800 research outputs found
Flory-Huggins theory for athermal mixtures of hard spheres and larger flexible polymers
A simple analytic theory for mixtures of hard spheres and larger polymers
with excluded volume interactions is developed. The mixture is shown to exhibit
extensive immiscibility. For large polymers with strong excluded volume
interactions, the density of monomers at the critical point for demixing
decreases as one over the square root of the length of the polymer, while the
density of spheres tends to a constant. This is very different to the behaviour
of mixtures of hard spheres and ideal polymers, these mixtures although even
less miscible than those with polymers with excluded volume interactions, have
a much higher polymer density at the critical point of demixing. The theory
applies to the complete range of mixtures of spheres with flexible polymers,
from those with strong excluded volume interactions to ideal polymers.Comment: 9 pages, 4 figure
On the electrical double layer contribution to the interfacial tension of protein crystals
We study the electrical double layer at the interface between a protein
crystal and a salt solution or a dilute solution of protein, and estimate the
double layer's contribution to the interfacial tension of this interface. This
contribution is negative and decreases in magnitude with increasing salt
concentration. We also consider briefly the interaction between a pair of
protein surfaces.Comment: 6 pages, 3 figures, revtex
Does grandparental help mediate the relationship between kin presence and fertility?
Background: Previous research suggests that kin availability may be correlated with reproductive outcomes, but it is not clear that a causal relationship underlies these findings. Further, there is substantial variation in how kin availability is measured. Objective: We attempt to identify whether different measures of kin availability influence how kin affect reproductive outcomes and whether the effect of kin on reproductive outcomes is driven by the help that they provide. Methods: Using data from the Indonesia Family Life Survey (1993, 1997, 2000, 2007), we compare the survival of parents and parents-in-law, their co-residence, geographic proximity, contact frequency, and helping behavior in predicting fertility outcomes, and test a hypothesized causal pathway linking kin availability to reproduction via helping behavior. Results: We find different results if we operationalize parental availability as survival or co-residence, suggesting that these measures cannot be used interchangeably. Receiving help from parents or parents-in-law has a positive effect on progression to birth when women have fewer than three living children. Path analyses show that geographic proximity is associated with contact frequency, which in turn influences helping behavior. Kin help has a positive effect on progression to giving birth for all parental categories, but the effects are strongest for mothers-in-law. Conclusions: In Indonesia, kin availability has a positive effect on fertility only when kin provide help, suggesting that there is a causal relationship between kin availability and fertility which is mediated via the provision of help
Local environmental quality positively predicts breastfeeding in the UK’s Millennium Cohort Study
Background and Objectives: Breastfeeding is an important form of parental investment with clear health benefits. Despite this, rates remain low in the UK; understanding variation can therefore help improve interventions. Life history theory suggests that environmental quality may pattern maternal investment, including breastfeeding. We analyse a nationally representative dataset to test two predictions: (i) higher local environmental quality predicts higher likelihood of breastfeeding initiation and longer duration; (ii) higher socioeconomic status (SES) provides a buffer against the adverse influences of low local environmental quality.Methodology: We ran factor analysis on a wide range of local-level environmental variables. Two summary measures of local environmental quality were generated by this analysis—one ‘objective’ (based on an independent assessor’s neighbourhood scores) and one ‘subjective’ (based on respondent’s scores). We used mixed-effects regression techniques to test our hypotheses.Results: Higher objective, but not subjective, local environmental quality predicts higher likelihood of starting and maintaining breastfeeding over and above individual SES and area-level measures of environmental quality. Higher individual SES is protective, with women from high-income households having relatively high breastfeeding initiation rates and those with high status jobs being more likely to maintain breastfeeding, even in poor environmental conditions.Conclusions and Implications: Environmental quality is often vaguely measured; here we present a thorough investigation of environmental quality at the local level, controlling for individual- and area-level measures. Our findings support a shift in focus away from individual factors and towards altering the landscape of women’s decision making contexts when considering behaviours relevant to public health
Homogeneous nucleation near a second phase transition and Ostwald's step rule
Homogeneous nucleation of the new phase of one transition near a second phase
transition is considered. The system has two phase transitions, we study the
nucleation of the new phase of one of these transitions under conditions such
that we are near or at the second phase transition. The second transition is an
Ising-like transition and lies within the coexistence region of the first
transition. It effects the formation of the new phase in two ways. The first is
by reducing the nucleation barrier to direct nucleation. The second is by the
system undergoing the second transition and transforming to a state in which
the barrier to nucleation is greatly reduced. The second way occurs when the
barrier to undergoing the second phase transition is less than that of the
first phase transition, and is in accordance with Ostwald's rule.Comment: 11 pages, 5 figure
Long-Lived Non-Equilibrium Interstitial-Solid-Solutions in Binary Mixtures
We perform particle resolved experimental studies on the heterogeneous
crystallisation process of two compo- nent mixtures of hard spheres. The
components have a size ratio of 0.39. We compared these with molecular dynamics
simulations of homogenous nucleation. We find for both experiments and
simulations that the final assemblies are interstitial solid solutions, where
the large particles form crystalline close-packed lattices, whereas the small
particles occupy random interstitial sites. This interstitial solution
resembles that found at equilibrium when the size ratios are 0.3 [Filion et
al., Phys. Rev. Lett. 107, 168302 (2011)] and 0.4 [Filion, PhD Thesis, Utrecht
University (2011)]. However, unlike these previous studies, for our system sim-
ulations showed that the small particles are trapped in the octahedral holes of
the ordered structure formed by the large particles, leading to long-lived
non-equilibrium structures in the time scales studied and not the equilibrium
interstitial solutions found earlier. Interestingly, the percentage of small
particles in the crystal formed by the large ones rapidly reaches a maximum of
around 14% for most of the packing fractions tested, unlike previous
predictions where the occupancy of the interstitial sites increases with the
system concentration. Finally, no further hopping of the small particles was
observed
Specific protein-protein binding in many-component mixtures of proteins
Proteins must bind to specific other proteins in vivo in order to function.
The proteins must bind only to one or a few other proteins of the of order a
thousand proteins typically present in vivo. Using a simple model of a protein,
specific binding in many component mixtures is studied. It is found to be a
demanding function in the sense that it demands that the binding sites of the
proteins be encoded by long sequences of bits, and the requirement for specific
binding then strongly constrains these sequences. This is quantified by the
capacity of proteins of a given size (sequence length), which is the maximum
number of specific-binding interactions possible in a mixture. This calculation
of the maximum number possible is in the same spirit as the work of Shannon and
others on the maximum rate of communication through noisy channels.Comment: 13 pages, 3 figures (changes for v2 mainly notational - to be more in
line with notation in information theory literature
Phase separation in mixtures of colloids and long ideal polymer coils
Colloidal suspensions with free polymer coils which are larger than the
colloidal particles are considered. The polymer-colloid interaction is modeled
by an extension of the Asakura-Oosawa model. Phase separation occurs into
dilute and dense fluid phases of colloidal particles when polymer is added. The
critical density of this transition tends to zero as the size of the polymer
coils diverges.Comment: 5 pages, 3 figure
Demixing in a single-peak distributed polydisperse mixture of hard spheres
An analytic derivation of the spinodal of a polydisperse mixture is
presented. It holds for fluids whose excess free energy can be accurately
described by a function of a few moments of the size distribution. It is shown
that one such mixture of hard spheres in the Percus-Yevick approximation never
demixes, despite its size distribution. In the
Boublik-Mansoori-Carnahan-Starling-Leland approximation, though, it demixes for
a sufficiently wide log-normal size distribution. The importance of this result
is twofold: first, this distribution is unimodal, and yet it phase separates;
and second, log-normal size distributions appear in many experimental contexts.
The same phenomenon is shown to occur for the fluid of parallel hard cubes.Comment: 4 pages, 2 figures, needs revtex, multicol, epsfig and amstex style
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