36 research outputs found
Effective rheology across the fragmentation transition for sea ice and ice shelves
Funding was provided by the NERC grant NE/P011365/1 Calving Laws for Ice Sheet Models CALISMO. Data files for the plots are found at: https://doi.org/10.5285/76D7D3CA-7B83-4BB0-AAE5-A8E92C7DA5B0Sea ice and ice shelves can be described by a viscoelastic rheology that is approximately linear elastic and brittle at high strain rates, and viscously shearâthinning at low strain rates. Brittle ice easily fractures under compressive shear and forms shear bands as the material undergoes a transition to a fragmented, granular state. This transition plays a central role in the mechanical behaviour at large scales of seaâice in the Arctic Ocean or Antarctic ice shelves. Here we demonstrate that the fragmentation transition is characterized by an essentially discontinuous drop of 3â5 orders of magnitude in effective viscosity and stressârelaxation time. Beyond the fragmentation transition, grinding in shear zones further reduces both effective viscosity and shear stiffness, but with an essentially constant relaxation time of âŒ10second. These results are relevant for iceârheology implementation in largeâscale climateârelated models of sea ice and thin ice shelves.Publisher PDFPeer reviewe
Scaling of impact fragmentation near the critical point
We investigated two-dimensional brittle fragmentation with a flat impact
experimentally, focusing on the low impact energy region near the
fragmentation-critical point. We found that the universality class of
fragmentation transition disagreed with that of percolation. However, the
weighted mean mass of the fragments could be scaled using the pseudo-control
parameter multiplicity. The data for highly fragmented samples included a
cumulative fragment mass distribution that clearly obeyed a power-law. The
exponent of this power-law was 0.5 and it was independent of sample size. The
fragment mass distributions in this regime seemed to collapse into a unified
scaling function using weighted mean fragment mass scaling. We also examined
the behavior of higher order moments of the fragment mass distributions, and
obtained multi-scaling exponents that agreed with those of the simple biased
cascade model.Comment: 6 pages, 6 figure
Non-universality of elastic exponents in random bond-bending networks
We numerically investigate the rigidity percolation transition in
two-dimensional flexible, random rod networks with freely rotating cross-links.
Near the transition, networks are dominated by bending modes and the elastic
modulii vanish with an exponent f=3.0\pm0.2, in contrast with central force
percolation which shares the same geometric exponents. This indicates that
universality for geometric quantities does not imply universality for elastic
ones. The implications of this result for actin-fiber networks is discussed.Comment: 4 pages, 3 figures, minor clarifications and amendments. To appear in
PRE Rap. Com
Possible origins of macroscopic left-right asymmetry in organisms
I consider the microscopic mechanisms by which a particular left-right (L/R)
asymmetry is generated at the organism level from the microscopic handedness of
cytoskeletal molecules. In light of a fundamental symmetry principle, the
typical pattern-formation mechanisms of diffusion plus regulation cannot
implement the "right-hand rule"; at the microscopic level, the cell's
cytoskeleton of chiral filaments seems always to be involved, usually in
collective states driven by polymerization forces or molecular motors. It seems
particularly easy for handedness to emerge in a shear or rotation in the
background of an effectively two-dimensional system, such as the cell membrane
or a layer of cells, as this requires no pre-existing axis apart from the layer
normal. I detail a scenario involving actin/myosin layers in snails and in C.
elegans, and also one about the microtubule layer in plant cells. I also survey
the other examples that I am aware of, such as the emergence of handedness such
as the emergence of handedness in neurons, in eukaryote cell motility, and in
non-flagellated bacteria.Comment: 42 pages, 6 figures, resubmitted to J. Stat. Phys. special issue.
Major rewrite, rearranged sections/subsections, new Fig 3 + 6, new physics in
Sec 2.4 and 3.4.1, added Sec 5 and subsections of Sec
Development and Validation of a Tokamak Skin Effect Transformer model
A control oriented, lumped parameter model for the tokamak transformer
including the slow flux penetration in the plasma (skin effect transformer
model) is presented. The model does not require detailed or explicit
information about plasma profiles or geometry. Instead, this information is
lumped in system variables, parameters and inputs. The model has an exact
mathematical structure built from energy and flux conservation theorems,
predicting the evolution and non linear interaction of the plasma current and
internal inductance as functions of the primary coil currents, plasma
resistance, non-inductive current drive and the loop voltage at a specific
location inside the plasma (equilibrium loop voltage). Loop voltage profile in
the plasma is substituted by a three-point discretization, and ordinary
differential equations are used to predict the equilibrium loop voltage as
function of the boundary and resistive loop voltages. This provides a model for
equilibrium loop voltage evolution, which is reminiscent of the skin effect.
The order and parameters of this differential equation are determined
empirically using system identification techniques. Fast plasma current
modulation experiments with Random Binary Signals (RBS) have been conducted in
the TCV tokamak to generate the required data for the analysis. Plasma current
was modulated in Ohmic conditions between 200kA and 300kA with 30ms rise time,
several times faster than its time constant L/R\approx200ms. The model explains
the most salient features of the plasma current transients without requiring
detailed or explicit information about resistivity profiles. This proves that
lumped parameter modeling approach can be used to predict the time evolution of
bulk plasma properties such as plasma inductance or current with reasonable
accuracy; at least in Ohmic conditions without external heating and current
drive sources
Spontaneous formation of densely packed shear bands of rotating fragments
Appearance of self-similar space-filling ball bearings has been suggested to provide the explanation for seismic gaps, shear weakness, and lack of detectable frictional heat formation in mature tectonic faults (shear zones). As the material in a shear zone fractures and grinds, it could be thought to eventually form a conformation that allows fragments to largely roll against each other without much sliding. This type of space-filling âball bearingâ can be constructed artificially, but so far how such delicate structures may appear spontaneously has remained unexplained. It is demonstrated here that first-principles simulations of granular packing with fragmenting grains indeed display spontaneous formation of shear bands with fragment conformations very similar to those of densely packed ball bearings
Cell aggregation : packing soft grains
Cellular aggregates may be considered as collections of membrane enclosed units with a pressure difference between the internal and external liquid phases. Cells are kept together by membrane adhesion and/or confined space compression. Pattern formation and, in particular, intercellular spacing have important roles in controlling solvent diffusion within such aggregates. A physical approach is used to study generic aspects of cellular packings in a confined space. Average material properties are derived from the free energy. The appearance of penetrating intercellular void channels is found to be critically governed by the cell wall adhesion mechanisms during the formation of dense aggregates. A fully relaxed aggregate efficiently hinders solvent diffusion at high hydrostatic pressures, while a small fraction (~0.1) of adhesion related packing frustration is sufficient for breaking such a blockage even at high a pressure
Cell aggregation : packing soft grains
Cellular aggregates may be considered as collections of membrane enclosed units with a pressure difference between the internal and external liquid phases. Cells are kept together by membrane adhesion and/or confined space compression. Pattern formation and, in particular, intercellular spacing have important roles in controlling solvent diffusion within such aggregates. A physical approach is used to study generic aspects of cellular packings in a confined space. Average material properties are derived from the free energy. The appearance of penetrating intercellular void channels is found to be critically governed by the cell wall adhesion mechanisms during the formation of dense aggregates. A fully relaxed aggregate efficiently hinders solvent diffusion at high hydrostatic pressures, while a small fraction (~0.1) of adhesion related packing frustration is sufficient for breaking such a blockage even at high a pressure
Fragmentation dynamics within shear bands-a model for aging tectonic faults?
A numerical model for packing of fragmenting
blocks in a shear band is introduced, and its dynamics is
compared with that of a tectonic fault.
The shear band undergoes a slow aging process in which
the blocks are being grinded by the shear motion and
the compression.
The dynamics of the model have the same
statistical characteristics as the seismic activity in
faults. The characteristic magnitude distribution
of earthquakes appears to result from frictional slips at
small and medium magnitudes, and from fragmentation of blocks at the
largest magnitudes. Aftershocks to large-magnitude earthquakes
are local recombinations of the fragments before
they reach a new quasi-static equilibrium. The aftershocks
satisfy Omori's law. Local precursor activity at a few times the
normal background level appears at a short time
before a major earthquake. Seismic gaps appear
as a natural consequence of the aging process of a fault.
Explanation of the heat flux and principal stress direction
anomalies at the faults both involve the value of fracture stress
of the blocks in the gouge.
The final form of a tectonic fault is predicted to involve
a gouge dominated by fine-grained and rather rounded blocks
so that it cannot withstand large shear stresses
Aster formation and rupture transition in semi-flexible fiber networks with mobile cross-linkers
Fibrous active network structures whose properties are regulated by motor proteins, or simply motors, are fundamental to life. Here, a full elastic and three dimensional model for such networks and motors is presented. The effects of surface anchoring are accounted for and we demonstrate that for unidirectional motors two basic contractile phases emerge in these systems. The transition is governed by a single parameter (tb/tc) which is the ratio of the breaking strain (tb) and the motility limiting strain (tc) of the motors. For tb/tc [less, similar] 2 and clamped boundaries, the network ruptures and formation of local asters occurs with a high density of motors at the centre and the fibers radially spanning out. This phase displays contraction strain during the formation of asters but the network stress is relaxed once the asters have emerged, demonstrating that the formation of aster-like structures provides a mechanism for stress relaxation. For 2.7 [less, similar] tb/tc the network remains intact, but reaches a force equilibrium with a high contraction strain in the case of clamped boundaries. Between these two limits the network is partly ruptured. Experimental measurements (e.g. J. T. Nishizaka, H. Miyata, H. Yoshikawa, S. Ishiwata and K. Kinosita Jr., Nature, 1995, 377, 251 and J. F. Finer, R. M. Simmons, J. A. Spudich, Nature, 1994, 368, 113) indicate that actin filament and myosin motors interact with tb/tc Ë 2.7 which is right at the limit of motor induced fracture for a random network, indicating that e.g. a cytoskeleton with active myosin is susceptible to rupture. This is perhaps not a coincidence and may well be an important factor contributing to cellular dynamics. In the case of free boundaries the network collapses onto one single aster. We also show that the distribution of energy on the motors is a power-law, below the motility limit energy, with the exponent -0.5