13 research outputs found
Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution
The nanoscopic adhesive and frictional behaviour of end-grafted poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) films (brushes) in contact with gold- or PDMAEMA-coated atomic force microscope tips in potassium halide solutions with different concentrations up to 300 mM is a strong function of salt concentration. The conformation of the polymers in the brush layer is sensitive to salt concentration, which leads to large changes in adhesive forces and the contact mechanics at the tip–sample contact, with swollen brushes (which occur at low salt concentrations) yielding large areas of contact and friction–load plots that fit JKR behaviour, while collapsed brushes (which occur at high salt concentrations) yield sliding dominated by ploughing, with conformations in between fitting DMT mechanics. The relative effect of the different anions follows the Hofmeister series, with I − collapsing the brushes more than Br − and Cl − for the same salt concentration
Effect of triaxial in situ stresses and heterogeneities on absolute permeability of laminated rocks
Rheological characterization of commercial highly viscous alginate solutions in shear and extensional flows
The rheological properties of sodium alginate in salt-free solutions were studied by steady shear, dynamic
oscillatory and extensional measurements. This biopolymer
consists of mannuronic and guluronic acid residues that give
a polyelectrolyte character. We applied the scaling theories
and checked their accordance with polyelectrolyte behaviour
for low concentrations with a shift to neutral polymer behaviour
at larger concentrations. This nature was supported by
the effect of the concentration on the specific viscosity, the
relaxation times from steady shear and the longest relaxation
times from small amplitude oscillatory shear (SAOS) measurements.
To analyze the extensional behaviour of the samples,
we conducted a study of dimensionless numbers and
time scales where filament thinning driven by viscous, capillary
or elastic forces is at play. We conclude that an exponential
filament thinning followed by breakup results in the
best regimes that describe the experimental data. Besides, the
data pointed out that alginate in salt-free concentrated solutions
shows strain thinning of the extensional viscosity and
chain rigidity, behaviours that cannot be inferred from the
shear rheometry.This research was supported by funds from the European Research Council (ERC). Project MYCAP (258984) STAR TING GRANTS 2010