3,624 research outputs found
A Rare Nasopharyngeal Foreign Body
Nasopharynx is an exceptionally rare anatomical location for foreign body impaction. We present a rare case of nasopharyngeal foreign body (NFB) in a 7 years old child. The diagnosis was confirmed by nasal endoscopy. Immediate removal of foreign body (FB) in the nasopharynx was performed under general anesthesia. This rare situation is potentially dangerous, since its dislodgment may cause fatal airway obstruction. Therefore, in all cases with missing foreign bodies in the aerodigestive system, nasopharyngeal impaction should be kept in mind and endoscopic examination of the region should be considere
Nucleation of a stable solid from melt in the presence of multiple metastable intermediate phases: Wetting, Ostwald step rule and vanishing polymorphs
In many systems, nucleation of a stable solid may occur in the presence of
other (often more than one) metastable phases. These may be polymorphic solids
or even liquid phases. In such cases, nucleation of the solid phase from the
melt may be facilitated by the metastable phase because the latter can "wet"
the interface between the parent and the daughter phases, even though there may
be no signature of the existence of metastable phase in the thermodynamic
properties of the parent liquid and the stable solid phase. Straightforward
application of classical nucleation theory (CNT) is flawed here as it
overestimates the nucleation barrier since surface tension is overestimated (by
neglecting the metastable phases of intermediate order) while the thermodynamic
free energy gap between daughter and parent phases remains unchanged. In this
work we discuss a density functional theory (DFT) based statistical mechanical
approach to explore and quantify such facilitation. We construct a simple order
parameter dependent free energy surface that we then use in DFT to calculate
(i) the order parameter profile, (ii) the overall nucleation free energy
barrier and (iii) the surface tension between the parent liquid and the
metastable solid and also parent liquid and stable solid phases. The theory
indeed finds that the nucleation free energy barrier can decrease significantly
in the presence of wetting. This approach can provide a microscopic explanation
of Ostwald step rule and the well-known phenomenon of "disappearing polymorphs"
that depends on temperature and other thermodynamic conditions. Theory reveals
a diverse scenario for phase transformation kinetics some of which may be
explored via modern nanoscopic synthetic methods
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