4,759 research outputs found
Characterization of the Noise in Secondary Ion Mass Spectrometry Depth Profiles
The noise in the depth profiles of secondary ion mass spectrometry (SIMS) is
studied using different samples under various experimental conditions. Despite
the noise contributions from various parts of the dynamic SIMS process, its
overall character agrees very well with the Poissonian rather than the Gaussian
distribution in all circumstances. The Poissonian relation between the measured
mean-square error (MSE) and mean can be used to describe our data in the range
of four orders. The departure from this relation at high counts is analyzed and
found to be due to the saturation of the channeltron used. Once saturated, the
detector was found to exhibit hysteresis between rising and falling input flux
and output counts.Comment: 14 pages, 4 postscript figures, to appear on J. Appl. Phy
Influence of particle composition and thermal cycling on bijel formation
Colloidal particles with appropriate wetting properties can become very
strongly trapped at an interface between two immiscible fluids. We have
harnessed this phenomenon to create a new class of soft materials with
intriguing and potentially useful characteristics. The material is known as a
bijel: bicontinuous interfacially-jammed emulsion gel. It is a
colloid-stabilized emulsion with fluid-bicontinuous domains. The potential to
create these gels was first predicted using computer simulations.
Experimentally we use mixtures of water and 2,6-lutidine at the composition for
which the system undergoes a critical demixing transition on warming. Colloidal
silica, with appropriate surface chemistry, is dispersed while the system is in
the single-fluid phase; the composite sample is then slowly warmed well beyond
the critical temperature. The liquids phase separate via spinodal decomposition
and the particles become swept up on the newly created interfaces. As the
domains coarsen the interfacial area decreases and the particles eventually
become jammed together. The resulting structures have a significant yield
stress and are stable for many months. Here we begin to explore the complex
wetting properties of fluorescently-tagged silica surfaces in water-lutidine
mixtures, showing how they can be tuned to allow bijel creation. Additionally
we demonstrate how the particle properties change with time while they are
immersed in the solvents.Comment: Proceedings of the 7th Liquid Matter Conference, held in Lund
(Sweden) in June 200
Emulsification in binary liquids containing colloidal particles: a structure-factor analysis
We present a quantitative confocal-microscopy study of the transient and
final microstructure of particle-stabilised emulsions formed via demixing in a
binary liquid. To this end, we have developed an image-analysis method that
relies on structure factors obtained from discrete Fourier transforms of
individual frames in confocal image sequences. Radially averaging the squared
modulus of these Fourier transforms before peak fitting allows extraction of
dominant length scales over the entire temperature range of the quench. Our
procedure even yields information just after droplet nucleation, when the
(fluorescence) contrast between the two separating phases is scarcely
discernable in the images. We find that our emulsions are stabilised on
experimental time scales by interfacial particles and that they are likely to
have bimodal droplet-size distributions. We attribute the latter to coalescence
together with creaming being the main coarsening mechanism during the late
stages of emulsification and we support this claim with (direct)
confocal-microscopy observations. In addition, our results imply that the
observed droplets emerge from particle-promoted nucleation, possibly followed
by a free-growth regime. Finally, we argue that creaming strongly affects
droplet growth during the early stages of emulsification. Future investigations
could clarify the link between quench conditions and resulting microstructure,
paving the way for tailor-made particle-stabilised emulsions from binary
liquids.Comment: http://iopscience.iop.org/0953-8984/22/45/455102
Spin-Correlation Coefficients and Phase-Shift Analysis for p+He Elastic Scattering
Angular Distributions for the target spin-dependent observables A,
A, and A have been measured using polarized proton beams at
several energies between 2 and 6 MeV and a spin-exchange optical pumping
polarized He target. These measurements have been included in a global
phase-shift analysis following that of George and Knutson, who reported two
best-fit phase-shift solutions to the previous global p+He elastic
scattering database below 12 MeV. These new measurements, along with
measurements of cross-section and beam-analyzing power made over a similar
energy range by Fisher \textit{et al.}, allowed a single, unique solution to be
obtained. The new measurements and phase-shifts are compared with theoretical
calculations using realistic nucleon-nucleon potential models.Comment: Submitted to Phys. Rev.
The Slowly Formed Guiselin Brush
We study polymer layers formed by irreversible adsorption from a polymer
melt. Our theory describes an experiment which is a ``slow'' version of that
proposed by Guiselin [Europhys. Lett., v. 17 (1992) p. 225] who considered
instantaneously irreversibly adsorbing chains and predicted a universal density
profile of the layer after swelling with solvent to produce the ``Guiselin
brush.'' Here we ask what happens when adsorption is not instantaneous. The
classic example is chemisorption. In this case the brush is formed slowly and
the final structure depends on the experiment's duration, . We find
the swollen layer consists of an inner region of thickness with approximately constant density and an outer region
extending up to height which has the same density decay as for the Guiselin case.Comment: 7 pages, submitted to Europhysics Letter
The burden and characteristics of peripheral arterial disease in patients undergoing amputation in Korle Bu Teaching Hospital, Accra, Ghana
Background: To determine the prevalence of Peripheral Arterial Disease (PAD) and associated risk factors in patients undergoing amputation at the Korle Bu Teaching Hospital (KBTH), Accra, Ghana.Objectives: A cross- sectional study of all patients undergoing lower extremity amputation at the Department ofSurgery, KBTH.Materials: A coded questionnaire was used to ascertain risk factors for PAD. The Edinburgh Claudication Questionnaire was used to determine symptomatic PAD and a 5 mmHz hand held Summit® Doppler together with an Accoson® sphygmomanometer was used to determine PAD and its severity.Method: Clinical diagnosis of symptomatic PAD was made using a symptom-based questionnaire and signs of PAD determined by measuring the ankle brachial pressure index (ABPI) by means of a handheld Doppler and sphygmomanometer. Risk factors were determined using the coded questionnaire and related to the occurrence and severity of PAD.Results: The prevalence of PAD among recruited participants was 71%. Twenty-eight per cent of participants with PAD in the index limb also showed signs of PAD in the other limb. The diagnosis of PAD was made in 71%, using ABPI, and 13%, using ECQ. Twenty-seven per cent of patient with hypertension, seventeen per cent with diabetes and all patients with hypercholesterolemia were not on any form of medication.Conclusion: There is a high prevalence of PAD among patients undergoing lower extremity amputation at the KBTH. The majority of PAD patients presented with moderate to severe PAD. Instituting measures to identify and control risk factors of PAD may reduce this high burden.Funding: Not declaredKeywords: Peripheral arterial disease, ABPI, Prevalence, Ghana, AmputationFunding source: Authors funded the stud
The dust envelope of the pre-planetary nebula IRAS19475+3119
We present the spectral energy distribution (SED) of the pre-planetary
nebula, IRAS 19475+3119 (I19475), from the optical to the far-infrared. We
identify emission features due to crystalline silicates in the ISO SWS spectra
of the star. We have fitted the SED of I19475 using a 1-D radiative transfer
code, and find that a shell with inner and outer radii of 8.8X10^{16} and
4.4X10^{17}cm, and dust temperatures ranging from about 94K to 46K provide the
best fit. The mass of this shell is greater than/equal to
1[34cm^{2}g^{-1}/kappa(100micron)][delta/200]M_Sun, where kappa(100micron) is
the 100micron dust mass absorption coefficient (per unit dust mass), and delta
is the gas-to-dust ratio. In agreement with results from optical imaging and
millimeter-wave observations of CO emission of I19475, our model fits support
an r^{-3} density law for its dust shell, with important implications for the
interaction process between the fast collimated post-AGB winds and the dense
AGB envelopes which results in the observed shapes of PPNs and PNs. We find
that the observed JCMT flux at sub-millimeter wavelengths (850micron) is a
factor ~ 2 larger than our model flux, suggesting the presence of large dust
grains in the dust shell of I19475 which are not accounted for by our adopted
standard MRN grain size distribution.Comment: 38 pages, 8 figures. Accepted for publication in Ap
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