Effect of cationic polyelectrolyte and surfactant on cohesion and friction in contacts between cellulose fibers

New insights into the mechanisms of water-stress-induced cavitation in conifers

Cavitation resistance is a key parameter to understand tree drought tolerance but little is known about the mechanisms of air entry into xylem conduits. For conifers three mechanisms have been proposed: (1) a rupture of pit margo microfibrils, (2) a displacement of the pit torus from its normal sealing position over the pit aperture, and (3) a rupture of an air-water menisci in a pore of the pit margo. In this article, we report experimental results on three coniferous species suggesting additional mechanisms. First, when xylem segments were injected with a fluid at a pressure sufficient to aspirate pit tori and well above the pressure for cavitation induction we failed to detect the increase in sample conductance that should have been caused by torus displacement from blocking the pit aperture or by membrane rupture. Second, by injecting xylem samples with different surfactant solutions, we found a linear relation between sample vulnerability to cavitation and fluid surface tension. This suggests that cavitation in conifers could also be provoked by the capillary failure of an air-water meniscus in coherence with the prediction of Young-Laplace's equation. Within the bordered pit membrane, the exact position of this capillary seeding is unknown. The possible Achilles' heel could be the seal between tori and pit walls or holes in the torus. The mechanism of water-stress-induced cavitation in conifers could then be relatively similar to the one currently proposed for angiosperms.

The influence of surface-active media on the mechanical properties of materials

This survey summarizes studies dealing with the influence of surface-active media on the mechanical behavior of materials that have been carried out at the Institute of Physical Chemistry of the Russian Academy of Sciences, at the Department of Chemistry of Moscow State University and at the Department of Geography and Environmental Engineering of the Johns Hopkins University, and presented partially at the SIS Symposia. The phenomena of the environment-sensitive mechanical behavior take place for all kinds of solids under the effect of adsorption layers and due to the contact with liquid phases containing some specific components which are physically-chemically akin with respect to a given solid and cause lowering of the surface energy of the solid and weakening of the cohesive forces in the surface (interfacial) layer. Studies of these effects and their atomic-molecular mechanisms provide basis for control, prevention or utilization of these phenomena.