1,883 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
Self-healing slip pulses and the friction of gelatin gels
We present an extensive experimental study and scaling analysis of friction
of gelatin gels on glass. At low driving velocities, sliding occurs via
propagation of periodic self-healing slip pulses whose velocity is limited by
collective diffusion of the gel network. Healing can be attributed to a
frictional instability occurring at the slip velocity . For ,
sliding is homogeneous and friction is ruled by the shear-thinning rheology of
an interfacial layer of thickness of order the (nanometric) mesh size,
containing a semi-dilute solution of polymer chain ends hanging from the
network. Inspite of its high degree of confinement, the rheology of this system
does not differ qualitatively from known bulk ones. The observed ageing of the
static friction threshold reveals the slow increase of adhesive bonding between
chain ends and glass. Such structural ageing is compatible with the existence
of a velocity-weakening regime at velocities smaller than , hence with the
existence of the healing instability.Comment: 9 pages, 16 figure
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
Studies on the respiratory pigment of Urechis eggs
Experiments previously reported (1) have shown that the eggs of the Pacific marine worm, Urechis caupo, contain a reversible oxidation-reduction pigment. The pigment, called urechrome, is autoxidizable and changes color from red to yellow on oxidation. It is soluble in water (reduced form insoluble below pH 5) and in acidified methanol, but insoluble in ether, acetone, chloroform, and neutral alcohol. Evidence for participation of the pigment in cellular respiration has been previously given
Potential of Lignins as Antioxidant Additive in Active Biodegradable Packaging Materials
Lien vers la version éditeur: http://link.springer.com/article/10.1007/s10924-013-0570-6Due to their polyphenolic structure lignins bear a number of interesting functional properties, such as antioxidant activity. Natural antioxidants are very much looked for in the aim of protection of light or oxygen sensitive goods and are being used in active packaging. Poly(lactide) (PLA)-lignin films were prepared by twin screw extrusion followed by thermo-compression using two different commercial sources of alkali lignins obtained from gramineous plants. A good dispersion of lignin in the matrix was observed. Mechanical properties of the compounded material were merely diminished and oxygen barrier properties slightly enhanced. The chromatographic study of the lignins revealed that the low molecular weight fraction of both lignins increased during the polymer processing. The migration of low molecular weight compounds in an ethanol/water solution simulating fatty foodstuff was performed and the antioxidant activity of the extract was analysed. It was found that the activity increases with increasing severity of the heat treatment because of the generation of free phenolic monomers during processing. These results open an interesting way for application of lignins as an active compound in packaging materials. Lignins do not impair the mechanical and barrier performance of the polymer and the plastics processing even allows for the generation of active substances
Types of understanding: Their nature and their relation to knowledge
What does it mean to understand something? I approach this question by comparing understanding with knowledge. Like knowledge, understanding comes, at least prima facia, in three varieties: propositional, interrogative and objectual. I argue that explanatory understanding (this being the most important form of interrogative understanding) and objectual understanding are not reducible to one another and are neither identical with, nor even a form of, the corresponding type of knowledge (nor any other type of knowledge). My discussion suggests that definitions of the concepts of explanatory and of objectual understanding must include a commitment condition, a grasping condition, an answering-the-facts condition, and an epistemically internal justification condition, but no further external anti-luck condition. On this basis I argue against reducing explanatory understanding to propositional understanding, and in favour of identifying propositional understanding with propositional knowledg
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