Indranil Saha Dalal

Molecular dynamics simulation of phase transitions in model lung surfactant monolayers.

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.

Multiple regimes of deformation in shearing flow of isolated polymers

Using Brownian dynamics simulations, without excluded volume and hydrodynamic interactions, on single polymer molecules represented by bead-spring models with stiff Fraenkel springs mimicking a single Kuhn step, we find multiple nonlinear regimes of deformation in shear flows that are controlled by the Peclet number (Pe), which is the shear rate times the relaxation time of a Kuhn step. We observe that, for all chain lengths investigated, the average stretch in the flow direction initially increases with increasing Pe, followed by saturation in chain stretch, as observed in previous studies. At even higher Pe, the stretch begins to decrease with increasing shear rate, in accordance with similar simulations of Sendner and Netz, Eur. Phys. J. E 30, 75–81 (2009). At these rates, the trajectories reveal “premature” recoiling of the chain before attaining a fully extended state during the phase of the “tumbling orbit” in which the chain is stretching. An increasing stretch at even higher Pe is characterized by...