We explore the rheology during a startup flow of well-characterized polyelectrolyte microgel suspensions, which form soft glasses above the jamming concentration. We present and discuss results measured using different mechanical histories focusing on the variations of the static yield stress and yield strain. The behavior of the shear stress growth function is affected by long-lived residual stresses and strains that imprint a slowly decaying mechanical memory inside the materials. The startup flow response is not reversible upon flow reversal and the amplitude of the static yield stress increases with the time elapsed after rejuvenation. We propose an experimental protocol that minimizes the directional memory and we analyze the effect of aging. The static yield strain γ p and the reduced static yield stress σ p / σ y , where σ y is the dynamic yield stress measured from steady flow measurements, are in good agreement with our previous simulations [Khabaz et al., “Transient dynamics of soft particle glasses in startup shear flow. Part I: Microstructure and time scales,” J. Rheol. 65, 241 (2021)]. Our results demonstrate the need to consider memory and aging effects in transient measurements on soft particle glasses.
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Dispersing nano- and micro-sized portlandite particulates via electrosteric exclusion at short screening lengths
In spite of their high surface charge (zeta potential ζ = +34 mV), aqueous suspensions of portlandite (calcium hydroxide: Ca(OH) 2 ) exhibit a strong tendency to aggregate, and thereby present unstable suspensions. While a variety of commercial dispersants seek to modify the suspension stability and rheology ( e.g. , yield stress, viscosity), it remains unclear how the performance of electrostatically and/or electrosterically based additives is affected in aqueous environments having either a high ionic strength and/or a pH close to the particle's isoelectric point (IEP). We show that the high native ionic strength (pH ≈ 12.6, IEP: pH ≈ 13) of saturated portlandite suspensions strongly screens electrostatic forces (Debye length: κ −1 = 1.2 nm). As a result, coulombic repulsion alone is insufficient to mitigate particle aggregation and affect rheology. However, a longer-range geometrical particle–particle exclusion that arises from electrosteric hindrance caused by the introduction of comb polyelectrolyte dispersants is very effective at altering the rheological properties and fractal structuring of suspensions. As a result, comb-like dispersants that stretch into the solvent reduce the suspension's yield stress by 5× at similar levels of adsorption as compared to linear dispersants, thus enhancing the critical solid loading ( i.e. , at which jamming occurs) by 1.4×. Significantly, the behavior of diverse dispersants is found to be inherently related to the thickness of the adsorbed polymer layer on particle surfaces. These outcomes inform the design of dispersants for concentrated suspensions that present strong charge screening behavior.
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- Award ID(s):
- 1922167
- NSF-PAR ID:
- 10195544
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 16
- Issue:
- 14
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 3425 to 3435
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0SM00045K
