Duane T. Johnson

A tutorial on the Rayleigh–Marangoni–Bénard problem with multiple layers and side wall effects

A brief review in the form of a tutorial is presented on convective instabilities that arise from thermocapillary and buoyancy effects. This tutorial primarily focuses on the effect of multiple layers and side walls on the nature of the convective flows and associated patterns. A comprehensive explanation of the physics of this type of convection is followed by a discussion of the mathematical features of bifurcation associated with the problem and some of the recent experimental studies. (c) 1999 American Institute of Physics.

Formation of FePt nanoparticles by organometallic synthesis

Our interest in determining the mechanism of FePt nanoparticle formation has led to this study of the evolution of particle size and composition during synthesis. FePt nanoparticles were prepared by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl. During the course of the reaction, samples were removed and the particle structure, size, and composition were determined using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy–energy dispersive x-ray spectrometry. Early in the reaction the particles were Pt rich (greater than 95at.% Pt) and as the reaction proceeded the Fe content increased to the target of 50%. The particle diameter increased from 3.1to4.6nm during the reaction. Energy dispersive x-ray spectrometry measurements of individual particle compositions using a high resolution TEM showed a broad distribution of particle compositions with a standard deviation greater than 15% of the average composition.

The Study of the Surface Tension of Charge‐Stabilized Colloidal Dispersions

Abstract Our measurements of the surface tension of charge‐stabilized TiO2 and SiO2 dispersions show that the surface tension of the dispersions is a function of the particle concentration. At lower concentrations, the adsorption of particles to the interface decreases the total free energy of the system which causes a decrease in the surface tension. At higher particle concentrations, the attractive capillary forces between particles at the surface increases the amount of work it takes to deform the surface, which increases the surface tension. Compared with the surface tension of TiO2 dispersions, the decrease of the surface tension of SiO2 dispersions is slightly less and the minimum surface tension occurs at a higher particle concentration. The reason is believed to be the larger particle size of the SiO2 particles.

Interfacial tension measurements of colloidal suspensions: An explanation of colloidal particle-driven interfacial flows

In previous work, we have shown that a dilute suspension of colloidal particles will spontaneously flow when placed in a container. The flow is caused by interfacial tension gradients created by aggregates of the colloidal particles on the interface. In this paper, we present some results that show the interfacial tension of charged stabilized titania suspensions significantly decreases within certain volume fractions. The data show that the interfacial tension is indeed a strong function of the particle concentration and help explain the unusual results seen in the flow experiments.

Rheological and magnetic properties of a metal particle dispersion exposed to magnetic fields

We present a series of experiments in which a DC magnetic field is applied to a magnetic dispersion of acicular ferromagnetic iron particles. For DC fields above the lower limit of the switching field distribution, the magnetic susceptibility and the storage modulus of the sample increase linearly. We believe that this increase is caused by the break-up of doublets and other aggregates that are normally present in a magnetic dispersion. The break-up increases the number of free particles in the dispersion, which in turn increases the magnetic interactions and elasticity of the dispersion.

Observations on Interfacial Convection in Multiple Layers without and with Evaporation

A tutorial is presented on convective instabilities that arise from thermocapillary and buoyancy effects in the presence of evaporation. It focuses on the effect of multiple layers on the nature of the instabilities. A comprehensive explanation of the physics of this type of convection is accompanied by results from calculations and their interpretation.

Viscous effects in liquid encapsulated liquid bridges

Abstract An analytical derivation of the surface deflections and the streamfunctions for the flow inside a liquid encapsulated liquid bridge has been derived using an asymptotic expansion about a small capillary number. The model assumes an initially flat and cylindrical interface under the assumption that the densities of both fluids are equal. To simplify the analysis, the top and bottom walls are assumed to be stress-free and the Reynolds number is assumed to be negligible. Flow is generated either by a moving outer wall (shear-driven flow) or by applying a temperature difference across the top and bottom walls (Marangoni-driven flow). The resulting equations show that for the shear-driven flow, as the viscosity ratio increases, the surface deflections increase monotonically. For the Marangoni-driven flow there exist values of the viscosity ratio where the surface deflections reach a minimum and then switch signs. This investigation shows that it may be possible in more realistic systems to use an outer encapsulating liquid of the proper viscosity ratio to stabilize the liquid–liquid interface during float zone crystal growth.

Small voltage electrocapillary flows in the presence of ionic surfactants

Rapid flows were observed by applying a small voltage across an organic-aqueous interface in the presence of an ionic surfactant. These flows are in contrast to the experiments of G. I. Taylor and J. R. Melcher [Annu. Rev. Fluid Mech. 1, 111 (1969); D. A. Saville, Annu. Rev. Fluid Mech. 29, 27 (1997)], where no ionic surfactant was used. In their experiments, a large voltage difference was necessary to induce a surface charge between two leaky dielectric fluids. Here, we intentionally create a surface charge by adding a cationic surfactant. The result is that a much lower voltage is necessary to cause substantial flows. Two experiments are performed to compare these results with classical thermocapillary flow experiments.

A NOTE ON THE STABILITY OF A SOLIDIFYING MATERIAL IN AN UNDER­ COOLED MELT IN THE PRESENCE OF CONVECTION

A calculation shows that convection provides slight stability to the morphology of a pure substance growing in an under-cooled melt provided that the surface tension is not very small or if the under-cooling is very small. A surprise of sorts turns up when gravity is reversed and the liquid melt acts like it is heated from above for here the perturbed convective fields destabilize the interface further moving the critical wavelengths to smaller values.