112 research outputs found
Stable Drop Shapes under Disjoining Pressure: II. Multiplicity and Stability
No full textThe stability of the contact line region as affected by the disjoining pressure has been analyzed by solving the augmented Young-Laplace equation. Because of the results in Part I (Zhang, X., Neogi, P., and Ybarra, R. M., J. Colloid Interface Sci.), we have concentrated on obtaining multiple solutions for the same set of conditions. as many as five solutions were obtained: drops that end in a thin film with uniform thickness and where the film shape oscillates, drops that end with microscopic contact angles, as well as uniform thin films of two different thicknesses. the results of linear stability analysis were used to show that most cases were unstable to infinitesimal disturbances. Only two stable drop shapes for the particular disjoining pressure investigated are stable, a thin film of constant thickness and a thin drop that ends in a film of same thickness. Both multiplicity and stability have been discussed here for the first time and shed considerable light on the role of the attractive and repulsive forces. © 2002 Elsevier Science (USA)
« The Desert and the Sown. Imperial Supremacy and Local Culture in Partho-Roman Mesopotamia ». Parthica, 7, 2005, pp. 235-246.
No full textThis paper deals with some aspects of the institutional history of Partho-Roman Mesopotamia, with a restricted focus on the city of Hatra, rightly pointed out as “a strategic key-stone” in the wide context of the Eastern Jezirah. In his further reading of well-known epigraphic evidence, the Author discusses the political environment of this particular area of the Parthian kingdom and the chronology of the city’s sovereigns, recalling their titles in the inscriptions which qualify them either ..
Hole-Filling Theory of Anomalous Diffusion in Glassy Polymers. Effects of Microvoids
No full textNon-Fickian diffusion is often observed at low concentrations and without any obvious irreversible changes. This contradicts some of the predictions of the existing theories, which in general work quite well. For these cases, it is suggested that the filling of microvoids give rise to such effects. the effective diffusivity and a memory-dependent source/sink term are obtained for describing the overall transport in heterogeneous media under local volume averaging. Some comparisons with experiments are shown. This mechanism is confined to low concentrations and supplements the existing theories which are effective at higher concentrations
Dynamics of an Adsorbed Patch and a Model for Spreading of Films of Ultralow Thicknesses
No full textThe dynamics of ultrathin films of liquids with vanishing 3D volatility, on solid surfaces, has been modeled by assuming that the films can be taken to be adsorbed patches spreading under surface diffusion. when the phase behavior of the adsorbed films is taken into account, it is seen that a vapor patch will spread without limits and evolve into a Gaussian profile. For a liquid film which has low 2D volatility, a patch will appear to equilibrate and the surface ahead will remain dry. This is the pancake, a case where a wetting liquid stops spreading. the spreading kinetics is generally seen to be the usual square-root-of-time type, although exceptions are also seen. a simple case of non-Fickian diffusion has been included, but fails to give anticipated results at least in a simple model
The Relation Between Dynamics of Wetting and Nonwetting Liquids
No full textThe relation between the dynamics of wetting and nonwetting liquids parameterized by non-zero contact angle was studied. Mathematics of singular solutions, blow-off experiments and lubrication theory approximation were used for the analysis. Voinov-Hoffman-Tanner rule was derived for a spreading drop of a wetting liquid
Mechanism of Pore Formation in Reverse Osmosis Membranes During the Casting Process
No full textThe process of pore formation during the casting of reverse osmosis membranes is analyzed. The process consists of two steps. The first step is the evaporation step, where the cast polymer solution is allowed to dry for 1 ∼ 100 s. The second step is the gel formation step, where the cast is soaked in water leaving behind the membrane in form of a gel. The evaporation step gives rise to a thin (∼0.1 μm) skin of high density and very small pores which is chiefly responsible for desalination. The gel forms the backing (∼100 to 250 μm) and contains large pores.
It is shown that low evaporation rates accompanied by shrinkage during evaporation gives rise to an instability leading to the formation of the skin region. The evaporation effect is fast, is confined to the skin region, and gives rise to very small pores. The gel formation is shown to be a very slow process which cannot interfere with the skin formation due to the vast differences in their rates of formation. It also gives rise to larger pores. All key features of the above experimental observations are explained.
The kinetics of the process depend on the diffusion coefficients D and Dp of the solvent and the polymer. However, the main factor is the solution chemistry of the polymer‐solvent system which controls both the effectiveness of the skin and the gel formed. For the first time, the relevant thermodynamic parameters which determine the extent and sizes of pore formation have been obtained
Interfacial Stability of Electrodeposition of Cuprous Oxide Thin Films
No full textExperiments on deposition of Cu2O films from basic cupper sulfate solution show that copper also deposits. At low, but basic values of pH only copper deposits and at high pH only cuprous oxide deposits. In the intermediate range where both compete the system shows oscillations at constant current. Linear stability analysis has been conducted for such an electrochemical cell to show that oscillations can take place in the parameter space identified in the experiments. The results are keeping with most of the experimental observations, which are many, but not with all. The physical mechanisms behind the oscillations are explained in terms of competing reactions
Steady-State Transport in Diluted Catalysts
No full textThe transport in composite media obtained by dispersing a porous active catalyst inside an inert porous medium is considered. Since such systems show a wide variety of advantages, it is necessary to obtain the correct transport equations. Very dilute systems are considered, a condition that any one dispersed catalyst microsphere cannot feel the influence of others arises. an averaged conservation equation at steady state is obtained under the above condition as well as the condition that any one microsphere can be randomly placed in the medium. Similarly, an effective diffusion coefficient has been obtained. the results differ from the ones previously in use. the enhancement in the reaction rate on diluting a catalyst is seen to be much less than the values previously suggested although a qualitative similarity exists
Condensation of a Non-Wetting Fluid on a Solid Surface
No full textTheoretical studies of a drop moving under condensation from the surrounding vapor, have been provided. Two cases are considered. In the first, the rate of condensation is large that the drop “moves” because condensation has changed its dimensions. The model provided here shows that the rate of spreading is a constant, proportional to the heat flux and inversely proportional to the macroscopic contact angle. This compares well with available experimental data. The other model where the rate of condensation is small, is taken from existing results and comes close to explaining one set of experimental data. It is based on the use of viscous forces as the primary rate mechanism. Its shortcomings have been discussed
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