473 research outputs found
A convective instability mechanism for quasistatic crack branching in a hydrogel
Experiments on quasistatic crack propagation in gelatin hydrogels reveal a
new branching instability triggered by wetting the tip opening with a drop of
aqueous solvent less viscous than the bulk one. We show that the emergence of
unstable branches results from a balance between the rate of secondary crack
growth and the rate of advection away from a non-linear elastic region of size
where is the fracture energy and the small
strain Young modulus. We build a minimal, predictive model that combines
mechanical characteristics of this mesoscopic region and physical features of
the process zone. It accounts for the details of the stability diagram and
lends support to the idea that non-linear elasticity plays a critical role in
crack front instabilities
Rheological aging and rejuvenation in solid friction contacts
We study the low-velocity (0.1--100 m.s) frictional properties of
interfaces between a rough glassy polymers and smooth silanized glass, a
configuration which gives direct access to the rheology of the adhesive joints
in which shear localizes. We show that these joints exhibit the full
phenomenology expected for confined quasi 2D soft glasses: they strengthen
logarithmically when aging at rest, and weaken (rejuvenate) when sliding.
Rejuvenation is found to saturate at large velocities. Moreover, aging at rest
is shown to be strongly accelerated when waiting under finite stress below the
static threshold
From thermally activated to viscosity controlled fracture of biopolymer hydrogels
We report on rate-dependent fracture energy measurements over three decades
of steady crack velocities in alginate and gelatin hydrogels. We evidence that,
irrespective of gel thermo-reversibility, thermally activated "unzipping" of
the non-covalent cross-link zones results in slow crack propagation, prevaling
against the toughening effect of viscous solvent drag during chain pull-out,
which becomes efficient above a few mm.s. We extend a previous model
[Baumberger {\it et al.} Nature Materials, {\bf 5}, 552 (2006)] to account for
both mechanisms, and estimate the microscopic unzipping rates
Interplay between shear loading and structural aging in a physical gel
We show that the aging of the mechanical relaxation of a gelatin gel exhibits
the same scaling phenomenology as polymer and colloidal glasses. Besides,
gelatin is known to exhibit logarithmic structural aging (stiffening). We find
that stress accelerates this process. However, this effect is definitely
irreducible to a mere age shift with respect to natural aging. We suggest that
it is interpretable in terms of elastically-aided elementary (coilhelix)
local events whose dynamics gradually slows down as aging increases geometric
frustration
Fracture of a biopolymer gel as a viscoplastic disentanglement process
We present an extensive experimental study of mode-I, steady, slow crack
dynamics in gelatin gels. Taking advantage of the sensitivity of the elastic
stiffness to gel composition and history we confirm and extend the model for
fracture of physical hydrogels which we proposed in a previous paper (Nature
Materials, doi:10.1038/nmat1666 (2006)), which attributes decohesion to the
viscoplastic pull-out of the network-constituting chains. So, we propose that,
in contrast with chemically cross-linked ones, reversible gels fracture without
chain scission
Self healing slip pulses along a gel/glass interface
We present an experimental evidence of self-healing shear cracks at a
gel/glass interface. This system exhibits two dynamical regimes depending on
the driving velocity : steady sliding at high velocity (> Vc = 100-125 \mu
m/s), caracterized by a shear-thinning rheology, and periodic stick-slip
dynamics at low velocity. In this last regime, slip occurs by propagation of
pulses that restick via a ``healing instability'' occuring when the local
sliding velocity reaches the macroscopic transition velocity Vc. At driving
velocities close below Vc, the system exhibits complex spatio-temporal
behavior.Comment: 4 pages, 6 figure
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