83 research outputs found
Dilatancy in slow granular flows
When walking on wet sand, each footstep leaves behind a temporarily dry impression. This counterintuitive
observation is the most common illustration of the Reynolds principle of dilatancy: that is, a
granular packing tends to expand as it is deformed, therefore increasing the amount of porous space.
Although widely called upon in areas such as soil mechanics and geotechnics, a deeper understanding of
this principle is constrained by the lack of analytical tools to study this behavior. Using x-ray radiography,
we track a broad variety of granular flow profiles and quantify their intrinsic dilatancy behavior. These
measurements frame Reynolds dilatancy as a kinematic process. Closer inspection demonstrates, however,
the practical importance of flow induced compaction which competes with dilatancy, leading more
complex flow properties than expected
Viscoelastic shear banding in foam
Shear banding is an important feature of flow in complex fluids. Essentially,
shear bands refer to the coexistence of flowing and non-flowing regions in
driven material. Understanding the possible sources of shear banding has
important implications for a wide range of flow applications. In this regard,
quasi-two dimensional flow offers a unique opportunity to study competing
factors that result in shear bands. One proposal is the competition between
intrinsic dissipation and an external source of dissipation. In this paper, we
report on the experimental observation of the transition between different
classes of shear-bands that have been predicted to exist in cylindrical
geometry as the result of this competition [R. J. Clancy, E. Janiaud, D.
Weaire, and S. Hutzlet, Eur. J. Phys. E, {\bf 21}, 123 (2006)]
X-ray observation of micro-failures in granular piles approaching an avalanche
An X-ray imaging technique is used to probe the stability of 3-dimensional
granular packs in a slowly rotating drum. Well before the surface reaches the
avalanche angle, we observe intermittent plastic events associated with
collective rearrangements of the grains located in the vicinity of the free
surface. The energy released by these discrete events grows as the system
approaches the avalanche threshold. By testing various preparation methods, we
show that the pre-avalanche dynamics is not solely controlled by the difference
between the free surface inclination and the avalanche angle. As a consequence,
the measure of the pre-avalanche dynamics is unlikely to serve as a tool for
predicting macroscopic avalanches
Strain-induced alignment in collagen gels
Collagen is the most abundant extracellular-network-forming protein in animal
biology and is important in both natural and artificial tissues, where it
serves as a material of great mechanical versatility. This versatility arises
from its almost unique ability to remodel under applied loads into anisotropic
and inhomogeneous structures. To explore the origins of this property, we
develop a set of analysis tools and a novel experimental setup that probes the
mechanical response of fibrous networks in a geometry that mimics a typical
deformation profile imposed by cells in vivo. We observe strong fiber alignment
and densification as a function of applied strain for both uncrosslinked and
crosslinked collagenous networks. This alignment is found to be irreversibly
imprinted in uncrosslinked collagen networks, suggesting a simple mechanism for
tissue organization at the microscale. However, crosslinked networks display
similar fiber alignment and the same geometrical properties as uncrosslinked
gels, but with full reversibility. Plasticity is therefore not required to
align fibers. On the contrary, our data show that this effect is part of the
fundamental non-linear properties of fibrous biological networks.Comment: 12 pages, 7 figures. 1 supporting material PDF with 2 figure
Tumour heterogeneity promotes collective invasion and cancer metastatic dissemination.
Heterogeneity within tumour cell populations is commonly observed in most cancers. However, its impact on metastatic dissemination, one of the primary determinants of the disease prognosis, remains poorly understood. Working with a simplified numerical model of tumour spheroids, we investigated the impact of mechanical heterogeneity on the onset of tumour invasion into surrounding tissues. Our work establishes a positive link between tumour heterogeneity and metastatic dissemination, and recapitulates a number of invasion patterns identified in vivo, such as multicellular finger-like protrusions. Two complementary mechanisms are at play in heterogeneous tumours. A small proportion of stronger cells are able to initiate and lead the escape of cells, while collective effects in the bulk of the tumour provide the coordination required to sustain the invasive process through multicellular streaming. This suggests that the multicellular dynamics observed during metastasis is a generic feature of mechanically heterogeneous cell populations and might rely on a limited and generic set of attributes
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Micro-scale visualization of Microbial-Induced Calcium Carbonate Precipitation (MICP) processes
Microbial-Induced Calcium Carbonate (CaCO₃ ) Precipitation (MICP) has been explored for its potential engineering applications such as soil stabilization, but our understanding of the fundamental MICP processes at the microscale is limited. In this study, real-time in situ micro-scale experiments were conducted using glass slides and microfluidic chips (synthetic porous media which simulate soil matrices to model the conditions similar to actual MICP treatments) to visualize the CaCO₃ precipitation process. The results of this study show that irregularly-shaped CaCO₃ precipitates initially emerged on bacterial aggregates and subsequently dissolved with time as regularly-shaped CaCO₃ crystals started growing; less stable and smaller CaCO₃ crystals may dissolve at the expense of growth of more stable and larger CaCO₃ crystals. The time-dependent phase transformation of CaCO₃ precipitates makes the size of the crystals formed during MICP highly dependent on the time interval between cementation solution injections during a staged injection procedure. When the injection interval was 3-5 hours, a larger number of crystals (200-1000 per 10⁶ μm³) with smaller sizes (5-10 μm) was produced. When the injection interval was longer (23-25 hours), the crystals were larger (10-80 μm) and fewer in number (5-20 per 10⁶ μm³). The direct observation of MICP processes in this study improves the understanding of MICP fundamentals and the effect of MICP processes on the properties of CaCO₃ crystals formed after MICP treatment. These observations will therefore be useful for designing future MICP treatment protocols which improve the properties and sustainability of MICP-treated samples
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