1,623 research outputs found
Registered and antiregistered phase separation of mixed amphiphilic bilayers
We derive a mean-field free energy for the phase behaviour of coupled bilayer
leaflets, which is implicated in cellular processes and important to the design
of artificial membranes. Our model accounts for amphiphile-level structural
features, particularly hydrophobic mismatch, which promotes antiregistration
(AR), in competition with the `direct' trans-midplane coupling usually studied,
promoting registration (R). We show that the phase diagram of coupled leaflets
allows multiple \textit{metastable} coexistences, then illustrate the kinetic
implications with a detailed study of a bilayer of equimolar overall
composition. For approximate parameters estimated to apply to phospholipids,
equilibrium coexistence is typically registered, but metastable antiregistered
phases can be kinetically favoured by hydrophobic mismatch. Thus a bilayer in
the spinodal region can require nucleation to equilibrate, in a novel
manifestation of Ostwald's `rule of stages'. Our results provide a framework
for understanding disparate existing observations, elucidating a subtle
competition of couplings, and a key role for phase transition kinetics in
bilayer phase behaviour.Comment: Final authors' version. Important typo in Eq. A24 corrected. To
appear in Biophysical Journa
A Minimal Model for Vorticity and Gradient Banding in Complex Fluids
A general phenomenological reaction-diffusion model for flow-induced phase
transitions in complex fluids is presented. The model consists of an equation
of motion for a nonconserved composition variable, coupled to a Newtonian
stress relations for the reactant and product species. Multivalued reaction
terms allow for different homogeneous phases to coexist with each other,
resulting in banded composition and shear rate profiles. The one-dimensional
equation of motion is evolved from a random initial state to its final
steady-state. We find that the system chooses banded states over homogeneous
states, depending on the shape of the stress constitutive curve and the
magnitude of the diffusion coefficient. Banding in the flow gradient direction
under shear rate control is observed for shear-thinning transitions, while
banding in the vorticity direction under stress control is observed for
shear-thickening transitions.Comment: 11 pages, submitted to Eur Phys J
A non-monotonic constitutive model is not necessary to obtain shear banding phenomena in entangled polymer solutions
In 1975 Doi and Edwards predicted that entangled polymer melts and solutions
can have a constitutive instability, signified by a decreasing stress for shear
rates greater than the inverse of the reptation time. Experiments did not
support this, and more sophisticated theories incorporated Marrucci's idea
(1996) of removing constraints by advection; this produced a monotonically
increasing stress and thus stable constitutive behavior. Recent experiments
have suggested that entangled polymer solutions may possess a constitutive
instability after all, and have led some workers to question the validity of
existing constitutive models. In this Letter we use a simple modern
constitutive model for entangled polymers, the non-stretching Rolie-Poly model
with an added solvent viscosity, and show that (1) instability and shear
banding is captured within this simple class of models; (2) shear banding
phenomena is observable for weakly stable fluids in flow geometries that impose
a sufficiently inhomogeneous total shear stress; (3) transient phenomena can
possess inhomogeneities that resemble shear banding, even for weakly stable
fluids. Many of these results are model-independent.Comment: 5 figure
Method for Computing Short-Range Forces between Solid-Liquid Interfaces Driving Grain Boundary Premelting
We present a molecular dynamics based method for computing accurately
short-range structural forces resulting from the overlap of spatially diffuse
solid-liquid interfaces at wetted grain boundaries close to the melting point.
The method is based on monitoring the fluctuations of the liquid layer width at
different temperatures to extract the excess interfacial free-energy as a
function of this width. The method is illustrated for a high energy Sigma 9
twist boundary in pure Ni. The short-range repulsion driving premelting is
found to be dominant in comparison to long-range dispersion and entropic forces
and consistent with previous experimental findings that nanometer-scale layer
widths may only be observed very close to the melting point.Comment: 5 pages, four figure
A Comparative Study of Knots of Star Formation in Interacting vs. Spiral Galaxies
Interacting galaxies are known to have higher global rates of star formation
on average than normal galaxies, relative to their stellar masses. Using UV and
IR photometry combined with new and published H-alpha images, we have compared
the star formation rates of ~700 star forming complexes in 46 nearby
interacting galaxy pairs with those of regions in 39 normal spiral galaxies.
The interacting galaxies have proportionally more regions with high star
formation rates than the spirals. The most extreme regions in the interacting
systems lie at the intersections of spiral/tidal structures, where gas is
expected to pile up and trigger star formation. Published Hubble Telescope
images show unusually large and luminous star clusters in the highest
luminosity regions. The star formation rates of the clumps correlate with
measures of the dust attenuation, consistent with the idea that regions with
more interstellar gas have more star formation. For the clumps with the highest
star formation rates, the apparent dust attenuation is consistent with the
Calzetti starburst dust attenuation law. This suggests that the high luminosity
regions are dominated by a central group of young stars surrounded by a shell
of clumpy interstellar gas. In contrast, the lower luminosity clumps are bright
in the UV relative to H-alpha, suggesting either a high differential
attenuation between the ionized gas and the stars, or a post-starburst
population bright in the UV but faded in H-alpha. The fraction of the global
light of the galaxies in the clumps is higher on average for the interacting
galaxies than for the spirals. Thus the star forming regions in interacting
galaxies are more luminous, dustier, or younger on average.Comment: Astronomical Journal, in pres
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