured it in polymers [31,72], a model colloidal gel [73], and molecular glasses [32,74,75].…”

“…In startup of steady simple shearing flow (shear startup), for example, the (near ubiquitous) presence of an overshoot in the shear stress startup signal has been identified as a possible trigger for the formation of shear bands, at least transiently, en route to a steady flowing state [7,8,13,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34]. A declining time-dependent viscosity has been similarly identified as a trigger for banding following the imposition of a step stress [16][17][18][25][26][27]29,[35][36][37][38].…”

Wall slip across the jamming transition of soft thermoresponsive particles

“…Although historically described as a mere artifact, wall slip appears as an intrinsic feature of complex fluids that has been reported in polymeric fluids [3][4][5] as well as in soft glassy materials such as colloidal suspensions [6,7], foams [8][9][10], emulsions [11][12][13], microgels [14,15], attractive gels [16], etc. Moreover, wall slip is often non-trivially coupled to the bulk dynamics of complex fluids, especially during the yielding transition where it may even govern the nature of the subsequent steady-state flow [17][18][19].…”

Ultrasound Imaging Velocimetry: a review

“…turbulent flows. Nevertheless, in all of these studies the velocity fields provided insight in the underlying physics of the observed overall rheological behaviour, hereby "linking global rheology to the local dynamics" (Kurokawa et al 2015). This makes ultrasound-based techniques a very powerful tool in rheology.…”

“…In this case the torque, rather than the pressure drop, is used to quantify viscous losses. Ultrasound speckle velocimetry was used to characterise fatigue due to shear in gels (Gibaud et al 2016), flow banding in cellulose nanofibril suspensions (Martoïa et al 2015), flow instabilities and shear banding in a non-Newtonian micellar solution (Gallot et al 2013) and avalanche-like fluidisation events in suspensions (Kurokawa et al 2015). Again, most of these systems are inherently echogenic and do thus not require seeding.…”