The relationship between induced fluid structure and boundary slip in nanoscale polymer films

The molecular mechanism of slip at the interface between polymer melts and weakly attractive smooth surfaces is investigated using molecular dynamics simulations. In agreement with our previous studies on slip flow of shear-thinning fluids, it is shown that the slip length passes through a local minimum at low shear rates and then increases rapidly at higher shear rates. We found that at sufficiently high shear rates, the slip flow over atomically flat crystalline surfaces is anisotropic. It is demonstrated numerically that the friction coefficient at the liquid-solid interface (the ratio of viscosity and slip length) undergoes a transition from a constant value to the power-law decay as a function of the slip velocity. The characteristic velocity of the transition correlates well with the diffusion velocity of fluid monomers in the first fluid layer near the solid wall at equilibrium. We also show that in the linear regime, the friction coefficient is well described by a function of a single variable, which is a product of the magnitude of surface-induced peak in the structure factor and the contact density of the adjacent fluid layer. The universal relationship between the friction coefficient and induced fluid structure holds for a number of material parameters of the interface: fluid density, chain length, wall-fluid interaction energy, wall density, lattice type and orientation, thermal or solid walls.Comment: 33 pages, 14 figure

Mbd1 is recruited to both methylated and nonmethylated CpGs via distinct DNA binding domains

MBD1 is a vertebrate methyl-CpG binding domain protein (MBD) that can bring about repression of methylated promoter DNA sequences. Like other MBD proteins, MBD1 localizes to nuclear foci that in mice are rich in methyl-CpG. In methyl-CpG-deficient mouse cells, however, Mbd1 remains localized to heterochromatic foci whereas other MBD proteins become dispersed in the nucleus. We find that Mbd1a, a major mouse isoform, contains a CXXC domain (CXXC-3) that binds specifically to nonmethylated CpG, suggesting an explanation for methylation-independent localization. Transfection studies demonstrate that the CXXC-3 domain indeed targets nonmethylated CpG sites in vivo. Repression of nonmethylated reporter genes depends on the CXXC-3 domain, whereas repression of methylated reporters requires the MBD. Our findings indicate that MBD1 can interpret the CpG dinucleotide as a repressive signal in vivo regardless of its methylation status

Questionnaires

The aims of this chapter are to present the potential uses of questionnaires and the principles involved in developing an effective and valid questionnaire. It will consider some of the issues involved with the effective development and use of questionnaires as a research tool, their administration and ethical considerations. The information provided in this chapter is not set out to be definitive, but rather, it is presented as an introduction in the development and use of questionnaires in health and physical activity research. Depending on the intended research methods, the reader may wish to read this chapter alongside those on surveys, focus groups and questionnaires, as much of the information presented in these chapters relates and informs the others to provide a more comprehensive coverage. These chapters have been presented in this way to provide an informative coverage of the topic without excessive duplication and repetition of material

Polymeric filament thinning and breakup in microchannels

The effects of elasticity on filament thinning and breakup are investigated in microchannel cross flow. When a viscous solution is stretched by an external immiscible fluid, a low 100 ppm polymer concentration strongly affects the breakup process, compared to the Newtonian case. Qualitatively, polymeric filaments show much slower evolution, and their morphology features multiple connected drops. Measurements of filament thickness show two main temporal regimes: flow- and capillary-driven. At early times both polymeric and Newtonian fluids are flow-driven, and filament thinning is exponential. At later times, Newtonian filament thinning crosses over to a capillary-driven regime, in which the decay is algebraic. By contrast, the polymeric fluid first crosses over to a second type of flow-driven behavior, in which viscoelastic stresses inside the filament become important and the decay is again exponential. Finally, the polymeric filament becomes capillary-driven at late times with algebraic decay. We show that the exponential flow thinning behavior allows a novel measurement of the extensional viscosities of both Newtonian and polymeric fluids.Comment: 7 pages, 7 figure