2,256 research outputs found
Rate of Homogeneous Crystal Nucleation in molten NaCl
We report a numerical simulation of the rate of crystal nucleation of sodium
chloride from its melt at moderate supercooling. In this regime nucleation is
too slow to be studied with "brute-force" Molecular Dynamics simulations. The
melting temperature of ("Tosi-Fumi") NaCl is K. We studied crystal
nucleation at =800K and 825K. We observe that the critical nucleus formed
during the nucleation process has the crystal structure of bulk NaCl.
Interestingly, the critical nucleus is clearly faceted: the nuclei have a
cubical shape. We have computed the crystal-nucleation rate using two
completely different approaches, one based on an estimate of the rate of
diffusive crossing of the nucleation barrier, the other based on the Forward
Flux Sampling and Transition Interface Sampling (FFS-TIS) methods. We find that
the two methods yield the same result to within an order of magnitude. However,
when we compare the extrapolated simulation data with the only available
experimental results for NaCl nucleation, we observe a discrepancy of nearly 5
orders of magnitude. We discuss the possible causes for this discrepancy
Clinical Features of Pediatric Idiopathic Intracranial Hypertension and Applicability of New ICHD-3 Criteria
Idiopathic intracranial hypertension (IIH) is characterized by intracranial pressure >28 cmH2O in the absence of identifiable causes. Aim of this paper is to describe the clinical phenotype of pediatric IIH and to analyze the applicability of ICHD-3 criteria in comparison to the ICHD-2. We conducted a retrospective analysis of full clinical data of pediatric patients diagnosed with IIH between January 2007 and June 2018. Diagnostic evaluation included neuroimaging (all patients) and ultrasound-based optic nerve sheath diameter measurement (9 patients). Diagnosis of IIH was verified according to both ICHD-2 and ICHD-3 criteria for headache attributed to IIH, to verify the degree of concordance. We identified 41 subjects with suspected IIH; 14 were excluded due a diagnosis of secondary IH or lack of data. We therefore selected 27 subjects (age 4-15 years, mean 11). All patients presented with headache and bilateral papilloedema. Headache was daily in 22% cases, with diffuse gravative pain in 41%. In 4%, pain was exacerbated by cough, stress or tension. The most common presentation symptoms, in addition to headache, were blurred vision or diplopia (70%), vomiting (33%), and dizziness (15%). Twenty patients (74%) were obese. In 6 patients (22%) neuroimaging showed empty sella. Optic nerve sheath distension was detected in 6 out of 9 patients. Regarding the applicability of the ICHD-2 criteria, 18/27 (71%) patients have criterion A; 24/27 (89%) criterion B; 27/27 (100%) criterion C; 27/27 (100%) criterion D. When the ICHD-3 criteria were used, 27/27 (100%) fitted criterion A; 24/27 (89%) criterion B; 27/27 (100%) criterion C; and 27/27 (100%) criterion D. Our study suggests that, as compared with the ICHD-2, the new ICHD-3 criteria for headache attributed to IIH are better satisfied by pediatric patients with IIH. This is mainly due to the fact that qualitative headache characteristics are no longer considered in ICHD-3. Although the risk of under-rating the symptom of headache in IIH should not be disregarded, in pediatric population headache characteristics are usually less defined than in adults and obtaining a precise description of them is often very difficult
Nucleation in a sheared Ising model: effects of external field
Simulations using the Forward Flux Sampling method have shown a nonmonotonic
de- pendence of the homogeneous nucleation rate on the shear rate for a sheared
two dimensional Ising model [R. J. Allen et al, arXiv cond-mat/0805.3029]. For
quasi-equilibrium systems (i.e. in the absence of shear), Classical Nucleation
Theory (CNT) predicts the dependence of the critical cluster size and the
nucleation rate on the external magnetic field. We investigate the behaviour of
the sheared Ising model as a function of the external field. At low exter- nal
field strength, the same nonmonotonic behaviour holds and the peak in the
nucleation rate is remarkably insensitive to the field strength. This suggests
that the same external field-dependence holds for the enhancement of nucleation
by shear at low shear rates and the suppression of shear at high shear rates.
At high field strength, the nucleation behaviour is qualitatively different. We
also analyse the size and shape of the largest cluster in the transition state
configurations, as a function of the external field. In the sheared system, the
transition state cluster becomes larger and more elongated as the field
strength decreases. We compare our results for the sheared system to the
predictions of the CNT for the quasi- equilibrium case, and find that the CNT
cannot easily be used to describe nucleation in the system under shear
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