A crystal growing from its aqueous solution creates a three-dimensional solute distribution in its vicinity. The mechanism involved is the withdrawal of the solute from the solution, causing the solute concentration to decline in the neighboring solution. The solutal concentration gradients, and hence the gradients in the density of the solution are responsible for the evolution of buoyancy-driven convection currents in the growth chamber. The convection currents are the main drivers for the solute from the solution rich in salt as it is transported to the surface of the growing crystal. In the absence of convection, solute transport is mainly by molecular diffusion. Since diffusion coefficients in liquids are small, small fluid velocities greatly increase mass transfer (over the diffusion values) and hence the growth rate of the crystal. It is understandable that changes in the symmetry pattern of the flow field and unsteadiness can lead to a lowering of the crystal quality and the growth rate. Hence an optimum strength of convection currents is desirable to maintain a balance between the quality of the growing crystal and its growth rate. The strength and orientation of buoyancy driven convection currents are intricately linked with the size and morphology of the growing crystal. These in turn depend upon the process parameters namely the supersaturation level of the solution and the rate at which it is cooled. More recently, convection has been found to be quality-limiting mechanism in the growth of protein crystals from their solution. Here, convection has the capability of distorting the effects of mechanisms such as surface tension and magnetic fields and thus influences the crystal growth process.

Buoyancy-driven convection currents influence the magnitude of the concentration gradients prevailing along the growth interfaces. In turn, the gradients control the stability of the growth process and the overall crystal quality. It has been experimentally noted that growth in free convection regime is often limited by density stratification in the vicinity of the growing crystal leading subsequently to unwanted nucleation of solute in the growth chamber. The concentration gradients in the growth chamber are significantly altered when the crystal is given a rotation. An optimum rotation rate tends to stabilize the perturbations along the rotational axis eventually leading to an axisymmetric concentration distribution over the crystal. The stirring of the solution also reduces the natural convection-induced temperature oscillations by homogenizing the bulk solution. Rotation can hence be viewed as a method for controlling convection during the growth process by diminishing the impact of buoyancy.

To ensure the growth of high-quality large crystals, it is important to understand from a fundamental point-of-view, the transport phenomena involved during solute deposition from the solution to the crystal surfaces. Simultaneously, visualization techniques are required to monitor the crystal growth process itself during its progress. Unlike growth from melt and vapor, growth from an aqueous solution is particularly amenable to flow visualization, since it is transparent. It is possible to generate images of the convective field by exploiting changes in the refractive index that accompany changes in the density of the medium. Optical visualization techniques are thus useful for online monitoring of the growth process.