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This review summarizes the optical, electromagnetic, and acoustic imaging techniques that are in use or available for mapping convection, temperature, and solute concentration in the solution around a growing crystal and for measuring the growth rate and the facial micromorphology of the crystal itself. By way of introduction, the need for such mapping, the value of optical, electromagnetic, and acoustic techniques, and the comparative value of two- vs. three-dimensional mapping are discussed. Then, the phenomenology of crystal growth from solution is briefly reviewed with emphasis on the effect of convection and gravity. Next, for each type of measurement, the techniques are described and representative examples of application to crystal growth research are cited. Convection can be mapped by measuring refractive index differences (shadowgraphy, schlieren, interferometry and holography), solute concentration (absorption photometry), displacements (particles, laser speckle, bubbles, and dye markers), or velocity (laser Doppler velocimetry and particle image velocimetry). Since the profound effects of buoyant convection were the primary motive for NASA's spaceflight experiments on crystal growth, the optical techniques used in this program and results thereof are discussed in some detail for both inorganic and protein crystallization. Solution temperature can be mapped by physical probes, liquid crystal thermography, thermochromism, fluorescence, interferometric, or ultrasonic techniques. Solution concentration can be mapped by phase shifting interferometry, electronic speckle pattern interferometry, or optical absorption. Simultaneous measurement of temperature and concentration can be performed using dual-wavelength interferometry or a combination of absorption and interferometry. There has to date been relatively little use of these techniques in crystal growth research. The measurement of the growth rate and the mapping of facial micromorphology are feasible by low-precision techniques (acoustic reflection, optical rotation, and cathetometry), by various forms and combinations of microscopy and interferometry, and by atomic-level techniques such as atomic force microscopy and X-ray or electron microscopy. Post-growth techniques (optical, electron, and scanning probe microscopy, X-ray diffraction topography, growth band analysis, and light-scattering tomography) are briefly mentioned. The application of these techniques to crystal growth research is briefly summarized. Some modes of measurement can be extended to three-dimensional imaging by holography, point-by-point scanning, planar slice imaging, or computerized axial tomography (CAT). Of these, the most promising appear to be magnetic resonance imaging (MRI), magnetic gradient discrimination of optical image depth, and optical CAT. These techniques are therefore discussed in some detail. In particular, the recent work of one of the authors (SV) on the application of shadowgraph and interferometric CAT techniques to crystal growth experiments is herein presented in some detail. This review appears to show that, although some optical techniques have been extensively used to study crystal growth from solution, many other promising techniques have yet to be explored. A representative bibliography of over 600 publications and websites is included. |
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