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Curvature defects in lamellar phases of amphiphile–water systems (1991)

Bagdassarian, Carey K. ; Roux, Didier ; Ben-Shaul, Avinoam ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 94 (1991), S. 3030-3041

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Within the framework of two complementary models, we show that the densities and patterns of defects in amphiphile–water systems with lamellar organization are coupled to the strength of the bilayer–bilayer interactions and hence to the overall surfactant concentration. We consider defects which introduce curvature (i.e., larger head-group area per molecule) while preserving the integrity of stacked bilayers at surfactant volume fractions of several tenths. These features are favored if the molecules comprising the lamellae are preferentially packed with a nonplanar aggregate–water interface: curvature defects lower the local free energy in systems constrained by aggregate–aggregate interactions to lamellar geometry. As the amphiphile volume fraction is increased—and the bilayer–bilayer spacing thereby decreased—we predict phase transitions between lamellar phases of different defect patterns on the bilayer surface, with concurrent decrease in the defect area fraction per bilayer. Specifically, there is a progression from a stripe-like pattern of parallel channels to a random network of line defects to a pore phase, with the latter appearing at the highest amphiphile concentrations but characterized by the lowest density of defects. Connection is made with experimental work which has recently suggested various departures from classical lamellar structure.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.459826

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Computer simulations of the growth of breath figures (1988)

Fritter, Daniela ; Knobler, Charles M. ; Roux, Didier ; [et al.]

Springer

Journal of statistical physics 52 (1988), S. 1447-1459

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ISSN: 1572-9613

Keywords: Droplet growth ; coalescence ; simulation ; surfaces ; patterns

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract We describe computer simulations of the growth of breath figures, the patterns formed when droplets condense on a cold surface. The focus is on the coalescence of droplets, which is an important growth mechanism, and the conditions for self-similar patterns, which are experimentally observed. It is assumed that individual droplets grow according to a power law; droplets that touch coalesce instantly and are replaced by a new droplet at the center of gravity of the coalescing pair. The average droplet radius, distribution of droplet sizes, surface coverage, and radial distribution function are determined as a function of the time for a variety of initial coverages and polydispersities. These quantities are compared to those determined by experiment, and our simple model is found to be in good accord with the observed behavior. It is observed that the process of coalescence induces spatial correlation between droplets.