3,178 research outputs found
Dynamics of Spreading of Small Droplets of Chainlike Molecules on Surfaces
Dynamics of spreading of small droplets on surfaces has been studied by the
molecular dynamics method. Simulations have been performed for mixtures of
solvent and dimer, and solvent and tetramer droplets. For solvent particles and
dimers, layering occurs leading to stepped droplet shapes. For tetramers such
shapes occur for relatively deep and strong surface potentials only. For wider
and more shallow potentials, more rapid spreading and rounded droplet shapes
occur. These results are in accordance with experimental data on small non -
volatile polymer droplets. PACS numbers: 68.10Gw, 05.70.Ln, 61.20.Ja, 68.45GdComment: to appear in Europhys. Letters (1994), Latex, 12 page
NIEL Dose Dependence for Solar Cells Irradiated with Electrons and Protons
The investigation of solar cells degradation and the prediction of its
end-of-life performance is of primary importance in the preparation of a space
mission. In the present work, we investigate the reduction of solar-cells'
maximum power resulting from irradiations with electrons and protons. Both GaAs
single junction and GaInP/GaAs/Ge triple junction solar cells were studied. The
results obtained indicate how i) the dominant radiation damaging mechanism is
due to atomic displacements, ii) the relative maximum power degradation is
almost independent of the type of incoming particle, i.e., iii) to a first
approximation, the fitted semi-empirical function expressing the decrease of
maximum power depends only on the absorbed NIEL dose, and iv) the actual
displacement threshold energy value (Ed=21 eV) accounts for annealing
treatments, mostly due to self-annealing induced effects. Thus, for a given
type of solar cell, a unique maximum power degradation curve can be determined
as a function of the absorbed NIEL dose. The latter expression allows one to
predict the performance of those solar cells in space radiation environment.Comment: To appear on the Proceedings of the 13th ICATPP Conference on
Astroparticle, Particle, Space Physics and Detectors for Physics
Applications, Villa Olmo (Como, Italy), 23--27 October, 2013, to be published
by World Scientific (Singapore
Associations between allelic variation at major histocompatibility complex I and inbreeding and fitness components in the house sparrow (Passer domesticus)
Abstract. Major histocompatibility complex (MHC) is a widely studied multigene family that is found in all vertebrates. The main role of MHC genes is in the adaptive immune system, but they are also known to have a role in other processes, for example in mate choice. In this thesis the main goal is to find out how inbreeding affects the number of MHC class I alleles (exon 3 sequences) in house sparrow (Passer domesticus) and whether individual allelic variation at MHC is associated to components of individual fitness. The data used in this thesis was collected between the years 1998 and 2013 in six house sparrow populations in an archipelago of northern Norway. Associations between the nucleotide and amino acid sequence numbers and inbreeding, lifespan and lifetime reproductive success were studied using regression analyses. In total of 127 unique nucleotide and 113 unique amino acid sequences (i.e., alleles) were found at MHC class I exon 3, with on average 12.6 (SD ± 3.5) nucleotide and 11.4 (SD ± 2.9) amino acid alleles per individual. Positive associations were found between the number of nucleotide alleles per individual and lifetime reproductive success, but there was only weak evidence for the association between number of amino acid alleles and reproductive success. There was no evidence for associations between the individual numbers of either type of alleles and inbreeding or lifespan. The positive association found between the number of MHC alleles and reproductive success could be explained by ecological traits that may be positively affected by a higher number of MHC alleles; for example, the ability to defend a wider range of pathogens and thus produce more and healthier offspring. It is also possible that there could be associations between MHC alleles and other fitness-components that are not detected using my analysis methods, for example if specific MHC alleles are advantageous or an intermediate number of alleles is the most beneficial
Molecular ordering of precursor films during spreading of tiny liquid droplets
In this work we address a novel feature of spreading dynamics of tiny liquid
droplets on solid surfaces, namely the case where the ends of the molecules
feel different interactions to the surface. We consider a simple model of
dimers and short chain--like molecules which cannot form chemical bonds with
the surface. We study the spreading dynamics by Molecular Dynamics techniques.
In particular, we examine the microscopic structure of the time--dependent
precursor film and find that in some cases it can exhibit a high degree of
local order. This order persists even for flexible chains. Our results suggest
the possibility of extracting information about molecular interactions from the
structure of the precursor film.Comment: 4 pages, revtex, no figures, complete file available from
ftp://rock.helsinki.fi/pub/preprints/tft/ or at
http://www.physics.helsinki.fi/tft/tft_preprints.html (to appear in Phys.
Rev. E Rapid Comm.
Quantized evolution of the plasmonic response in a stretched nanorod
Quantum aspects, such as electron tunneling between closely separated
metallic nanoparticles, are crucial for understanding the plasmonic response of
nanoscale systems. We explore quantum effects on the response of the
conductively coupled metallic nanoparticle dimer. This is realized by
stretching a nanorod, which leads to the formation of a narrowing atomic
contact between the two nanorod ends. Based on first-principles time-dependent
density-functional-theory calculations, we find a discontinuous evolution of
the plasmonic response as the nanorod is stretched. This is especially
pronounced for the intensity of the main charge-transfer plasmon mode. We show
the correlation between the observed discontinuities and the discrete nature of
the conduction channels supported by the formed atomic-sized junction.Comment: Main text: 6 pages, 2 figures; Supplemental Material: 5 pages, 4
figure
Estimating seasonal variations in cloud droplet number concentration over the boreal forest from satellite observations
Seasonal variations in cloud droplet number concentration (NCD) in low-level stratiform clouds over the boreal forest are estimated from MODIS observations of cloud optical and microphysical properties, using a sub-adiabatic cloud model to interpret vertical profiles of cloud properties. An uncertainty analysis of the cloud model is included to reveal the main sensitivities of the cloud model. We compared the seasonal cycle in NCD, obtained using 9 yr of satellite data, to surface concentrations of potential cloud activating aerosols, measured at the SMEAR II station at Hyytiälä in Finland. The results show that NCD and cloud condensation nuclei (CCN) concentrations have no clear correlation at seasonal time scale. The fraction of aerosols that actually activate as cloud droplet decreases sharply with increasing aerosol concentrations. Furthermore, information on the stability of the atmosphere shows that low NCD is linked to stable atmospheric conditions. Combining these findings leads to the conclusion that cloud droplet activation for the studied clouds over the boreal forest is limited by convection. Our results suggest that it is important to take the strength of convection into account when studying the influence of aerosols from the boreal forest on cloud formation, although they do not rule out the possibility that aerosols from the boreal forest affect other types of clouds with a closer coupling to the surfac
Internal Migration and Regional Population Dynamics in Europe: Finland Case Study
Both natural increase and internal migration have played roles in the shaping of population distribution of Finland since 1900. Far reaching recent changes in the economy have brought about massive shift of jobs from agriculture to manufacturing and services. As a result people have relocated from rural to urban areas. Both natural change and net migration have distinct geographical patterns, resulting in serious depopulation in remote areas in the east and north of the country. Internal migration benefits the south, the west, coastal areas, urban agglomerations and suburban areas. International migration is a marginal phenomenon in Finland and has little impact on population dynamics. Net migration losses in the past were offset by high natural increase and in recent decades Finnish emigrants have returned.
Urban concentration is a dominant feature of the Finnish migration system. At the subregional level, suburbanisation is visible, but is not as strong as in the overcrowded metropolises of Western Europe. The relationships between migration and size of municipality, migration and population density and migration and urban/rural class of municipalities show that the process of concentration is the strongest force at work in shifting people to urban agglomerations and their suburban rings.
Regional patterns of migration show strong transfers of population from north and east to south and to lesser extent to west of the country. The Baltic Sea coast has a strong attraction to migrants. Migration is sex-selective, with a much higher propensity of females to leave remote and rural areas and migrate to urban centres and the southern part of the country. The result is a significant gender imbalance: a deficiency of females in rural areas and in the north and east of the country and a surplus in urban and semi-urban areas. However, the economic indicator unemployment has a rather weak and imprecise effect on migrants
Optical application and measurement of torque on microparticles of isotropic nonabsorbing material
We show how it is possible to controllably rotate or align microscopic
particles of isotropic nonabsorbing material in a TEM00 Gaussian beam trap,
with simultaneous measurement of the applied torque using purely optical means.
This is a simple and general method of rotation, requiring only that the
particle is elongated along one direction. Thus, this method can be used to
rotate or align a wide range of naturally occurring particles. The ability to
measure the applied torque enables the use of this method as a quantitative
tool--the rotational equivalent of optical tweezers based force measurement. As
well as being of particular value for the rotation of biological specimens,
this method is also suitable for the development of optically-driven
micromachines.Comment: 8 pages, 6 figure
Kohn-Sham decomposition in real-time time-dependent density-functional theory: An efficient tool for analyzing plasmonic excitations
The real-time-propagation formulation of time-dependent density-functional
theory (RT-TDDFT) is an efficient method for modeling the optical response of
molecules and nanoparticles. Compared to the widely adopted linear-response
TDDFT approaches based on, e.g., the Casida equations, RT-TDDFT appears,
however, lacking efficient analysis methods. This applies in particular to a
decomposition of the response in the basis of the underlying single-electron
states. In this work, we overcome this limitation by developing an analysis
method for obtaining the Kohn-Sham electron-hole decomposition in RT-TDDFT. We
demonstrate the equivalence between the developed method and the Casida
approach by a benchmark on small benzene derivatives. Then, we use the method
for analyzing the plasmonic response of icosahedral silver nanoparticles up to
Ag. Based on the analysis, we conclude that in small nanoparticles
individual single-electron transitions can split the plasmon into multiple
resonances due to strong single-electron-plasmon coupling whereas in larger
nanoparticles a distinct plasmon resonance is formed.Comment: 11 pages, 3 figure
Nanoplasmonics simulations at the basis set limit through completeness-optimized, local numerical basis sets
We present an approach for generating local numerical basis sets of improving
accuracy for first-principles nanoplasmonics simulations within time-dependent
density functional theory. The method is demonstrated for copper, silver, and
gold nanoparticles that are of experimental interest but computationally
demanding due to the semi-core d-electrons that affect their plasmonic
response. The basis sets are constructed by augmenting numerical atomic orbital
basis sets by truncated Gaussian-type orbitals generated by the
completeness-optimization scheme, which is applied to the photoabsorption
spectra of homoatomic metal atom dimers. We obtain basis sets of improving
accuracy up to the complete basis set limit and demonstrate that the
performance of the basis sets transfers to simulations of larger nanoparticles
and nanoalloys as well as to calculations with various exchange-correlation
functionals. This work promotes the use of the local basis set approach of
controllable accuracy in first-principles nanoplasmonics simulations and
beyond.Comment: 11 pages, 6 figure
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