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To explore the role of individual lung surfactant components in liquid-condensed (LC)/liquid-expanded (LE) phase transitions the MARTINI coarse-grained (CG) model is used to simulate monolayers containing DPPC and additional lipid or peptide components. Our analysis suggests that the LC phase forms from the LE phase via a nucleation and growth mechanism, while the LC-LE transition occurs by melting that originates from defects in the monolayer. On the time scale of our simulations, DPPC monolayers display a substantial hysteresis between the ordering and disordering transitions, which is decreased by the addition of a second component. In binary mixtures of DPPC with lung surfactant peptide fragment SP-B(1-25), the ordered side of the hysteresis loop is abolished altogether, suggesting that SP-B(1-25) effectively nucleates disorder in the monolayer on heating. SP-B(1-25) is observed to perturb the packing of the surrounding lipids leading to local fluidization of the monolayer and to aggregate within the LE phase. In 1:1 DPPC:POPC monolayers, a high concentration of unsaturated phospholipid leads to a substantial decrease in the LC-LE and LE-LC transition temperatures. Adding cholesterol to pure DPPC increases the LC-LE and LE-LC transition temperatures and increases the order on the disordered side of the hysteresis loop leading to a phase of intermediate order, which could be the liquid-disordered (Ld) phase. Cholesterol is also observed to show a preference for LC-LE domain boundaries. The results of our molecular dynamics simulations coincide with many experimental observations and can help provide insight into the physiological roles of individual surfactant components.

Using Brownian dynamics simulations of polymer chains over a wide range of resolution, we find universal scaling laws for polymer coil dimensions and tumbling time. At high shear rates γ, the coil thickness in the gradient direction becomes independent of chain length, scaling as N K 0 γ̇− 1/4 , where N K denotes the numbers of Kuhn steps, and the tumbling time scales as N K γ̇− 3/4 , correcting scaling laws presented in prior studies. We find this to be a consequence of the formation of loops whose length is limited by the time required to stretch them and derive scaling laws from a balance of convection and diffusion of monomers. We find that, in the absence of hydrodynamic interaction (HI) and excluded volume (EV), for wormlike chains, the shrinkage in chain stretch observed at ultrahigh shear rates is pushed out to arbitrarily high shear rates if the chain is resolved increasingly finely below the persistence length. Finally, scaling laws in the presence of excluded volume and hydrodynamic interactions are derived that are expected to be valid for long chains at high shear rates. ■ INTRODUCTIONThe dynamics of isolated flexible and semiflexible polymer chains in shear flow has attracted considerable attention and is thought to be well understood as a result of single-molecule imaging of DNA molecules 1−3 and Brownian dynamics simulations. 4,5 These studies have shown that the average molecular extension of a polymer chain in the flow direction (x) increases with shear rate (γ) and reaches a plateau at high shear rates. The failure to reach full extension at high shear rates is a consequence of repeated end-over-end tumbling of the polymer molecule, due to the equal strength of the extensional and rotational components in a shear flow. The plateau of polymer extension at high shear rate was assumed to be the asymptotic response at high shear rates, until some recent simulations, 6,9,12 performed over a wide range of shear rates, revealed a highly nonmonotonic response in chain stretch. Quite surprisingly, without excluded volume (EV) and hydrodynamic interactions (HI), the average chain stretch was shown to reach a maximum and then decrease at the highest shear rates. (This effect almost vanished in the presence of EV but became more pronounced again when both HI and EV were included. 6 ) We show in what follows that this nonmonotonic behavior disappears in the absence of EV and HI when a bending potential is included in the chain model to represent more accurately the behavior of a semiflexible wormlike chain, such as double-stranded DNA or other biopolymers.In addition, coarse-grained simulations 7,8 have indicated that the tumbling time decreases with shear rate as γ̇2 /3 at high shear rates, which was further confirmed by a theoretical analysis. 11 For a DNA chain, the measured tumbling time approximately followed this power law for shear flows with Wi > 10, 7,8 where Wi is the Weissenberg number, which is the product of shear rate γ̇and the relaxation time of the polymer chain. It seems doubtful, however, ...

A detailed study of the effects of hydrodynamic interaction (HI) and excluded volume (EV), on isolated bead‐rod chains in shear flows, where the “rods” are mimicked by stiff Fraenkel springs is presented. It is observed that the deformation behavior at weak and intermediate shear rates is qualitatively similar to that observed for polymer chains in the absence of EV and HI, while that at high‐shear rates is sensitive to modeling details and chain resolution. Our simulations with varying degrees of resolution reveal universality in chain behavior in the presence of EV (without HI), while the onset of the transition to a compressed chain in the presence of HI (without EV) shifts to higher shear rates with increasing chain resolution. The results also highlight the success of the bead‐spring models in predicting the chain behavior in shear flows in the presence of HI, when the HI parameter is appropriately chosen. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1400–1412, 2014

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