67 research outputs found
Concentration fluctuations and phase transitions in coupled modulated bilayers
We consider the formation of finite-size domains in lipid bilayers consisting
of saturated and hybrid lipids. First, we describe a monolayer model that
includes a coupling between a compositional scalar field and a two-dimensional
vectorial order-parameter. Such a coupling yields an effective two-dimensional
microemulsion free-energy for the lipid monolayer, and its characteristic
length of compositional modulations can be considered as the origin of
finite-size domains in biological membranes. Next, we consider a coupled
bilayer composed of two modulated monolayers, and discuss the static and
dynamic properties of concentration fluctuations above the transition
temperature. We also investigate the micro-phase separation below the
transition temperature, and compare the micro-phase separated structures with
statics and dynamics of concentration fluctuations above the transition.Comment: 14 pages, 12 figures, 1 tabl
Electrostatic Interactions of Asymmetrically Charged Membranes
We predict the nature (attractive or repulsive) and range (exponentially
screened or long-range power law) of the electrostatic interactions of
oppositely charged and planar plates as a function of the salt concentration
and surface charge densities (whose absolute magnitudes are not necessarily
equal). An analytical expression for the crossover between attractive and
repulsive pressure is obtained as a function of the salt concentration. This
condition reduces to the high-salt limit of Parsegian and Gingell where the
interaction is exponentially screened and to the zero salt limit of Lau and
Pincus in which the important length scales are the inter-plate separation and
the Gouy-Chapman length. In the regime of low salt and high surface charges we
predict - for any ratio of the charges on the surfaces - that the attractive
pressure is long-ranged as a function of the spacing. The attractive pressure
is related to the decrease in counter-ion concentration as the inter-plate
distance is decreased. Our theory predicts several scaling regimes with
different scaling expressions for the pressure as function of salinity and
surface charge densities. The pressure predictions can be related to surface
force experiments of oppositely charged surfaces that are prepared by coating
one of the mica surfaces with an oppositely charged polyelectrolyte
Phase Transitions in Chemisorbed Systems
Contains reports on two research projects.Joint Services Electronics Program (Contract DAAG29-80-C-0104
Effective zero-thickness model for a conductive membrane driven by an electric field
The behavior of a conductive membrane in a static (DC) electric field is
investigated theoretically. An effective zero-thickness model is constructed
based on a Robin-type boundary condition for the electric potential at the
membrane, originally developed for electrochemical systems. Within such a
framework, corrections to the elastic moduli of the membrane are obtained,
which arise from charge accumulation in the Debye layers due to capacitive
effects and electric currents through the membrane and can lead to an
undulation instability of the membrane. The fluid flow surrounding the membrane
is also calculated, which clarifies issues regarding these flows sharing many
similarities with flows produced by induced charge electro-osmosis (ICEO).
Non-equilibrium steady states of the membrane and of the fluid can be
effectively described by this method. It is both simpler, due to the zero
thickness approximation which is widely used in the literature on fluid
membranes, and more general than previous approaches. The predictions of this
model are compared to recent experiments on supported membranes in an electric
field.Comment: 14 pages, 5 figure
Phase Transitions in Chemisorbed Systems
Contains report on five research projects.Joint Services Electronics Program (Contract DAAG29-83-K-0003
Phase Transitions in Chemisorbed Systems
Contains reports on six research projects.Joint Services Electronics Program (Contract DAAG29-80-C-0104
Interfaces of Modulated Phases
Numerically minimizing a continuous free-energy functional which yields
several modulated phases, we obtain the order-parameter profiles and
interfacial free energies of symmetric and non-symmetric tilt boundaries within
the lamellar phase, and of interfaces between coexisting lamellar, hexagonal,
and disordered phases. Our findings agree well with chevron, omega, and
T-junction tilt-boundary morphologies observed in diblock copolymers and
magnetic garnet films.Comment: 4 page
Limits To The Use Of Threatened Species Lists
Threatened species lists are designed primarily to provide an easily understood qualitative estimate of risk of extinction. Although these estimates of risk can be accurate, the lists have inevitably become linked to several decision-making processes. There are four ways in which such lists are commonly used: to set priorities for resource allocation for species recovery; to inform reserve system design; to constrain development and exploitation; and to report on the state of the environment. The lists were not designed for any one of these purposes, and consequently perform some of them poorly. We discuss why, if and how they should be used to achieve these purposes
- âŠ