823 research outputs found
Premicellar aggregation of amphiphilic molecules: Aggregate lifetime and polydispersity
A recently introduced thermodynamic model of amphiphilic molecules in
solution has yielded, under certain realistic conditions, a significant
presence of metastable aggregates well below the critical micelle concentration
-- a phenomenon that has been reported also experimentally. The theory is
extended in two directions pertaining to the experimental and technological
relevance of such premicellar aggregates. (a) Combining the thermodynamic model
with reaction rate theory, we calculate the lifetime of the metastable
aggregates. (b) Aggregation number fluctuations are examined. We demonstrate
that, over most of the metastable concentration range, the premicellar
aggregates should have macroscopic lifetimes and small polydispersity.Comment: 7 pages, 2 figure
Micellization in the presence of polyelectrolyte
We present a simple model to study micellization of amphiphiles condensed on
a rodlike polyion. Although the mean field theory leads to a first order
micellization transition for sufficiently strong hydrophobic interactions, the
simulations show that no such thermodynamic phase transition exists. Instead,
the correlations between the condensed amphiphiles can result in a structure
formation very similar to micelles.Comment: 8 pages, 7 figure
Hydrodynamic interaction in quasi-two-dimensional suspensions
Confinement between two parallel surfaces is found, theoretically and
experimentally, to drastically affect the hydrodynamic interaction between
colloid particles, changing the sign of the coupling, its decay with distance
and its concentration dependence. In particular, we show that three-body
effects do not modify the coupling at large distances as would be expected from
hydrodynamic screening.Comment: 8 pages, 2 figure
Retrospective analysis of esophageal imaging features in brachycephalic vs. non-brachycephalic dogs based on videofluoroscopic swallowing studies
Critical swelling of particle-encapsulating vesicles
We consider a ubiquitous scenario where a fluctuating, semipermeable vesicle
is embedded in solution while enclosing a fixed number of solute particles. The
swelling with increasing number of particles or decreasing concentration of the
outer solution exhibits a continuous phase transition from a fluctuating state
to the maximum-volume configuration, whereupon appreciable pressure difference
and surface tension build up. This criticality is unique to
particle-encapsulating vesicles, whose volume and inner pressure both
fluctuate. It implies a universal swelling behavior of such vesicles as they
approach their limiting volume and osmotic lysis.Comment: 4 pages, 1 figur
LIF Measurements of the Cylindrical Hall Thruster Plume
This paper presents a Laser Induced Fluorescence (LIF) investigation of ion velocity profiles in the plume of the Fully Cylindrical Hall Thruster (FCHT) recently developed at the Princeton Plasma Physics Laboratory. These measurements confirm a previously measured ∼ 7% increase in the exhaust velocity when the cathode keeper draws an excess current (overrun mode). Furthermore, it was found that velocity directions in the plume remain relatively unchanged for the cusped and direct magnetic field configuration in both overrun and non-overrun modes. It is shown that the reported plume narrowing in the overrun mode is due to the shift of the acceleration and ionization regions inward toward the anode. This conclusion is supported by the potential profiles extracted from the LIF measurements, which indicate that in the overrun mode a larger fraction of potential fall occurs inside the thruster. Recent probe measurements further substantiate these findings [Raitses et al. , Phys. Plasma 16, 057106 (2009)]. The ratio of the potential fall experienced by ions inside the thruster over the actual potential drop within the thruster increases from 20% in the non-overrun regime with magnets in the direct configuration to 70% in the overrun mode with magnets in the cusped configuration. Magnetic field lines outside the thruster channel are found to be not equipotential, with the degree of equipotentiality gradually increasing at the thruster exit toward the edge
Low-dose glucocorticoid treatment affects multiple aspects of intermediary metabolism in healthy humans: a randomised controlled trial
AIM/HYPOTHESIS: To assess whether low-dose glucocorticoid treatment induces adverse metabolic effects, as is evident for high glucocorticoid doses. METHODS: In a randomised placebo-controlled double-blind (participants and the investigators who performed the studies and assessed the outcomes were blinded) dose-response intervention study, 32 healthy men (age 22 +/- 3 years; BMI 22.4 +/- 1.7 kg/m(2)) were allocated to prednisolone 7.5 mg once daily (n = 12), prednisolone 30 mg once daily (n = 12), or placebo (n = 8) for 2 weeks using block randomisation. Main outcome measures were glucose, lipid and protein metabolism, measured by stable isotopes, before and at 2 weeks of treatment, in the fasted state and during a two-step hyperinsulinaemic clamp conducted in the Clinical Research Unit of the Academic Medical Centre, Amsterdam, the Netherlands RESULTS: Prednisolone, compared with placebo, dose dependently and significantly increased fasting plasma glucose levels, whereas only prednisolone 30 mg increased fasting insulin levels (29 +/- 15 pmol/l). Prednisolone 7.5 mg and prednisolone 30 mg decreased the ability of insulin to suppress endogenous glucose production (by 17 +/- 6% and 46 +/- 7%, respectively, vs placebo). Peripheral glucose uptake was not reduced by prednisolone 7.5 mg, but was decreased by prednisolone 30 mg by 34 +/- 6% (p < 0.0001). Compared with placebo, prednisolone treatment tended to decrease lipolysis in the fasted state (p = 0.062), but both prednisolone 7.5 mg and prednisolone 30 mg decreased insulin-mediated suppression of lipolysis by 11 +/- 5% and 34 +/- 6%, respectively. Finally, prednisolone treatment increased whole-body proteolysis during hyperinsulinaemia, which tended to be driven by prednisolone 30 mg (5 +/- 2%; p = 0.06). No side effects were reported by the study participants. All participants completed the study and were analysed. CONCLUSIONS/INTERPRETATION: Not only at high doses but also at low doses, glucocorticoid therapy impaired intermediary metabolism by interfering with the metabolic actions of insulin on liver and adipose tissue. These data indicate that even low-dose glucocorticoids may impair glucose tolerance when administered chronically. TRIAL REGISTRATION: ISRCTN83991850
Multiple-length-scale elastic instability mimics parametric resonance of nonlinear oscillators
Spatially confined rigid membranes reorganize their morphology in response to
the imposed constraints. A crumpled elastic sheet presents a complex pattern of
random folds focusing the deformation energy while compressing a membrane
resting on a soft foundation creates a regular pattern of sinusoidal wrinkles
with a broad distribution of energy. Here, we study the energy distribution for
highly confined membranes and show the emergence of a new morphological
instability triggered by a period-doubling bifurcation. A periodic
self-organized focalization of the deformation energy is observed provided an
up-down symmetry breaking, induced by the intrinsic nonlinearity of the
elasticity equations, occurs. The physical model, exhibiting an analogy with
parametric resonance in nonlinear oscillator, is a new theoretical toolkit to
understand the morphology of various confined systems, such as coated materials
or living tissues, e.g., wrinkled skin, internal structure of lungs, internal
elastica of an artery, brain convolutions or formation of fingerprints.
Moreover, it opens the way to new kind of microfabrication design of
multiperiodic or chaotic (aperiodic) surface topography via self-organization.Comment: Submitted for publicatio
Topography and instability of monolayers near domain boundaries
We theoretically study the topography of a biphasic surfactant monolayer in
the vicinity of domain boundaries. The differing elastic properties of the two
phases generally lead to a nonflat topography of ``mesas'', where domains of
one phase are elevated with respect to the other phase. The mesas are steep but
low, having heights of up to 10 nm. As the monolayer is laterally compressed,
the mesas develop overhangs and eventually become unstable at a surface tension
of about K(dc)^2 (dc being the difference in spontaneous curvature and K a
bending modulus). In addition, the boundary is found to undergo a
topography-induced rippling instability upon compression, if its line tension
is smaller than about K(dc). The effect of diffuse boundaries on these features
and the topographic behavior near a critical point are also examined. We
discuss the relevance of our findings to several experimental observations
related to surfactant monolayers: (i) small topographic features recently found
near domain boundaries; (ii) folding behavior observed in mixed phospholipid
monolayers and model lung surfactants; (iii) roughening of domain boundaries
seen under lateral compression; (iv) the absence of biphasic structures in
tensionless surfactant films.Comment: 17 pages, 9 figures, using RevTeX and epsf, submitted to Phys Rev
Free energy of colloidal particles at the surface of sessile drops
The influence of finite system size on the free energy of a spherical
particle floating at the surface of a sessile droplet is studied both
analytically and numerically. In the special case that the contact angle at the
substrate equals a capillary analogue of the method of images is
applied in order to calculate small deformations of the droplet shape if an
external force is applied to the particle. The type of boundary conditions for
the droplet shape at the substrate determines the sign of the capillary
monopole associated with the image particle. Therefore, the free energy of the
particle, which is proportional to the interaction energy of the original
particle with its image, can be of either sign, too. The analytic solutions,
given by the Green's function of the capillary equation, are constructed such
that the condition of the forces acting on the droplet being balanced and of
the volume constraint are fulfilled. Besides the known phenomena of attraction
of a particle to a free contact line and repulsion from a pinned one, we
observe a local free energy minimum for the particle being located at the drop
apex or at an intermediate angle, respectively. This peculiarity can be traced
back to a non-monotonic behavior of the Green's function, which reflects the
interplay between the deformations of the droplet shape and the volume
constraint.Comment: 24 pages, 19 figure
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