Rheology of microgels in single particle confinement

Abstract

In this work, we investigate the shear rheology of Carbopol 981 microgel particle suspensions, confined between shearing plates with gap separations from 5 to 100 μm. We show that even for confining gaps smaller than that of the gel particle size, the yielding of concentrated microgel suspensions is delayed to stress levels above the bulk yield stress. Furthermore, for stresses below this new yield point, slip is described by elastohydrodynamic lubrication theory as long as the direct confinement of the single gel particles between the shearing surfaces is limited to a Hertzian deformation. For a strong, non-Hertzian particle deformation, the slip layer breaks down and leads to a frictional interaction of the single confined particle with the two shearing surfaces, depending on their surface roughness. Lubrication pressures and friction coefficients have been quantified with in situ normal force measurements on the confined particles, which have also been utilized to unambiguously determine the relevant swollen particle dimensions.

Particle–wall tribology of slippery hydrogel particle suspensions

“…29 Slip in concentrated soft particle suspensions, such as colloidal pastes or emulsions, has attracted a lot of attention in the recent years. [33][34][35][36][37][38][39][40][41][42] These materials have a close-packed amorphous structure and flow past one another appreciably only when a large enough stress is applied, greater than the so-called yield stress. For steady shear flows, Meeker et al proposed a model where the particle deformation is coupled to the flow through the pressure field so that the flat contacts existing between the particles and the bounding surfaces are deformed asymmetrically.…”

On the inseparability of slip and gap-error

“…For the case of only apparent slip the slope iṡ b is another possibility. However, it should be pointed out that this might result in a strong increase in the gap error ε, and that the degree of reducing b will depend on the interplay of the roughness and the specific sample [23,28]. Also in this case an error determination (and eventual correction) would still be good practice.…”

“…While regular gap settings on the order of a millimeter are most of the time sufficient, lower gap separations can be desirable. This might be the case if only small sample volumes are available [2][3][4][5][6][7][8], when slip needs to be determined via a Mooney analysis (since measurements at small gap settings lead to a higher resolution for the determination of slip velocities) [9][10][11][12][13][14], for measurements at higher rates for a given (maximum) velocity difference of the shearing plates [15][16][17][18][19][20], or if confinement effects should be introduced and measured [21][22][23][24][25].…”

Effect of D-Mannitol on the Microstructure and Rheology of Non-Aqueous Carbopol Microgels

“…Similar to most polyelectrolytes, the final swollen Carbopol configuration depends on the internal density of uncompensated ions on the polymer backbone and, therefore, can be controlled through the pH of the solution [ 12 ]. For this reason, researchers have investigated extensively, for many types of Carbopol molecules and alkaline compounds, how changing the solution pH affects the rheological properties of water-based Carbopol microgels [ 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. In addition to typical bases, the effect of other additives, such as surfactants and polyols, commonly used in the formulation of pharmaceutical and personal care products, has also been partly examined.…”