1,218 research outputs found
To cope or not to cope- are we meeting the needs of women experiencing pain during child birth?
Verwandertes Leben, verwandertes Gehirn? On Else Lasker-Schüler, Neurodiversity and Migration
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Strand plasticity governs fatigue in colloidal gels
Repeated loading of a solid leads to microstructural damage that ultimately
results in catastrophic material failure. While posing a major threat to the
stability of virtually all materials, the microscopic origins of fatigue,
especially for soft solids, remain elusive. Here we explore fatigue in
colloidal gels as prototypical inhomogeneous soft solids by combining
experiments and computer simulations. Our results reveal how mechanical loading
leads to irreversible strand stretching, which builds slack into the network
that softens the solid at small strains and causes strain hardening at larger
deformations. We thus find that microscopic plasticity governs fatigue at much
larger scales. This gives rise to a new picture of fatigue in soft thermal
solids and calls for new theoretical descriptions of soft gel mechanics in
which local plasticity is taken into account.Comment: 5 pages, 4 figure
Brownian particles in transient polymer networks
We discuss the thermal motion of colloidal particles in transient polymer networks. For particles that are physically bound to the surrounding chains, light-scattering experiments reveal that the submillisecond dynamics changes from diffusive to Rouse-like upon crossing the network formation threshold. Particles that are not bound do not show such a transition. At longer time scales the mean-square displacement (MSD) exhibits a caging plateau and, ultimately, a slow diffusive motion. The slow diffusion at longer time scales can be related to the macroscopic viscosity of the polymer solutions. Expressions that relate the caging plateau to the macroscopic network elasticity are found to fail for the cases presented here. The typical Rouse scaling of the MSD with the square root of time, as found in experiments at short time scales, is explained by developing a bead-spring model of a large colloidal particle connected to several polymer chains. The resulting analytical expressions for the MSD of the colloidal particle are shown to be consistent with experimental findings
Looking through a glass comorbidly : towards a stylometric estimate of vulnerability in writings by Sylvia Plath and Ingeborg Bachmann
Relaxation dynamics at different time scales in electrostatic complexes: Time-salt superposition
In this Letter we show that in the rheology of electrostatically assembled soft materials, salt concentration plays a similar role as temperature for polymer melts, and as strain rate for soft solids. We rescale linear and nonlinear rheological data of a set of model electrostatic complexes at different salt concentrations to access a range of time scales that is otherwise inaccessible. This provides new insights into the relaxation mechanisms of electrostatic complexes, which we rationalize in terms of a microscopic mechanism underlying salt-enhanced activated processe
Stress management in composite biopolymer networks
Living tissues show an extraordinary adaptiveness to strain, which is crucial
for their proper biological functioning. The physical origin of this mechanical
behaviour has been widely investigated using reconstituted networks of collagen
fibres, the principal load-bearing component of tissues. However, collagen
fibres in tissues are embedded in a soft hydrated polysaccharide matrix which
generates substantial internal stresses whose effect on tissue mechanics is
unknown. Here, by combining mechanical measurements and computer simulations,
we show that networks composed of collagen fibres and a hyaluronan matrix
exhibit synergistic mechanics characterized by an enhanced stiffness and
delayed strain-stiffening. We demonstrate that the polysaccharide matrix has a
dual effect on the composite response involving both internal stress and
elastic reinforcement. Our findings elucidate how tissues can tune their
strain-sensitivity over a wide range and provide a novel design principle for
synthetic materials with programmable mechanical properties
Cracking up: symmetry breaking in cellular systems
The shape of animal cells is, to a large extent, determined by the cortical actin network that underlies the cell membrane. Because of the presence of myosin motors, the actin cortex is under tension, and local relaxation of this tension can result in cortical flows that lead to deformation and polarization of the cell. Cortex relaxation is often regulated by polarizing signals, but the cortex can also rupture and relax spontaneously. A similar tension-induced polarization is observed in actin gels growing around beads, and we propose that a common mechanism governs actin gel rupture in both systems
Dynamics of polymer bridge formation and disruption
In this Letter we show, with colloidal probe AFM measurements, that the formation and subsequent disruption of polymer bridges between two solid surfaces is characterized by slow relaxation times. This is due to the retardation of polymer dynamics near a surface. For colloidal particles, that are in constant (Brownian) motion, kinetic aspects are key. To understand these effects, we develop a model of polymer bridging and bridge disruption that agrees quantitatively with our experiment
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