Bulk and interfacial stresses in suspensions of soft and hard colloids

Abstract

Abstract.We explore the influence of particle softness and internal structure on both the bulk and interfacial rheological properties of colloidal suspensions. We probe bulk stresses by conventional rheology, by measuring the flow curves, shear stress vs strain rate, for suspensions of soft, deformable microgel particles and suspensions of near hard-sphere-like silica particles. A similar behavior is seen for both kind of particles in suspensions at concentrations up to the random close packing volume fraction, in agreement with recent theoretical predictions for sub-micron colloids. Transient interfacial stresses are measured by analyzing the patterns formed by the interface between the suspensions and their own solvent, due to a generalized Saffman-Taylor hydrodynamic instability. At odd with the bulk behavior, we find that microgels and hard particle suspensions exhibit vastly different interfacial stress properties. We propose that this surprising behavior results mainly from the difference in particle internal structure (polymeric network for microgels vs compact solid for the silica particles), rather than softness alone.

Nonequilibrium Interfacial Tension in Simple and Complex Fluids

“…Preliminary data for CP-Ludox at the higher volume fractions were reported in Ref. [41]. For most of the CP samples, the solvent is water, to which salt is added to partially screen the Coulombic repulsion due to surface charges: we use 50 mmol KCl for CP-Ludox and CP-silica, and 250 mmol NaCl for CP-Pema.…”

“…Simulations [33] and experiments [34,35] probing the Saffman-Taylor instability led to the conclusions that no interfacial tension exists between miscible fluids; in particular, the observation of a fractal-like interface between two miscible fluids was attributed to a vanishing interfacial energy cost, i.e., to Γ e ¼ 0 [34,35]. This has to be contrasted with experiments and simulations on miscible fluids probing capillary waves [14,15,36], the shape of drops and menisci [13,16,17,37,38], and that of patterns in hydrodynamic instabilities [18,[27][28][29][39][40][41], which all claimed the existence of an effective interfacial tension. The sign of Γ e is also debated.…”

“…No a priori arguments are available to quantitatively asses whether Korteweg's law would be adequate to model a given system. Moreover, deviations from Korteweg's Δφ 2 scaling have been clearly observed in molecular [13] and complex fluids alike [16,41], thus demonstrating the need for a more general theory. Even for those systems where Korteweg's law correctly captures the Δφ scaling of Γ e , no predictions are available for the magnitude of the square gradient coefficient κ, with the exception of microgel colloidal particles [18].…”

Off-equilibrium surface tension in miscible fluids

“…These questions are currently lively debated, as the result of systematic experimental investigations performed in the last years by several groups, who employed various experimental strategies, ranging from probing capillary waves by light scattering [8][9][10] , to observing the shape of drops [11][12][13] and menisci 14 , and studying hydrodynamic instabilities 15,16 . The interest in this topic is not purely academic: the existence of an effective surface tension, in spite of its transient character, is of great relevance in many fields, from geodynamics to polymer physics and multiphase flow, and has implications in a large number of practical applications.…”

Avalanche-like fluidization of a non-Brownian particle gel

“…67 In most of the previous studies, NaCl is added in a relatively small amount (typically 0.05-0.5 M) such that electrostatic repulsion is screened leading to the slow reversible aggregation of individual colloids until the formation of a colloidal gel. [68][69][70] Here, we add a much larger amount of salt (1.2 M) to the colloidal suspension, which leads to an ion exchange where protons are replaced by sodium ions. 71,72 The loss of hydrogen bonding between the colloids and the solvent triggers the fast and irreversible aggregation of the silica colloids through the formation of interparticle siloxane bonds, [73][74][75] resulting in the formation of the non-Brownian particles described above.…”