Solute Concentration Field Research Articles

Dendritic growth of ammonium chloride crystals: Measurements of the concentration field and a proposed nucleation model for growth.

We have developed a method of investigating the solute concentration field around a dendrite growing from a supersaturated ammonium chloride solution. It uses the refractive index of the solution as a measure of its concentration. The spatial dependence of refractive index is measured with the aid of an interference microscope. The field distributions obtained in this way indicate a nonlinear relationship between growth velocity and driving force. We propose a model for dendritic growth based on homogeneous nucleation at a temperature below the surface roughening temperature. We show that it is consistent with the measurements and also provides a mechanism of pattern selection.

CHEMICALLY INDUCED MIGRATION OF PARTICLES ACROSS FLUID STREAMLINES

The velocity of a colloidal particle that moves because of a gradient of concentration of a molecular solute depends on the concentration field at the surface of the particle. Effects of macroscopic convection of the suspending fluid on two such transport phenomena, capillary-driven movement of fluid particles and diffusiophoresis of rigid particles, are considered here. In the case of fluid particles our results also apply to motion caused by a temperature gradient. If the particles are in a laminar flow with the solute gradient directed perpendicular to the direction of flow, as might arise in the boundary layer near a surface to which the particles are being deposited, the local solute concentration field around each particle is disturbed from that of pure diffusion of the solute. Using published results for these concentration disturbances in a simple-shear flow, we determine the effect of the imposed velocity gradient on the speed of the particles in the direction of the solute gradient. For both flu...

Numerical simulation of diffusion-controlled droplet growth: Dynamical correlation effects.

Diffusion-controlled coarsening (Ostwald ripening) of precipitated solutions is studied by numerical simulation. An algorithm is devised which exploits the screening of solute concentration fields, thereby removing the restriction to small systems of previous work. Simulation of the coarsening of 5000 droplets at 10% volume fraction reveals long-ranged dynamical correlations which broaden the droplet size-distribution function and increase the coarsening-rate coefficient.

A unidirectional temperature gradient stage for solidification studies in aqueous solutions.

A low temperature light microscope stage has been designed and fabricated for quantitative analysis of the solidification of aqueous solutions. Thermal boundary conditions are controlled during freezing so as to enable direct comparison of data with complementary theoretical models. Evolution of the temperature distribution within the system is monitored by a one-dimensional thermocouple array, and growth of the phase interface is measured via a displacement transducer. The two-dimensional solution concentration field is assessed by computer densitometric analysis of the grey scale distribution for a light absorbing solute. Experimental and theoretical analyses of freezing processes with the stage correlate well, and the data base is shown to be internally consistent by independent thermal and mechanical measures of the interface velocity.