Influence of sedimentation on the threshold for Soret-driven convection in colloidal suspensions.

The onset of Soret-driven convection in a horizontal layer of a colloidal suspension is investigated by considering a particulate medium model. We consider a dilute suspension of spherical solid particles being subjected to convection in a Rayleigh-Bénard geometry setup. The mathematical model takes into account the effects of thermophoresis, particle sedimentation, and Brownian diffusion. The equations governing the convective motion consist of the momentum equation which includes an extra body force term to account for the thermophoretic force effect, the conservation of particles equation whose mass-flux term couples the Soret and particle diffusion effects and whose advective term includes the sedimentation force, and the heat and mass balance equations. The horizontal boundaries are assumed rigid, perfectly thermally conducting, and impervious to mass flow. Furthermore, the model makes use of the effective viscosity of the suspension whose dependence on the particle concentration is through Einstein's formula. Moreover, we take into account the decrease of both the coefficient of Brownian diffusion and the mixture thermal diffusion with particle concentration due to the particles hindrance effect. The nondimensionalization leads to the emergence of an experimental parameter, ß, which depicts the competition between the effects of thermophoresis, sedimentation, and particle diffusion. The parameter ß is a function of the particles radius, the shape of which is an inverted parabola having two zeros. A combination of asymptotic and numerical computations is used to determine the threshold for the onset of the mass dominated convection, which corresponds to 0<ß≪1. Our findings shed light on the role of particle sedimentation and particle size, as well as the influence of other processing variables on the fluid instability.

Axisymmetric shapes and forces resulting from the interaction of a particle with a solidifying interface.

A steady state model of the interaction of a foreign particle with a solidifying interface in a microgravity environment is presented in order to examine the phenomenon of particle engulfment. The model considers the interception of a spherical impurity particle by a deformable solid-liquid interface. Three forces are present whenever the particle is in near contact with the solid front, namely, the disjoining pressure force, the thermal force, and the hydrodynamic pressure force. These forces arise due to (i) the disjoining pressure gradients resulting from the deformation of the interface behind the particle, (ii) the interface distortion caused by the difference between the coefficients of thermal conductance of the particle and melt, and (iii) the hydrodynamic pressure resulting from the melt flow driven by the pressure gradients in the gap between the particle and the interface. The model accounts for the modification of the melt's freezing point by these pressure terms. The dependence of the interface morphology on the various physical and processing parameters is revealed. These include the rate of solidification, the thermal gradient, the gap width, and the coefficients of thermal conductivity of the particle and the melt. The gap profile is calculated and used in the evaluation of the three forces that characterize the interaction. The analysis shows that the outcome of the interaction, i.e., particle engulfment or rejection, depends primarily on the competition between the hydrodynamic and thermal forces.

Morphology of a solidifying interface near a spherical particle: disjoining pressure effects.

Numerical simulations were conducted to determine the morphology of a solid-liquid interface near an insoluble spherical particle. The model accounts for the undercoolings due to the front's curvature and to the nonretarded van der Waals interactions. Our numerical results show that, in the near-contact region, the interface profile develops a sharp peak whose curvature has a logarithmic singularity. This is in agreement with the asymptotic analysis of a previously published equation for the interface profile.

Thermal force induced by the presence of a particle near a solidifying interface.

The presence of a foreign particle in the melt, ahead of a solid-liquid interface, leads to the onset of interfacial deformations if the thermal conductivity of the particle, k(p), differs from that of the melt, k(l). In this paper, the influence of the thermal conductivity contrast on the interaction between the solidifying interface and the particle is quantified. We show that the interface distortion gives rise to a thermal force whose expression is given by F(th)=2piLGa3(1-alpha)/(2+alpha)T(m), where L is the latent heat of fusion per unit volume, T(m) is the melting point, a is the particle's radius, G the thermal gradient in the liquid phase and alpha=k(p)/k(l). The derivation makes use of the following assumptions: (i) the particle is small compared to the horizontal extent of the interface, (ii) the particle is placed in the near proximity of the deformable solid-liquid interface, and (iii) the interface is practically immobile in the calculation of the thermal field, i.e., V<

Asymptotic analysis of particle engulfmemt.

An asymptotic analysis is conducted on the interaction between an insoluble spherical particle and an advancing solid-liquid interface when the particle is in the near-contact region (gap thickness is much smaller than the particle's radius). The analysis considers only thermal effects in a pure substance. The interface equilibrium temperature includes the undercooling effects due to the front curvature and to the long-range intermolecular forces in the thin melt film behind the particle. An expression for the crystal-particle gap thickness is derived and used to calculate the threshold value for the front velocity V(T) which separates pushing from engulfment.

Release of a slow-reacting substance from rabbit platelets.

Washed rabbit platelets stimulated with platelet-activating factor, thrombin, or arachidonic acid, released a slow-reacting substance (SRS), whereas platelets aggregated by adenosine diphosphate did not. Production of platelet-derived SRS was neither affected by indomethacin nor aspirin but was reduced by large doses of eicosatetraynoic acid, an inhibitor of the cyclo-oxygenase and lipoxygenase. L-cysteine enhanced markedly the release of SRS from platelets. This SRS activity, which was antagonized by FPL 55712 and inactivated by arylsulfatase, followed the same elution pattern on Amberlite, silicic acid, and reverse phase high-pressure liquid chromatography columns as that described for the SRS from other origins. SRS activity released from platelets preincubated with [14C]arachidonic acid exhibited the same retention time as radioactivity in reverse phase high-pressure liquid chromatography. The release of a SRS from platelets is consistent with their implication in the pathogenesis of asthma and other lung diseases.

Partial characterization of human histaminopectic factor.

Sera from normal human subjects are pooled and fractions are isolated by salt precipitation. The fraction responsible for agglutinating histamine bound latex particles is further purified by column chromatography. The histaminopectic activity can thus be traced to a single protein present in the IgG fraction of normal human serum. Its absence or the presence of an inhibitor in serum from allergic subjects is also sought as well as correlations between plasma concentrations of known immunoglobulins and specific histaminopectic activity. Some of its physical-chemical properties are explored. Evidence was also presented pointing to the possible presence of an inhibitor of the histaminopectic factor in human atopic serum.