Tailoring flow behavior of colloidal dispersions with short-range repulsive interactions using depletion forces
Final Report Abstract
We have developed a new microrheology method, namely the so-called Micro-Rheo-Mapping (MRM) technique which allow us to obtain an accurate and direct visualisation of the microstructure of commercial colloidal dispersions namely acrylate thickeners of Carbopol-type of different degree of crosslinking. This new imaging technique combines the Voronoi triangulation technique results obtained from a conventional Multiple Particle Tracking (MPT) microrheology analysis discarding short particle trajectories with the image overlay technique including all trajectories generated during a tracking experiment. At low polymer concentration, i.e. in the diluted regime, we obtained structural information on agglomerates formed of primary Carbopol particles and estimated a diameter of 43±11, 56±14 and 10±2.5 µm for Carbopol Ultrez 10, ETD 2020 and ETD 2050, respectively. At higher, technically relevant, polymer concentrations, results indicate that all three thickeners are highly heterogeneous on a micrometer length scale with a microstructure composed of several regions characterised by different crosslink densities. A first region inaccessible for tracer particles which corresponds to a mixture of compact agglomerates of primary Carbopol particles and individual primary particles with a very dense, highly crosslinked core of mesh size < 200 nm and a second one, diluted enough to be accessible for the tracers, which comprises both elastic and viscous regions and for which we determined the local apparent elastic modulus G0,MPT and viscosity ηMPT. The study of the impact of pH, polymer concentration and crosslink density on both microstructural and macrorheological properties of the three types of Carbopol allowed us to establish a correlation between microstructure and macroelasticity. In particular, we found that the bulk shear modulus G 0 exhibits a power law scaling with the fraction of inaccessible area and exponents between 2.3 and 3.7 are found depending on Carbopol grade. This strong correlation clearly shows that the agglomerates and the highly crosslinked core of the Carbopol control the bulk elasticity of the solutions. This new MRM visualization method combined with rotational rheometry has also been used to investigate the impact of ionic strength on the phase behavior, structure and flow properties of colloidal dispersions consisting of poly (styrene-butyl acrylate) P(S/BA) stabilized by short-range repulsive interactions, but also when additional attractive depletion forces are present due to added non-absorbent polymer, namely polyethyleneoxide (PEO). We found that for dispersions with only repulsive electrosteric interactions, at low particle volume fractions (ϕ< 0.4), increasing ionic strength has almost no impact on the relative viscosity whereas at higher particle volume fractions, a decrease in viscosity has been observed, as expected due to a reduced range of electrosteric repulsion between particles, corresponding to a reduction of the effective particle volume fraction ϕeff. For dispersions including attractive interactions, we found that at volume fractions φ=0.45 below the hard sphere freezing point, irrespective of ionic strength, the bulk viscosity increases monotonically with increasing PEO concentration, i.e. attraction strength, due to the transition from a fluid to a fluid/crystalline and finally to a gel state. However, the increase in ionic strength shifts the concentrations of both phase transitions to lower polymer concentrations, indicating that in presence of salt a weaker attraction is required to induce these transitions. These investigations demonstrated how superimposed short range electrosteric repulsion and weak depletion attraction affect microstructure and flow behavior of colloidal dispersions.
Publications
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Imaging of the microstructure of Carbopol dispersions and correlation with their macroelasticity: A micro- and macrorheological study. Journal of Rheology, 66(4), 749-760.
Oelschlaeger, Claude; Marten, Jonas; Péridont, Florian & Willenbacher, Norbert
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Ionic strength effect on the structure and dynamics of colloidal dispersions with weak attractive interactions. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 700, 134694.
Oelschlaeger, Claude; Maciel, Bruna Regina; Ratel, Louise; Müller, Marc & Willenbacher, Norbert
