1,666 research outputs found
Critical jamming of frictional grains in the generalized isostaticity picture
While frictionless spheres at jamming are isostatic, frictional spheres at
jamming are not. As a result, frictional spheres near jamming do not
necessarily exhibit an excess of soft modes. However, a generalized form of
isostaticity can be introduced if fully mobilized contacts at the Coulomb
friction threshold are considered as slipping contacts. We show here that, in
this framework, the vibrational density of states (DOS) of frictional discs
exhibits a plateau when the generalized isostaticity line is approached. The
crossover frequency to elastic behavior scales linearly with the distance from
this line. Moreover, we show that the frictionless limit, which appears
singular when fully mobilized contacts are treated elastically, becomes smooth
when fully mobilized contacts are allowed to slip.Comment: 4 pages, 4 figures, submitted to PR
Force Dynamics in Weakly Vibrated Granular Packings
The oscillatory force F_b^ac on the bottom of a rigid, vertically vibrated,
grain filled column, reveals rich granular dynamics, even when the peak
acceleration of the vibrations is signicantly less than the gravitational
acceleration at the earth's surface. For loose packings or high frequencies,
F_b^ac 's dynamics are dominated by grain motion. For moderate driving
conditions in more compact samples, grain motion is virtually absent, but
F_b^ac nevertheless exhibits strongly nonlinear and hysteretic behavior,
evidencing a granular regime dominated by nontrivial force-network dynamics.Comment: 4 pages, 5 figure
The use of equine chondrogenic‐induced mesenchymal stem cells as a treatment for osteoarthritis : a randomised, double‐blinded, placebo‐controlled proof‐of‐concept study
Background: There is a need to improve therapies for osteoarthritis in horses. Objectives To assess the efficacy of equine allogeneic chondrogenic-induced mesenchymal stem cells combined with equine allogeneic plasma as a novel therapy for osteoarthritis in horses.
Study design: Randomised, double-blinded, placebo-controlled experiment.
Methods: In 12 healthy horses, osteoarthritis was induced in the metacarpophalangeal joint using an osteochondral fragment-groove model. Five weeks after surgery, horses were randomly assigned to either an intra-articular injection with chondrogenic-induced mesenchymal stem cells + equine allogeneic plasma (= intervention) or with 0.9% saline solution (= control). From surgery until the study end, horses underwent a weekly joint and lameness assessment. Synovial fluid was collected for cytology and biomarker analysis before surgery and at Weeks 5, 5 + 1d, 7, 9 and 11. At Week 11, horses were subjected to euthanasia, and the metacarpophalangeal joints were evaluated macroscopically and histologically.
Results: No serious adverse events or suspected adverse drug reactions occurred during the study. A significant improvement in visual and objective lameness was seen with the intervention compared with the control. Synovial fluid displayed a significantly higher viscosity and a significantly lower glycosaminoglycan concentration in the intervention group. Other biomarkers or cytology parameters were not significantly different between the treatment groups. Significantly less wear lines and synovial hyperaemia were present in the intervention group. The amount of cartilage oligomeric matrix protein, collagen type II and glycosaminoglycans were significantly higher in the articular cartilage of the intervention group.
Main limitations: This study assessed the short-term effect of the intervention on a limited number of horses, using an osteoarthritis model. This study also included multiple statistical tests, increasing the risk of type 1 error.
Conclusions: Equine allogeneic chondrogenic-induced mesenchymal stem cells combined with equine allogeneic plasma may be a promising treatment for osteoarthritis in horses
On elliptic solutions of the cubic complex one-dimensional Ginzburg-Landau equation
The cubic complex one-dimensional Ginzburg-Landau equation is considered.
Using the Hone's method, based on the use of the Laurent-series solutions and
the residue theorem, we have proved that this equation has neither elliptic
standing wave nor elliptic travelling wave solutions. This result amplifies the
Hone's result, that this equation has no elliptic travelling wave solutions.Comment: LaTeX, 12 page
Bounds on the shear load of cohesionless granular matter
We characterize the force state of shear-loaded granular matter by relating
the macroscopic stress to statistical properties of the force network. The
purely repulsive nature of the interaction between grains naturally provides an
upper bound for the sustainable shear stress, which we analyze using an
optimization procedure inspired by the so-called force network ensemble. We
establish a relation between the maximum possible shear resistance and the
friction coefficient between individual grains, and find that anisotropies of
the contact network (or the fabric tensor) only have a subdominant effect.
These results can be considered the hyperstatic limit of the force network
ensemble and we discuss possible implications for real systems. Finally, we
argue how force anisotropies can be related quantitatively to experimental
measurements of the effective elastic constants.Comment: 17 pages, 6 figures. v2: slightly rearranged, introduction and
discussion rewritte
Ensemble Theory for Force Networks in Hyperstatic Granular Matter
An ensemble approach for force networks in static granular packings is
developed. The framework is based on the separation of packing and force
scales, together with an a-priori flat measure in the force phase space under
the constraints that the contact forces are repulsive and balance on every
particle. In this paper we will give a general formulation of this force
network ensemble, and derive the general expression for the force distribution
. For small regular packings these probability densities are obtained in
closed form, while for larger packings we present a systematic numerical
analysis. Since technically the problem can be written as a non-invertible
matrix problem (where the matrix is determined by the contact geometry), we
study what happens if we perturb the packing matrix or replace it by a random
matrix. The resulting 's differ significantly from those of normal
packings, which touches upon the deep question of how network statistics is
related to the underlying network structure. Overall, the ensemble formulation
opens up a new perspective on force networks that is analytically accessible,
and which may find applications beyond granular matter.Comment: 17 pages, 17 figure
Cycling exposure and infrastructural correlates in a Flemish adolescent population
Introduction: Infrastructure adjusted to cyclists can be used as a tool to reduce cycling crashes and injuries. To increase cycling frequency and safety, a population's cycling exposure to different infrastructure needs to be known. Since cycling behavior of adolescents cannot be compared to adults, the objective of this study is to describe the cycling exposure and the characteristics of routes used by adolescents for bicycle commuting.
Methods: We used an online travel diary to collect bicycle route information and an exposure matrix to collect cycling frequency data. Data were collected in Flanders (Belgium) from an adolescent population (15-18 years).
Results: From the 1345 participating adolescents, 47% used their bicycle at least once a week. Infrastructural correlates were based on 467,000 km of cycled roads with a mean trip distance of 5.7 +/- 4.5 km; 91% of cycling exposure is on infrastructure with speed limits of 50 km/h or more and 32.7% on dedicated cycle lanes. Lower motorized traffic density was related to higher odds of daily bicycle commuting. Higher traffic densities or speed limits were related to lower odds of daily bicycle commuting, but this relationship disappeared after adjusting for presence of a dedicated cycle lane.
Conclusions: Many adolescents who commute by bicycle cycle along streets with high speed limits and no dedicated cycle lanes probably because of the low motorized traffic densities on these roads. Adolescents should avoid roads without dedicated cycling infrastructure and high speed limits since these roads have been identified as high risk. The results are region specific and therefore this study should be replicated in other contexts
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