34 research outputs found
Cavitation-induced force transition in confined viscous liquids under traction
We perform traction experiments on simple liquids highly confined between
parallel plates. At small separation rates, we observe a simple response
corresponding to a convergent Poiseuille flow. Dramatic changes in the force
response occur at high separation rates, with the appearance of a force plateau
followed by an abrupt drop. By direct observation in the course of the
experiment, we show that cavitation accounts for these features which are
reminiscent of the utmost complex behavior of adhesive films under traction.
Surprisingly enough, this is observed here in purely viscous fluids.Comment: Submitted to Physical Review Letters on May 31, 2002. Related
informations on http://www.crpp.u-bordeaux.fr/tack.htm
Spreading of Non-Newtonian and Newtonian Fluids on a Solid Substrate under Pressure
Strongly non-Newtonian fluids namely, aqueous gels of starch, are shown to
exhibit visco-elastic behavior, when subjected to a load. We study arrowroot
and potato starch gels. When a droplet of the fluid is sandwiched between two
glass plates and compressed, the area of contact between the fluid and plates
increases in an oscillatory manner. This is unlike Newtonian fluids, where the
area increases monotonically in a similar situation. The periphery moreover,
develops an instability, which looks similar to Saffman Taylor fingers. This is
not normally seen under compression. The loading history is also found to
affect the manner of spreading. We attempt to describe the non-Newtonian nature
of the fluid through a visco-elastic model incorporating generalized calculus.
This is shown to reproduce qualitatively the oscillatory variation in the
surface strain.Comment: 11 page
Adhesion between elastic solids with randomly rough surfaces: comparison of analytical theory with molecular dynamics simulations
The adhesive contact between elastic solids with randomly rough, self affine
fractal surfaces is studied by molecular dynamics (MD) simulations. The
interfacial binding energy obtained from the simulations of nominally flat and
curved surfaces is compared with the predictions of the contact mechanics
theory by Persson. Theoretical and simulation results agree rather well, and
most of the differences observed can be attributed to finite size effects and
to the long-range nature of the interaction between the atoms in the block and
the substrate in the MD model, as compared to the analytical theory which is
for an infinite system with interfacial contact interaction. For curved
surfaces (JKR-type of problem) the effective interfacial energy exhibit a weak
hysteresis which may be due to the influence of local irreversible detachment
processes in the vicinity of the opening crack tip during pull-off.Comment: 6 pages, 6 figure
Cavitation-induced force transition in confined viscous liquids under traction
We perform traction experiments on simple liquids highly confined between parallel plates. At small separation rates, we observe a simple response corresponding to a convergent Poiseuille flow. Dramatic changes in the force response occur at high separation rates, with the appearance of a force plateau followed by an abrupt drop. By direct observation in the course of the experiment, we show that cavitation accounts for these features which are reminiscent of the utmost complex behavior of adhesive films under traction. Surprisingly enough, this is observed here in purely viscous fluids
Force response of a viscous liquid in a probe-tack geometry: Fingering versus cavitation
We perform traction experiments on viscous liquids highly confined between parallel plates, a geometry known as the probe-tack test in the adhesion community. Direct observation during the experiment coupled to force measurement shows the existence of several mechanisms for releasing the stress: while fingering is favored for low traction velocities, low confinement and low viscosity, nucleation of bubbles occurs in the opposite conditions. It is possible to quantitatively predict the transition between the two regimes and, in many respects, describe the shape of the force response. Using a model for purely viscous fluids, we also present a phase diagram for the different force peak regimes that remarkably accounts for the data. Our results show that conspicuous features of the traction curve commonly thought to be characteristic of soft viscoelastic solids like adhesives are already encountered in liquid materials
Amphiphilic diblock copolymers with adhesive properties: I. Structure and swelling with water
We study asymmetric block copolymers with the simple diblock AB architecture, in the case where the longer block A is both hydrophobic and "soft", whereas the shorter block B is hydrophilic and "hard". Materials with such a particular combination of physico-chemical and mechanical properties have distinctive advantages, in particular for designing water-compatible adhesive materials. The phase diagram is established, combining NMR and SAXS characterisations of the materials. The swelling with water is monitored through gravimetry and "time-resolved" SAXS. Indications of maintained adhesive properties in a wet environment are given