Step propagation around surface defects and impurities
The movie (animated gif, 2.6MB) consists of 47 images separated by 21 seconds and displayed at a rate of one frame per second. The first image shows the surface of a growing ferritin crystal. The crystal is in solution with a ferritin concentration of 69 µg/mL, corresponding to 3x the equilibrium concentration. Three unfinished crystal layers are visible, with the higher layer colored in brighter yellow. The edges of these unfinished layers, also called growth steps are the only locations at which molecules from the solution can attach to the crystal and become a part of it. As a result of this molecular attachment, the steps move forward to build the crystal.
There are two highlights of this movie:
- In the first 10-15 frames, we see the molecules attach and detach from the step on the right. Although the supersaturation is quite high, these frames illustrates that growth consists of both attachment and detachment events.
- When the left step reaches the pair of surface vacancies, it is stopped by them. The reason is the elastic strain, created by these vacancies. The step tries to circumvent each individual vacancy, but the closeness of the two vacancies induces step curvature with a radius of about three molecule. This is obviously less than the characteristic capillarity length, the local supersaturation drops to zero, and the step stops. We see the dynamic character of the this equilibrium between the step segment held by the vacancies and the adjacent solution: on a molecular level, the step pursues several attempts to pass between the vacancies. Eventually, the step goes around the pair and proceeds its motions.
©1999 Copyright, S.-T. Yau and Peter G. Vekilov.
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