On the shear thinning of non-Brownian suspensions: Friction or adhesion?

Abstract

Shear thinning is fundamental to a broad range of particle suspensions, both in nature and in industrial applications. Yet the mechanisms governing it remain unclear. In particular, the distinct, and often competing, roles of the interparticle, particle-fluid interactions and the particle surface morphology need clarity. By using non-Brownian silica particles with different morphology, surface functional groups and suspending media, here we reveal two different shear thinning mechanisms, controlled either by frictional or adhesion forces between particles. Smooth glass sphere suspensions in a polar medium (glycerol), where particles interact strongly with the solvent, showed no shear thinning even at high volume fractions (φ≥0.5), while rough silica particles, with similar size distribution, induced shear thinning behaviour at φ values of 0.25 and above. The latter is attributed to the increased frictional contacts in rough and irregular particles. Considering surface irregularity as elastically deformable asperities enabled us to estimate the critical load above which two neighbouring rough particles experience frictional contacts giving rise to shear thinning. In contrast, in a non-polar (mineral oil) solvent, with which the particles do not interact strongly, both glass spheres and the rough silicas showed a pronounced shear thinning response and yield stress behaviour at volume fractions as low as 2% v/v. The rheology of these suspensions is dictated by the adhesion forces between the particles that lead to the formation of large agglomerates, which breakdown under increasing shear. The evolution of the sheared suspensions microstructure was captured using an optical shearing cell, which also enabled us to quantify the particle agglomeration characteristics using an aggregation index. To demonstrate the generality of our approach, we modified the surface chemistry of the glass spheres by introducing hydrophobic groups (e.g. a fluorosilane or palmitic acid) to inhibit inter-particle interactions and improve the dispersion of the otherwise inherently hydrophilic glass spheres in mineral oil; as expected, this suppressed the shear thinning behaviour of the suspensions. The present results clearly elucidate alternative design routes to controlling suspension rheology, whether to promote or suppress shear thinning, offering new insights for the manufacturing and manipulation of complex particle suspensions.

Effect of Particle Specific Surface Area on the Rheology of Non-Brownian Silica Suspensions

“…In our previous work [ 12 ], we showed that the strong shear thinning behaviour of the PS52 suspensions, as opposed to that of GS2, can be attributed to the surface asperities being elastically deformed due to the frictional contacts between the former particles. According to a theory introduced by Chatté et al [ 36 ] and Lobry et al [ 28 ], this leads to a decrease in the microscopic friction coefficient and shear thinning.…”

“…Unlike the glass sphere suspensions, the PS52 particles exhibit the onset of shear thinning response at much lower particle volume fractions ( ), attributed to a frictional shear thinning mechanism [ 12 ], based on the elastic deformation of the surface asperities during particle contact, resulting in a decrease in the microscopic friction coefficient with shear rate [ 9 , 28 , 36 ]. Thus, particle surface asperities seem to compete with the interparticle interaction screening induced by the glycerol layer, and in the case of PS52 particles, the surface asperities seem able to penetrate the solvation layer inducing particle contacts.…”

“…The lower effective volume fraction of PS226, estimated at as compared with the PS52 particles can be one reason for this behaviour. In addition, the PS226 suspensions do not show a critical stress which is independent of for the onset of the shear thinning response, whose values are shown in Appendix A , in contrast to the PS52 suspensions [ 12 ], and thus the friction driven mechanism is inadequate to fully explain their weak shear thinning. Shear thickening is also observed at , exhibiting a stronger extent (higher values, Figure 6 d) as compared with that of the glass spheres, even after scaling the data with .…”

“…Table A1 shows the relaxation time ( ) estimated from the Carreau fittings to the viscosity data of the filler silicas in Figure 2 c, alongside the viscosity values ( ) and the corresponding shear rates ( ) used to estimate the critical stress ( ) for the onset of shear thinning according to the theory presented in our recent work for the abrasive silica suspensions [ 12 ]. It can be seen that the values are highly dependent on the particle volume fraction highlighting the more complex rheology induced by the PS226, which cannot be described by a single .…”

“…In our recent studies, we also showed that the shear thinning response of suspensions of rough silica particles in glycerol could stem from the aforementioned friction-driven mechanism. It was also observed that particle surface roughness increased suspension viscosity and even promoted particle agglomeration due to increased interparticle surface contacts [ 12 , 13 ].…”