Effect of water-wall interaction potential on the properties of nanoconfined water

Pradeep Kumar — 2008-06-11T20:12:46-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 1. (2007)

Much of the understanding of bulk liquids has progressed through study of the limiting case in which molecules interact via purely repulsive forces, such as a hard-core or “repulsive ramp” potential. In the same spirit, we report progress on the understanding of confined water by examining the behavior of waterlike molecules interacting with planar walls via purely repulsive forces and compare our results with those obtained for Lennard-Jones (LJ) interactions between the molecules and the walls. Specifically, we perform molecular dynamics simulations of 512 waterlike molecules interacting via the TIP5P potential and confined between two smooth planar walls that are separated by 1.1 nm. At this separation, there are either two or three molecular layers of water, depending on density. We study two different forms of repulsive confinement, when the water-wall interaction potential is either (i) 1/r9 or (ii) a WCA-like repulsive potential. We find that the thermodynamic, dynamic, and structural properties of the liquid in purely repulsive confinements qualitatively match those for a system with a pure LJ attraction to the wall. In previous studies that include attractions, freezing into monolayer or trilayer ice was seen for this wall separation. Using the same separation as these previous studies, we find that the crystal state is not stable with 1/r9 repulsive walls but is stable with WCA-like repulsive confinement. However, by carefully adjusting the separation of the plates with 1/r9 repulsive interactions so that the effective space available to the molecules is the same as that for LJ confinement, we find that the same crystal phases are stable. This result emphasizes the importance of comparing systems only using the same effective confinement, which may differ from the geometric separation of the confining surfaces.

Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement

Clifford Brangwynne — 2006-11-20T22:19:20-00:00

J. Cell Biol., Vol. 173, No. 5. (5 June 2006), pp. 733-741.

Cytoskeletal microtubules have been proposed to influence cell shape and mechanics based on their ability to resist large-scale compressive forces exerted by the surrounding contractile cytoskeleton. Consistent with this, cytoplasmic microtubules are often highly curved and appear buckled because of compressive loads. However, the results of in vitro studies suggest that microtubules should buckle at much larger length scales, withstanding only exceedingly small compressive forces. This discrepancy calls into question the structural role of microtubules, and highlights our lack of quantitative knowledge of the magnitude of the forces they experience and can withstand in living cells. We show that intracellular microtubules do bear large-scale compressive loads from a variety of physiological forces, but their buckling wavelength is reduced significantly because of mechanical coupling to the surrounding elastic cytoskeleton. We quantitatively explain this behavior, and show that this coupling dramatically increases the compressive forces that microtubules can sustain, suggesting they can make a more significant structural contribution to the mechanical behavior of the cell than previously thought possible. 10.1083/jcb.200601060

CiteULike: dchen Kumar

Effect of water-wall interaction potential on the properties of nanoconfined water

Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement