Stick and Slip Behaviour of Confined Oligomer Melts under Shear. A Molecular-Dynamics Study.

The flow behaviour of melts of short chains, confined in molecularly thin Couette flow geometries, is studied with molecular-dynamics simulations. The effect of wall attraction and confinement on the density and velocity profiles is analysed. In these highly inhomogeneous films, a strong correlation between the density and velocity profile is found. Sticking of the interfacial layer on the wall and slip on the wall and inside the film is manifested by changes in the velocity profile. The location of the slip is determined by the strength of the wall attraction.

A Molecular-Dynamics Study

Abstract. -The flow behaviour of melts of short chains, confined in molecularly thin Couette flow geometries, is studied with molecular-dynamics simulations. The effect of wall attraction and confinement on the density and velocity profiles is analysed. In these highly inhomogeneous films, a strong correlation between the density and velocity profile is found. Sticking of the interfacial layer on the wall and slip on the wall and inside the film is manifested by changes in the velocity profile. The location of the slip is determined by the strength of the wall attraction. Even though the macroscopic phenomena of friction, lubrication and adhesion have been studied for a long time now, their molecular mechanisms have yet to be unveiled. Recent novel experimental techniques such as the surface forces apparatus together with the scanning probe microscopies are capable to give a vast amount of information on the nanometer level; these combined with computer simulations will provide further insight into the nanoscopic dynamics of friction, lubrication and adhesion. The structural and dynamical properties of ultrathin confined films between atomically flat surfaces undergoing shear are studied here with molecular-dynamics simulations. Simulations are performed on a fluid of short chains in a planar Couette flow geometry realized by confining the polymer system in the x-direction between two parallel planar f.c.c. (111) planes. Periodic boundary conditions are imposed in the other two directions. The Couette flow is introduced by moving the two walls with equal and constant velocities towards opposite directions keeping the wall-to-wall distance constant. In this way, a steady shear rate is introduced with the direction of flow parallel to the x-axis and the velocity gradient parallel to the x-axis, i.e. normal to the walls El]. The chains consist of six segments which are connected in a linear freely joined topology. Of course, the model is not expected to capture the real monomer response and segments correspond to several chemical monomers [2]. The segments of the same chain as well as segments belonging to different chains interact via a pairwise purely repulsive, shifted an

Adsorption-desorption kinetics in nanoscopically confined oligomer films under shear

The method of molecular dynamics computer simulations is employed to study oligomer melts confined in ultra-thin films and subjected to shear. The focus is on the self-diffusion of oligomers near attractive surfaces and on their desorption, together with the effects of increasing energy of adsorption and shear. It is found that the mobility of the oligomers near an attractive surface is strongly decreased. Moreover, although shearing the system forces the chains to stretch parallel to the surfaces and thus increase the energy of adsorption per chain, flow also promotes desorption. The study of chain desorption kinetics reveals the molecular processes responsible for the enhancement of desorption under shear. They involve sequences of conformations starting with a desorbed tail and proceeding in a very fast, correlated, segment-by-segment manner to the desorption of the oligomers from the surfaces.

The imperative of arachidonic acid in human reproduction.

We are presenting new evidence on essential fatty acids (EFA) in prenatal human development. We have demonstrated, for the first time, the detailed process of active selection of some fatty acids by the placenta (biomagnification) and rejection of others (bioreduction) and how this strategy is of supreme importance for understanding of the biology of human reproduction. The biomagnification process by the placenta is dominated by arachidonic acid (ArA) and its allies: di-homo-gamma-linolenic acid (DGLA), adrenic acid and ω6 docosapentaenoic acid. Stearic acid is similarly bio-magnified which is likely to provide for the sn-1 position in membrane synthesis. In contrast there is a bioreduction of oleic, linoleic and all ω3 precursors for docosahexaenoic acid (DHA), including eicosapentaenoic acid (EPA). Although DHA is biomagnified, the amplification from mother to fetus is small compared to ArA. We report on the dominant compartmentalisation of ArA from mother to fetal plasma, cell membranes of red cells, mono-nuclear cells, endothelium and the placenta. We conclude that ArA and its allies, play a paramount role in the development of the products of conception. It is plausible that inadequate provision of ArA may be relevant to the neuro-vascular complications of prematurity and neurodevelopmental disorders associated with premature birth. We present evidence of ArA's universal role from an identical arachidonic acid-based strategy observed in contrasting cultures. The dominance of ArA in the prenatal and in post-natal nutritional provision by human milk makes a compelling case for re-evaluation of its role, especially in reproductive biology

Radius and chirality dependent conformation of polymer molecule at nanotube interface

Temperature dependent conformations of linear polymer molecules adsorbed at carbon nanotube (CNT) interfaces are investigated through molecule dynamics simulations. Model polyethylene (PE) molecules are shown to have selective conformations on CNT surface, controlled by atomic structures of CNT lattice and geometric coiling energy. PE molecules form entropy driven assembly domains, and their preferred wrapping angles around large radius CNT (40, 40) reflect the molecule configurations with energy minimums on a graphite plane. While PE molecules prefer wrapping on small radius armchair CNT (5, 5) predominantly at low temperatures, their configurations are shifted to larger wrapping angle ones on a similar radius zigzag CNT (10, 0). A nematic transformation around 280 K is identified through Landau-deGennes theory, with molecule aligning along tube axis in extended conformationsComment: 19 pages, 7 figure2, submitted to journa

Capillary filling with wall corrugations] Capillary filling in microchannels with wall corrugations: A comparative study of the Concus-Finn criterion by continuum, kinetic and atomistic approaches

We study the impact of wall corrugations in microchannels on the process of capillary filling by means of three broadly used methods - Computational Fluid Dynamics (CFD), Lattice-Boltzmann Equations (LBE) and Molecular Dynamics (MD). The numerical results of these approaches are compared and tested against the Concus-Finn (CF) criterion, which predicts pinning of the contact line at rectangular ridges perpendicular to flow for contact angles theta > 45. While for theta = 30, theta = 40 (no flow) and theta = 60 (flow) all methods are found to produce data consistent with the CF criterion, at theta = 50 the numerical experiments provide different results. Whilst pinning of the liquid front is observed both in the LB and CFD simulations, MD simulations show that molecular fluctuations allow front propagation even above the critical value predicted by the deterministic CF criterion, thereby introducing a sensitivity to the obstacle heigth.Comment: 25 pages, 8 figures, Langmuir in pres