The development of the depletion zone during ceiling crystallization: phase shifting interferometry and simulation results

Abstract

The growth of high quality protein crystals is essential for the determination of their structure. This process is governed by many physical factors such as mass transport and solution flow. The quality of the crystals is usually better under diffusion-limited growth conditions, where a depleted zone of the solution encapsulates the crystal. We developed a Mach–Zehnder-based phase shifting interferometer coupled to image processing software to study the concentration gradients which develop around a crystal during its growth or dissolution. The depletion zones and the diffusion boundary layers around growing and dissolving KH₂PO₄ crystals are monitored and processed by a MATLAB based algorithm. Our main emphasis was to analyze the ceiling crystallization conditions in which the crystal is placed at the very top of the growth cell and therefore the solute transport is largely diffusion-limited. The experimental results are compared with simulations using finite element-based numerical calculations. The combined results clearly demonstrate the positive effect of the ceiling crystallization approach on crystal growth.