RESUMO
We study the particle scale response of a 2D frictionless disk system to bulk forcing via cyclic shear with reversal amplitude γ_{r}. We find a subdiffusive γ_{r}-dependent regime, which is consistent with models of anomalous diffusion with scale-invariant cage dynamics, and a crossover to diffusive grain motion at high γ_{r}. Analysis of local displacements of a particle relative to its cage of neighbors reveals a key distinction from thermal systems. Particles are moved by fluctuations of their cage of neighbors rather than rattling in their cage, indicating a distinct cage-breaking mechanism.
RESUMO
We numerically investigate the mechanical properties of static packings of frictionless ellipsoidal particles in two and three dimensions over a range of aspect ratio and compression Δφ. While amorphous packings of spherical particles at jamming onset (Δφ=0) are isostatic and possess the minimum contact number z_{iso} required for them to be collectively jammed, amorphous packings of ellipsoidal particles generally possess fewer contacts than expected for collective jamming (zAssuntos
Coloides/química
, Modelos Químicos
, Modelos Moleculares
, Reologia/métodos
, Simulação por Computador
, Movimento (Física)
RESUMO
We investigate the onset of irreversibility in a dense granular medium subjected to cyclic shear in a split-bottom geometry. To probe the micro- and mesoscale, we image bead trajectories in three dimensions throughout a series of shear strain oscillations. Although beads lose and regain contact with neighbors during a cycle, the global topology of the contact network exhibits reversible properties for low oscillation amplitudes. With increasing reversal amplitude, a transition to an irreversible diffusive regime occurs.
Assuntos
Coloides/química , Modelos Químicos , Modelos Moleculares , Termodinâmica , Simulação por Computador , Pressão , Resistência ao CisalhamentoRESUMO
The premature aging disorder, Hutchinson-Gilford progeria syndrome (HGPS), is caused by mutant lamin A, which affects the nuclear scaffolding. The phenotypic hallmark of HGPS is nuclear blebbing. Interestingly, similar nuclear blebbing has also been observed in aged cells from healthy individuals. Recent work has shown that treatment with rapamycin, an inhibitor of the mTOR pathway, reduced nuclear blebbing in HGPS fibroblasts. However, the extent of blebbing varies considerably within each cell population, which makes manual blind counting challenging and subjective. Here, we show a novel, automated and high throughput nuclear shape analysis that quantitatively measures curvature, area, perimeter, eccentricity and additional metrics of nuclear morphology for large populations of cells. We examined HGPS fibroblast cells treated with rapamycin and RAD001 (an analog to rapamycin). Our analysis shows that treatment with RAD001 and rapamycin reduces nuclear blebbing, consistent with blind counting controls. In addition, we find that rapamycin treatment reduces the area of the nucleus, but leaves the eccentricity unchanged. Our nuclear shape analysis provides an unbiased, multidimensional "fingerprint" for a population of cells, which can be used to quantify treatment efficacy and analyze cellular aging.
Assuntos
Forma do Núcleo Celular , Senescência Celular , Progéria/patologia , Forma do Núcleo Celular/efeitos dos fármacos , Células Cultivadas , Senescência Celular/efeitos dos fármacos , Everolimo , Fibroblastos/efeitos dos fármacos , Fibroblastos/metabolismo , Fibroblastos/patologia , Humanos , Processamento de Imagem Assistida por Computador , Lamina Tipo A , Proteínas Nucleares/metabolismo , Progéria/tratamento farmacológico , Progéria/metabolismo , Precursores de Proteínas/metabolismo , Sirolimo/análogos & derivados , Sirolimo/farmacologiaRESUMO
We study numerically frictionless ellipse packings versus the aspect ratio alpha, and find that the jamming transition is fundamentally different from that for spherical particles. The normal mode spectra possess two gaps and three distinct branches over a range of alpha. The energy from deformations along modes in the lowest-energy branch increases quartically, not quadratically. The quartic modes cause novel power-law scaling of the static shear modulus and their number matches the deviation from isostaticity. These results point to a new critical point at alpha>1 that controls jamming of aspherical particles.