In situ observation of crystal growth from solution

Abstract

Crystal growth from solution has been investigated by means of in situ observation. Phase-sensitive microscopy or microinterferometry has been employed in order to understand the kinetics of step motion and the effects of flow dynamics on the crystal growth. Both dislocated and dislocation-free crystal were used as the seed crystals. The growth of dislocation-free crystals was found to proceed via quasi-two-dimensional nucleation (Q2D) when the supersaturation is low and via two-dimensional nucleation (2D) when the supersaturation exceeds a critical value. Q2D nucleation was shown by in situ dissolution to be due to the presence of microdefects in the crystals which were probably formed by crystalline particles, impurities or bubbles transported from the bulk solution. The possibility that bubbles, which behave like growth centres, are pushed from the bulk solution with a concentration gradient towards the interface due to Marangoni convection is considered. In order to observe the direct reaction between crystalline particles in largely supersaturated solution, a seed crystal was chipped off with a glass rod to form small crystalline particles or fragments (ca. 10–20 µm in diameter after several s). It was observed by interferometry that, very soon after the fragments were formed (a few tens of s), small crystals were in some cases linked to each other to form a chain or were adsorbed to particular sites on the crystal, which might suggest the presence of interaction forces between particles and crystals.

In order to investigate this phenomenon from a different viewpoint, the exact concentration gradient near the growth interface was measured using the recently developed real-time phase-shift interferometry. This method is 100 times more sensitive than conventional interferometry. The concentration gradient near the interface was found to be much less steep than expected from theory, also supporting the possibility of driving forces other than the concentration gradient.