83 research outputs found
Shapes, contact angles, and line tensions of droplets on cylinders
Using an interface displacement model we calculate the shapes of
nanometer-size liquid droplets on homogeneous cylindrical surfaces. We
determine effective contact angles and line tensions, the latter defined as
excess free energies per unit length associated with the two contact lines at
the ends of the droplet. The dependences of these quantities on the cylinder
radius and on the volume of the droplets are analyzed.Comment: 26 pages, RevTeX, 10 Figure
Thermodynamics of heterogeneous crystal nucleation in contact and immersion modes
One of most intriguing problems of heterogeneous crystal nucleation in
droplets is its strong enhancement in the contact mode (when the foreign
particle is presumably in some kind of contact with the droplet surface)
compared to the immersion mode (particle immersed in the droplet). Many
heterogeneous centers have different nucleation thresholds when they act in
contact or immersion modes, indicating that the mechanisms may be actually
different for the different modes. Underlying physical reasons for this
enhancement have remained largely unclear. In this paper we present a model for
the thermodynamic enhancement of heterogeneous crystal nucleation in the
contact mode compared to the immersion one. To determine if and how the surface
of a liquid droplet can thermodynamically stimulate its heterogeneous
crystallization, we examine crystal nucleation in the immersion and contact
modes by deriving and comparing with each other the reversible works of
formation of crystal nuclei in these cases. As a numerical illustration, the
proposed model is applied to the heterogeneous nucleation of Ih crystals on
generic macroscopic foreign particles in water droplets at T=253 K. Our results
show that the droplet surface does thermodynamically favor the contact mode
over the immersion one. Surprisingly, our numerical evaluations suggest that
the line tension contribution to this enhancement from the contact of three
water phases (vapor-liquid-crystal) may be of the same order of magnitude as or
even larger than the surface tension contribution
Distribution of aluminium fractions in acid forest soils: influence of vegetation changes
This study examines aluminium as a potentially phytotoxic element in acidic forest soils. Concentrations of Al forms in soils are generally controlled by soil chemical conditions, such as pH, organic matter, base cation contents, etc. Moreover, soil conditions are influenced by the vegetation cover. This study analyzed the distribution of Al forms in soils after changes in vegetation. HPLC/IC was used for the separation of three Al fractions in two soil extracts according to their charge. An aqueous extract (AlH2O) simulated the natural soil conditions and bioavailable Al fractions. Potentially available Al form was represented by a 0.5 M KCl extract (AlKCl). We demonstrated that the vegetation type influences the concentrations of different Al fractions, mainly in the surface organic horizons. Differences were more common in the KCl extract. The trivalent fraction was less influenced by vegetation changes than the mono- and divalent fractions. Afforestation increased the concentrations of AlKCl and AlH2O. In contrast, grass expansion after deforestation led to significantly decreased concentrations of AlKCl and AlH2O. Concentrations of AlH2O in organic horizons were higher in spruce forest than in beech forest. A long-term effect of liming on soil pH and concentrations of potentially toxic Al fractions was not apparent. The results provide information on the variations of Al fractions distributions following vegetation type changes and indicate the existence of some natural mechanisms controlling Al toxicity. Furthermore, the results can be used in the management of forested areas endangered by soil acidification
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