11,210 research outputs found
Spatial gene drives and pushed genetic waves
Gene drives have the potential to rapidly replace a harmful wild-type allele
with a gene drive allele engineered to have desired functionalities. However,
an accidental or premature release of a gene drive construct to the natural
environment could damage an ecosystem irreversibly. Thus, it is important to
understand the spatiotemporal consequences of the super-Mendelian population
genetics prior to potential applications. Here, we employ a reaction-diffusion
model for sexually reproducing diploid organisms to study how a locally
introduced gene drive allele spreads to replace the wild-type allele, even
though it possesses a selective disadvantage . Using methods developed by
N. Barton and collaborators, we show that socially responsible gene drives
require , a rather narrow range. In this "pushed wave" regime, the
spatial spreading of gene drives will be initiated only when the initial
frequency distribution is above a threshold profile called "critical
propagule", which acts as a safeguard against accidental release. We also study
how the spatial spread of the pushed wave can be stopped by making gene drives
uniquely vulnerable ("sensitizing drive") in a way that is harmless for a
wild-type allele. Finally, we show that appropriately sensitized drives in two
dimensions can be stopped even by imperfect barriers perforated by a series of
gaps
Wetting morphologies on an array of fibers of different radii
We investigate the equilibrium morphology of a finite volume of liquid placed
on two parallel rigid fibers of different radii. As observed for identical
radii fibers, the liquid is either in a column morphology or adopts a drop
shape depending on the inter-fiber distance. However the cross-sectional area
and the critical inter-fiber distance at which the transition occurs are both
modified by the polydispersity of the fibers. Using energy considerations, we
analytically predict the critical inter-fiber distance corresponding to the
transition between the column and the drop morphologies occurs. This distance
depends both on the radii of the fibers and on the contact angle of the liquid.
We perform experiments using a perfectly wetting liquid on two parallel nylon
fibers: the results are in good agreement with our analytical model. The
morphology of the capillary bridges between fibers of different radii is
relevant to the modeling of large arrays of polydisperse fibers
Corrugated interfaces in multiphase core-annular flow
Microfluidic devices can be used to produce highly controlled and
monodisperse double or multiple emulsions. The presence of inner drops inside a
jet of the middle phase introduces deformations in the jet, which leads to
breakup into monodisperse double emulsions. However, the ability to generate
double emulsions can be compromised when the interfacial tension between the
middle and outer phases is low, leading to flow with high capillary and Weber
numbers. In this case, the interface between the fluids is initially deformed
by the inner drops but the jet does not break into drops. Instead, the jet
becomes highly corrugated, which prevents formation of controlled double
emulsions. We show using numerical calculations that the corrugations are
caused by the inner drops perturbing the interface and the perturbations are
then advected by the flow into complex shapes
Quasi-Chemical and Structural Analysis of Polarizable Anion Hydration
Quasi-chemical theory is utilized to analyze the roles of solute polarization
and size in determining the structure and thermodynamics of bulk anion
hydration for the Hofmeister series Cl, Br, and I. Excellent
agreement with experiment is obtained for whole salt hydration free energies
using the polarizable AMOEBA force field. The quasi-chemical approach exactly
partitions the solvation free energy into inner-shell, outer-shell packing, and
outer-shell long-ranged contributions by means of a hard-sphere condition.
Small conditioning radii, even well inside the first maximum of the
ion-water(oxygen) radial distribution function, result in Gaussian behavior for
the long-ranged contribution that dominates the ion hydration free energy. The
spatial partitioning allows for a mean-field treatment of the long-ranged
contribution, leading to a natural division into first-order electrostatic,
induction, and van der Waals terms. The induction piece exhibits the strongest
ion polarizability dependence, while the larger-magnitude first-order
electrostatic piece yields an opposing but weaker polarizability dependence. In
addition, a structural analysis is performed to examine the solvation
anisotropy around the anions. As opposed to the hydration free energies, the
solvation anisotropy depends more on ion polarizability than on ion size:
increased polarizability leads to increased anisotropy. The water dipole
moments near the ion are similar in magnitude to bulk water, while the ion
dipole moments are found to be significantly larger than those observed in
quantum mechanical studies. Possible impacts of the observed over-polarization
of the ions on simulated anion surface segregation are discussed.Comment: slight revision, in press at J. Chem. Phy
National Time Accounting: The Currency of Life
This monograph proposes a new approach for measuring features of society’s subjective well-being, based on time allocation and affective experience. We call this approach National Time Accounting (NTA). National Time Accounting is a set of methods for measuring, comparing and analyzing how people spend and experience their time -- across countries, over historical time, or between groups of people within a country at a given time. The approach is based on evaluated time use, or the flow of emotional experience during daily activities. After reviewing evidence on the validity of subjective well-being measures, we present and evaluate diary-based survey techniques designed to measure individuals’ emotional experiences and time use. We illustrate NTA with: (1) a new cross-sectional survey on time use and emotional experience for a representative sample of 4,000 Americans; (2) historical data on the amount of time devoted to various activities in the United States since 1965; and (3) a comparison of time use and wellbeing in the United States and France. In our applications, we focus mainly on the Uindex, a measure of the percentage of time that people spend in an unpleasant state, defined as an instance in which the most intense emotion is a negative one. The U-index helps to overcome some of the limitations of interpersonal comparisons of subjective well-being. National Time Accounting strikes us as a fertile area for future research because of advances in subjective measurement and because time use data are now regularly collected in many countries.
Paper Session I-B - Spacehab: Augmenting the Shuttle\u27s Middeck Capacity
SPACEHAB, Inc. is developing a pressurized laboratory which fits in the forward portion of the Space Shuttle payload bay and connects to the crew compartment through an airlock. SPACEHAB\u27s standard middeck type lockers offer crew-tended access to the microgravity environment for experimentation, technology development, and small-scale production. NASA\u27s Office of Commercial Programs has contracted with SPACEHAB to lease two-thirds of the SPACEHAB\u27s module capacity over the first six missions for commercial research initiatives. Use of SPACEHAB modules also offers an economical intermediate step prior to making a full commitment for commercial use of Space Station Freedom
Ab initio investigation of intermolecular interactions in solid benzene
A computational strategy for the evaluation of the crystal lattice constants
and cohesive energy of the weakly bound molecular solids is proposed. The
strategy is based on the high level ab initio coupled-cluster determination of
the pairwise additive contribution to the interaction energy. The
zero-point-energy correction and non-additive contributions to the interaction
energy are treated using density functional methods. The experimental crystal
lattice constants of the solid benzene are reproduced, and the value of 480
meV/molecule is calculated for its cohesive energy
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