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Abstract The streaming instability (SI) is a leading mechanism for concentrating solid particles into regions dense enough to form planetesimals. Its efficiency in clumping particles depends primarily on the dimensionless stopping time (τs, a proxy for particle size) and dust-to-gas surface density ratio (Z). Previous simulations identified a criticalZ(Zcrit) above which strong clumping occurs, where particle densities exceed the Hill density (thus satisfying a condition for gravitational collapse), over a wide range ofτs. These works found that, forτs≤ 0.01,Zcritwas above the interstellar medium value (∼0.01). In this work, we reexamine the clumping threshold using 2D axisymmetric, stratified simulations at high resolution and with relatively large (compared to many previous simulations) domain sizes. Our main results are as follows: First, whenτs = 0.01, strong clumping occurs even atZ ≲ 0.01, lower thanZcritfound in all previous studies. Consequently, we revise a previously published fit to theZcritcurve to account for this updatedZcrit. Second, higher resolution results in a thicker dust layer, which may result from other instabilities manifesting, such as the vertically shearing SI. Third, despite this thicker layer, higher resolution can lead to strong clumping even with a lower midplane dust-to-gas density ratios (which results from the thicker particle layer) so long asZ ≳ Zcrit. Our results demonstrate the efficiency of the SI in clumping small particles atZ ∼ 0.01, which is a significant refinement of the conditions for planetesimal formation by the SI.
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Impact of Salinity on the Erosion Threshold, Yield Stress, and Gelatinous State of a Cohesive Clay
Abstract Clay is the main component that contributes to sediment cohesiveness. Salinity impacts its transport, which controls the electrochemical force among the sediment grains. Here, we quantify the impacts of salinity on the erosion threshold, yield stress, and the microstructures of a fluorescently labeled smectite clay, laponite, by combining flume experiments, rheometer measurements, and macro‐ and microscopic imaging. We show that the critical shear stress for clay erosion,τb,crit, increases by one order of magnitude with increasing salinity when salinity <1.5 ppt and slightly decreases when salinity >1.5 ppt showing a weaker dependency upon salinity. We further show that the yield stress,τy, of the clay remains roughly a constant at salinity less than 1.5 ppt and decreases by over one order of magnitude at salinity larger than 1.5 ppt. This change in the dependency ofτb,critand yield stress on salinity corresponds to a change in the gelatinous state of clay, from gel‐like structures to phase‐separated structures as salinity increases. Our results provide a quantitative characterization of the dependency of clay erosion threshold and yield stress on salinity and highlight the importance of the clay gelatinous state in controlling clay transport.
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- Award ID(s):
- 2150796
- PAR ID:
- 10499676
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 129
- Issue:
- 3
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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