Surface roughness

A rough surface can be of interest for chemical reactivity and dissolution speed. Similarly, crystals attach to one another more readily without slipping, thereby ensuring greater cohesion for tablets.

However, broadly speaking, the rate of growth for a rough face is similar to that of a K-type face, that is, very high. As a consequence, the morphological significance of rough faces is low. These faces can become virtual.

Bennema and his team [GRI 98] explained why certain F faces of rough character can have fast growth. According to Onsager’s bidimensional theory [ONS 44], the linear step energy (J.m1) decreases linearly with the

temperature and is cancelled by a rough transition temperature Tc, known as the Ising temperature.

  • - where T < Tc growth is flat and slow;
  • - where T > Tc growth is rough and fast.

Nonetheless, other considerations must be taken into account. Bennema and his colleagues popularized the notion of layers forming a connected net. This net is obtained by the combination of the two non-parallel bonding chains. However, unlike a classic layer, a connected net is not necessarily stoichiometric, which means that polar net can exist.

A step corresponds to the development of a connected net comprising the breaking of bonds on its front, and the apparition of new bonds behind this front. When the difference between the two is zero, the linear energy Y is also zero, and the face can develop very quickly irrespective of temperature. Among the F faces able to develop quickly, the authors distinguish:

  • - rough faces, where Y is zero irrespective of the temperature;
  • - disordered faces, where two connected nets have different transition temperatures;
  • - flat but reconstructed faces, where the influence of the mother liquor can lead to rough growth.

Meekes et al. [MEE 98] explained the notion of roughness through symmetry (y = 0), while Grimbergen et al. [GRI 99] studied multiple connected nets. This is as far as we will discuss this subject.

 
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