188 research outputs found
Polycontinuous geometries for inverse lipid phases with more than two aqueous network domains
Inverse bicontinuous cubic phases with two aqueous network domains separated by a smooth bilayer are firmly established as equilibrium phases in lipid/water systems. The purpose of this article is to highlight the generalisations of these bicontinuous geometries to polycontinuous geometries, which could be realised as lipid mesophases with three or more network-like aqueous domains separated by a branched bilayer. An analysis of structural homogeneity in terms of bilayer width variations reveals that ordered polycontinuous geometries are likely candidates for lipid mesophase structures, with similar chain packing characteristics to the inverse micellar phases (that once were believed not to exist due to high packing frustration). The average molecular shape required by global geometry to form these multi-network phases is quantified by the surfactant shape parameter, v/(al); we find that it adopts values close to those of the known lipid phases. We specifically analyse the 3etc(187 193) structure of hexagonal symmetry P63 /mcm with three aqueous domains, the 3dia(24 220) structure of cubic symmetry I 3d composed of three distorted diamond networks, the cubic chiral 4srs(24 208) with cubic symmetry P4232 and the achiral 4srs(5 133) structure of symmetry P42/nbc, each consisting of four intergrown undistorted copies of the srs net (the same net as in the QGII gyroid phase). Structural homogeneity is analysed by a medial surface approach assuming that the head-group interfaces are constant mean curvature surfaces. To facilitate future experimental identification, we provide simulated SAXS scattering patterns that, for the 4srs(24 208) and 3dia(24 220) structures, bear remarkable similarity to those of bicontinuous QGII-gyroid and QDII-diamond phases, with comparable lattice parameters and only a single peak that cannot be indexed to the well-established structures. While polycontinuous lipid phases have, to date, not been reported, the likelihood of their formation is further indicated by the reported observation of a solid tricontinuous mesoporous silicate structure, termed IBN-9, which formed in the presence of surfactants [Han et al., Nat. Chem., 2009, 1, 123]
Wall roughness induces asymptotic ultimate turbulence
Turbulence is omnipresent in Nature and technology, governing the transport
of heat, mass, and momentum on multiple scales. For real-world applications of
wall-bounded turbulence, the underlying surfaces are virtually always rough;
yet characterizing and understanding the effects of wall roughness for
turbulence remains a challenge, especially for rotating and thermally driven
turbulence. By combining extensive experiments and numerical simulations, here,
taking as example the paradigmatic Taylor-Couette system (the closed flow
between two independently rotating coaxial cylinders), we show how wall
roughness greatly enhances the overall transport properties and the
corresponding scaling exponents. If only one of the walls is rough, we reveal
that the bulk velocity is slaved to the rough side, due to the much stronger
coupling to that wall by the detaching flow structures. If both walls are
rough, the viscosity dependence is thoroughly eliminated in the boundary layers
and we thus achieve asymptotic ultimate turbulence, i.e. the upper limit of
transport, whose existence had been predicted by Robert Kraichnan in 1962
(Phys. Fluids {\bf 5}, 1374 (1962)) and in which the scalings laws can be
extrapolated to arbitrarily large Reynolds numbers
Numerical observation of non-axisymmetric vesicles in fluid membranes
By means of Surface Evolver (Exp. Math,1,141 1992), a software package of
brute-force energy minimization over a triangulated surface developed by the
geometry center of University of Minnesota, we have numerically searched the
non-axisymmetric shapes under the Helfrich spontaneous curvature (SC) energy
model. We show for the first time there are abundant mechanically stable
non-axisymmetric vesicles in SC model, including regular ones with intrinsic
geometric symmetry and complex irregular ones. We report in this paper several
interesting shapes including a corniculate shape with six corns, a
quadri-concave shape, a shape resembling sickle cells, and a shape resembling
acanthocytes. As far as we know, these shapes have not been theoretically
obtained by any curvature model before. In addition, the role of the
spontaneous curvature in the formation of irregular crenated vesicles has been
studied. The results shows a positive spontaneous curvature may be a necessary
condition to keep an irregular crenated shape being mechanically stable.Comment: RevTex, 14 pages. A hard copy of 8 figures is available on reques
Palaeozoic giant dragonfies were hawker predators
The largest insects to have ever lived were the giant meganeurids of the Late Palaeozoic, ancient
stem relatives of our modern dragonfies. With wingspans up to 71cm, these iconic insects have been
the subject of varied documentaries on Palaeozoic life, depicting them as patrolling for prey through
coal swamp forests amid giant lycopsids, and cordaites. Such reconstructions are speculative as few
defnitive details of giant dragonfy biology are known. Most specimens of giant dragonfies are known
from wings or isolated elements, but Meganeurites gracilipes preserves critical body structures, most
notably those of the head. Here we show that it is unlikely it thrived in densely forested environments
where its elongate wings would have become easily damaged. Instead, the species lived in more
open habitats and possessed greatly enlarged compound eyes. These were dorsally hypertrophied, a
specialization for long-distance vision above the animal in fight, a trait convergent with modern hawker
dragonfies. Sturdy mandibles with acute teeth, strong spines on tibiae and tarsi, and a pronounced
thoracic skewness are identical to those specializations used by dragonfies in capturing prey while
in fight. The Palaeozoic Odonatoptera thus exhibited considerable morphological specializations
associated with behaviours attributable to ‘hawkers’ or ‘perchers’ among extant Odonata.This work benefted from a grant of the French ‘Agence Nationale de la Recherche’ via the program ‘Investissements d’avenir’ (ANR-11-INBS-0004-RECOLNAT)JP and MP gratefully acknowledge research support from the Grant Agency of the Czech Republic No. 18-03118 SThe work of MSE was supported by US National Science Foundation grant DEB-114416
Photonic band gaps in materials with triply periodic surfaces and related tubular structures
We calculate the photonic band gap of triply periodic bicontinuous cubic
structures and of tubular structures constructed from the skeletal graphs of
triply periodic minimal surfaces. The effect of the symmetry and topology of
the periodic dielectric structures on the existence and the characteristics of
the gaps is discussed. We find that the C(I2-Y**) structure with Ia3d symmetry,
a symmetry which is often seen in experimentally realized bicontinuous
structures, has a photonic band gap with interesting characteristics. For a
dielectric contrast of 11.9 the largest gap is approximately 20% for a volume
fraction of the high dielectric material of 25%. The midgap frequency is a
factor of 1.5 higher than the one for the (tubular) D and G structures
Numerical simulations of complex fluid-fluid interface dynamics
Interfaces between two fluids are ubiquitous and of special importance for
industrial applications, e.g., stabilisation of emulsions. The dynamics of
fluid-fluid interfaces is difficult to study because these interfaces are
usually deformable and their shapes are not known a priori. Since experiments
do not provide access to all observables of interest, computer simulations pose
attractive alternatives to gain insight into the physics of interfaces. In the
present article, we restrict ourselves to systems with dimensions comparable to
the lateral interface extensions. We provide a critical discussion of three
numerical schemes coupled to the lattice Boltzmann method as a solver for the
hydrodynamics of the problem: (a) the immersed boundary method for the
simulation of vesicles and capsules, the Shan-Chen pseudopotential approach for
multi-component fluids in combination with (b) an additional
advection-diffusion component for surfactant modelling and (c) a molecular
dynamics algorithm for the simulation of nanoparticles acting as emulsifiers.Comment: 24 pages, 12 figure
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