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 $\sim 1060$K. We studied crystal nucleation at $T$=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

Ipotesi sul ciclo in stucco di palazzo Altieri a Roma.

Il saggio contestualizza un ciclo significativo della produzione a stucco della Roma della seconda metà del 17. secolo

Non-equilibrium dynamics of an active colloidal "chucker"

We report Monte Carlo simulations of the dynamics of a "chucker": a colloidal particle which emits smaller solute particles from its surface, isotropically and at a constant rate k_c. We find that the diffusion constant of the chucker increases for small k_c, as recently predicted theoretically. At large k_c the chucker diffuses more slowly due to crowding effects. We compare our simulation results to those of a "point particle" Langevin dynamics scheme in which the solute concentration field is calculated analytically, and in which hydrodynamic effects can be included albeit in an approximate way. By simulating the dragging of a chucker, we obtain an estimate of its apparent mobility coefficient which violates the fluctuation-dissipation theorem. We also characterise the probability density profile for a chucker which sediments onto a surface which either repels or absorbs the solute particles, and find that the steady state distributions are very different in the two cases. Our simulations are inspired by the biological example of exopolysaccharide-producing bacteria, as well as by recent experimental, simulation and theoretical work on phoretic colloidal "swimmers".Comment: re-submission after referee's comment