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A recent experiment on zebrafish blastoderm morphogenesis showed that the viscosity (η) of a non-confluent embryonic tissue grows sharply until a critical cell packing fraction (ϕS). The increase inηup toϕSis similar to the behavior observed in several glass-forming materials, which suggests that the cell dynamics is sluggish or glass-like. Surprisingly,ηis a constant aboveϕS. To determine the mechanism of this unusual dependence ofηonϕ, we performed extensive simulations using an agent-based model of a dense non-confluent two-dimensional tissue. We show that polydispersity in the cell size, and the propensity of the cells to deform, results in the saturation of the available free area per cell beyond a critical packing fraction. Saturation in the free space not only explains the viscosity plateau aboveϕSbut also provides a relationship between equilibrium geometrical packing to the dramatic increase in the relaxation dynamics.
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Effect of polydispersity in concentrated magnetorheological fluids
Abstract Magnetorheological fluids (MRF) are smart materials of increasing interest due to their great versatility in mechanical and mechatronic systems. As main rheological features, MRFs must present low viscosity in the absence of magnetic field (0.1–1.0 Pa.s) and high yield stress (50–100 kPa) when magnetized, in order to optimize the magnetorheological effect. Such properties, in turn, are directly influenced by the composition, volume fraction, size, and size distribution (polydispersity) of the particles, the latter being an important piece in the improvement of these main properties. In this context, the present work aims to analyze, through experiments and simulations, the influence of polydispersity on the maximum packing fraction, on the yield stress under field (on-state) and on the plastic viscosity in the absence of field (off-state) of concentrated MRF (φ= 48.5 vol.%). Three blends of carbonyl iron powder (CIP) in polyalphaolefin oil were prepared. These blends have the same mode, but different polydispersity indexes (α), ranging from 0.46 to 1.44. Separate simulations show that the random close packing fraction increases from about 68% to 80% as the polydispersity indexes increase over this range. The on-state yield stress, in turn, is raised from 30 ± 0.5 kPa to 42 ± 2 kPa (B≈ 0.57 T) and the off-state plastic viscosity, is reduced from 4.8 Pa.s to 0.5 Pa.s. Widening the size distributions, as is well known in the literature, increases packing efficiency and reduces the viscosity of concentrated dispersions, but beyond that, it proved to be a viable way to increase the magnetorheological effect of concentrated MRF. The Brouwers model, which considers the void fraction in suspensions of particles with lognormal distribution, was proposed as a possible hypothesis to explain the increase in yield stress under magnetic field.
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- Award ID(s):
- 1804186
- PAR ID:
- 10511082
- Publisher / Repository:
- IOP
- Date Published:
- Journal Name:
- Smart Materials and Structures
- Volume:
- 32
- Issue:
- 4
- ISSN:
- 0964-1726
- Page Range / eLocation ID:
- 045014
- Subject(s) / Keyword(s):
- magnetorheology polydispersity rheology
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1088/1361-665X/acbb47
