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J A Aström

Publications and source records attributed to J A Aström.

11 recordsLinked to original sources
PubMed2006

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 (approximately 0.1) of adhesion related packing frustration is sufficient for breaking such a blockage even at high a pressure.

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PubMed2005

Dimensional effects in dynamic fragmentation of brittle materials.

It has been shown previously that dynamic fragmentation of brittle D -dimensional objects in a D -dimensional space gives rise to a power-law contribution to the fragment-size distribution with a universal scaling exponent 2-1/D . We demonstrate that in fragmentation of two-dimensional brittle objects in three-dimensional space, an additional fragmentation mechanism appears, which causes scale-invariant secondary breaking of existing fragments. Due to this mechanism, the power law in the fragment-size distribution has now a scaling exponent of approximately 1.17 .

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PubMed2004

Crumpling of a stiff tethered membrane.

A first-principles numerical model for crumpling of a stiff tethered membrane is introduced. This model displays wrinkles, ridge formation, ridge collapse, and initiation of stiffness divergence. The amplitude and wavelength of the wrinkles and the scaling exponent of the stiffness divergence are consistent with both theory and experiment. Close to the stiffness divergence further buckling is hindered by the nonzero thickness of the membrane, and its elastic behavior becomes similar to that of dry granular media. No change in the distribution of contact forces can be observed at the crossover, implying that the network of ridges is then simultaneously a granular force-chain network.

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PubMed2004

Carbon nanotube mats and fibers with irradiation-improved mechanical characteristics: a theoretical model.

We employ a theoretical model to calculate mechanical characteristics of macroscopic mats and fibers of single-walled carbon nanotubes. We further investigate irradiation-induced covalent bonds between nanotubes and their effects on the tensile strength of nanotube mats and fibers. We show that the stiffness and strength of the mats can be increased at least by an order of magnitude, and thus small-dose irradiation with energetic particles is a promising tool for making macroscopic nanotube materials with excellent mechanical characteristics.

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PubMed2004

Exponential and power-law mass distributions in brittle fragmentation.

Generic arguments, a minimal numerical model, and fragmentation experiments with gypsum disk are used to investigate the fragment-size distribution that results from dynamic brittle fragmentation. Fragmentation is initiated by random nucleation of cracks due to material inhomogeneities, and its dynamics are pictured as a process of propagating cracks that are unstable against side-branch formation. The initial cracks and side branches both merge mutually to form fragments. The side branches have a finite penetration depth as a result of inherent damping. Generic arguments imply that close to the minimum strain (or impact energy) required for fragmentation, the number of fragments of size s scales as s(-(2D-1)/D) f(1) (- (2/lambda)(D) s)+ f(2) (- s(-1 )(0 ) (lambda+ s(1/D) )(D) ), where D is the Euclidean dimension of the space, lambda is the penetration depth, and f(1) and f(2) can be approximated by exponential functions. Simulation results and experiments can both be described by this theoretical fragment-size distribution. The typical largest fragment size s(0) was found to diverge at the minimum strain required for fragmentation as it is inversely related to the density of initially formed cracks. Our results also indicate that scaling of s(0) close to this divergence depends on, e.g., loading conditions, and thus is not universal. At the same time, the density of fragment surface vanishes as L-1, L being the linear dimension of the brittle solid. The results obtained provide an explanation as to why the fragment-size distributions found in nature can have two components, an exponential as well as a power-law component, with varying relative weights.

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PubMed2004

Universal dynamic fragmentation in D dimensions.

A generic model is introduced for brittle fragmentation in D dimensions, and this model is shown to lead to a fragment-size distribution with two distinct components. In the small fragment-size limit a scale-invariant size distribution results from a crack branching-merging process. At larger sizes the distribution becomes exponential as a result of a Poisson process, which introduces a large-scale cutoff. Numerical simulations are used to demonstrate the validity of the distribution for D=2. Data from laboratory-scale experiments and large-scale quarry blastings of granitic gneiss confirm its validity for D=3. In the experiments the nonzero grain size of rock causes deviation from the ideal model distribution in the small-size limit. The size of the cutoff seems to diverge at the minimum energy sufficient for fragmentation to occur, but the scaling exponent is not universal.

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PubMed2003

Clustering and viscosity in a shear flow of a particulate suspension.

A shear flow of particulate suspension is analyzed for the qualitative effect of particle clustering on viscosity using a simple kinetic clustering model and direct numerical simulations. The clusters formed in a Couette flow can be divided into rotating chainlike clusters and layers of particles at the channel walls. The size distribution of the rotating clusters is scale invariant in the small-cluster regime and decreases rapidly above a characteristic length scale that diverges at a jamming transition. The behavior of the suspension can qualitatively be divided into three regimes. For particle Reynolds number Re(p) less than or approximately equal 0.1, viscosity is controlled by the characteristic cluster size deduced from the kinetic clustering model. For Re(p) approximately 1, clustering is maximal, but the simple kinetic model becomes inapplicable presumably due to onset of instabilities. In this transition regime viscosity begins to increase. For Re(p) greater than or approximately equal 10, inertial effects become important, clusters begin to breakup, and suspension displays shear thickening. This phenomenon may be attributed to enhanced contribution of solid phase in the total shear stress.

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PubMed2003

Dynamic rigidity transition.

An inflated closed loop (or membrane) is used to demonstrate a dynamic rigidity transition that occurs when impact energy is added to the loop in static equilibrium at zero temperature. The only relevant parameter in this transition is the ratio of the energy needed to collapse the loop and the impact energy. When this ratio is below a threshold value close to unity, the loop collapses into a high-entropy floppy state, and it does not return to the rigid state unless the impact energy can escape. The internal oscillations are in the floppy state dominated by 1/f(2) noise. When the ratio is above the threshold, the loop does not collapse, and the internal oscillations resulting from the impact are dominated by the eigenfrequencies of the stretched membrane. In this state, the loop can bounce for a long time. It is still an open question whether bouncing will eventually vanish or whether a stationary bouncing state will be reached. The dynamic transition between the floppy and the rigid state is discontinuous.

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PubMed2002

Fouling dynamics in suspension flows.

A particle suspension flowing in a channel in which fouling layers are allowed to form on the channel walls is investigated by numerical simulation. A two-dimensional phase diagram with at least four different behaviors is constructed. The fouling is modeled by attachment during collision with the deposits and by detachment caused by large enough hydrodynamic drag. For fixed total number of particles and small Reynolds numbers, the relevant parameters governing the fouling dynamics are the solid volume fraction of the suspension and the detachment drag force threshold. Below a critical curve in this 2D phase space only transient fouling takes place when the suspension is accelerated from rest by a pressure gradient. Above the fouling transition line, persistent fouling layers are formed via ballistic deposition for low and via homogeneous deposition for large solid volume fractions. Close to the fouling transition line, the flow path between the deposited layers meanders, while necking appears for increasing distance from the transition. Finally, another transition to a fully blocked flow path takes place. As determined by the estimated amount of deposited particles at saturation, both transitions seem to be discontinuous. Large fluctuations and long saturation times are typical of the dynamics of the system.

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PubMed2001

Fracture mechanics of snow avalanches.

Dense snow avalanches are analyzed by modeling the snow slab as an elastic and brittle plate, attached by static friction to the underlying ground. The grade of heterogeneity in the local fracture (slip) thresholds, and the ratio of the average substrate slip threshold to the average slab fracture threshold, are the decisive parameters for avalanche dynamics. For a strong pack of snow there appears a stable precursor of local slips when the frictional contacts are weakened (equivalent to rising temperature), which eventually trigger a catastrophic crack growth that suddenly releases the entire slab. In the opposite limit of very high slip thresholds, the slab simply melts when the temperature is increased. In the intermediate regime, and for a homogeneous slab, the model display features typical of real snow avalanches. The model also suggests an explanation to why avalanches are impossible to forecast reliably based on precursor observations. This explanation may as well be applicable to other catastrophic rupture phenomena such as earthquakes.

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