Individuals growing or dissolving in real crystal-forming system are often exposed to the mechanical impact, the evidence of which is scratches and cracks, or rather defects initiated by them and the follow-up "healing" in the form of regeneration surfaces and anatomic peculiarities of the structure detected by a researcher. With the appearance of new methods it became possible to experimentally model similar processes and to observe in detail a growing crystal responding to the impact on its surface.

 The purpose of the present work was to do an atomic-force microscopy (AFM) study of the evolution of a growing or dissolving crystal in the area of an intentional scratch on its surface. Dioxydine crystals (hydroxymethylquinoxalindioxyde, C₁₀H₁₀N₂O₄) served us as model objects.

In the contact mode, the AFM probe presses on a surface with the force of 10^-9N. In the operating mode, with the feedback regime on, this force of interaction is maintained stable and the needle-probe physically does not get in touch with the surface. When the scanning is off and the feedback is disconnected, the probe, nevertheless, can be moved intentionally over the surface for the purpose of mechanical impact, in particular, on high steps on its way. In this case, even short-term impact of this kind is capable to initiate, besides a visible scratch, an intense area round it. The influence of such contact has important crystal-genetic (morphological and kinetic) consequences and is perceptible even after a long time.

This way, in one of the experiments on a crystal surface in an undersaturated solution, we drew the letter "O" with the probe moving it counter-clockwise. Almost two hours of observations showed an intensive dissolution of the surface in the area of the scratch, which affects very little the surrounding surface.

           Statistical calculations show that to the mid of the experiment there occurred enlargement of dissolving steps, and then they began to split into more thin ones. The average roughness of the surface changes non-monotonously, generally increasing by the end of the experiment end. We have discovered anisotropy of dissolution rates, connected with one of the axes of the second order, along which the pits are eventually extended. 

The next experiment was carried out in conditions of crystals growth. We observed the process of regeneration of the surface damaged as a result of mechanical impact.

By means of AFM probe, two parallel furrows, each 5 mkm in length and 1 mkm in width, were drawn on two sides of the screw dislocation hillock from its top on face (100). The growth steps of the given hillock have height about 8Å that corresponds to the parameter a of an elementary cell. The AFM-images show how quickly the disturbed parts are healed on both sides (the numbers indicate a picture serial number in the experiment, time interval between consecutive pictures of 4.5 min.).

The healing rate of steps composing a scratch reaches 1.7 nm/sec, whereas the surrounding steps continue to grow with a very small rate from 0.1 to 0.6 nm/sec. Earlier [1] it was noted that for macroscale the mechanical impact on the surface, for example, pricking, increases the growth rate of faces more than twice. It was explained by the occurrence of more powerful centers of growth on the face. In this case, no new centers of growth are formed. Probably, it is energy factors that are responsible for the kinetic consequences at nano-level.

            Observations show that the morphological structure of similar hillocks of growth on dioxydine is rather stable. Even after two or three hours of growth they retain their original outlines, having somewhat increased or reduced the number of steps. It was especially unexpected that after the impact, the growth rate of steps, even on those parts of surface which had no direct impact, experiences great fluctuation. This results in the loss of morphological stability: by the 45th minute, the steps to the right of the top lose the initial strict order in their position and alignment, and large curves were formed. The hillock top becomes more and more flat and asymmetrical. The steps going out from its center grow with the rate of 1.3 nm/sec in the beginning, and decrease their rate to 0.4 nm/sec by the end of the observation. This, probably, is connected with approaching an equilibrium state. At the same time, we observe the phenomenon of simultaneous growth and dissolution at the neighbouring steps, which is difficult to explain. Thus, after the 50th minute, some areas on the hillock top grow at a rate of 0.9 nm/sec, and the neighbouring ones are noticably dissolved at a rate reaching 2 nm/sec.

The investigation was carried out with the financial support of the Programs of the Russian Academy of Sciences, 12-U-5-1026, 12-P-5-1027, 12-P-5-1011, Scientific School 1310.2012.5, RFBR 11-05-00432а .

1. Askhabov A.M. Influence of mechanical deformations on crystals growth // Proceedings of the Institute of Geology, Komi Branch, USSR Acad. Sci. Issue 21. Syktyvkar. 1975. P. 3-11.

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