The crystalization happens within the stirring time, the more time you do the stirring the more your crystals will grow bigger.

Check out this interesting article.

https://www.researchgate.net/publication/29600729_Asymptotic_Modelling_of_Crystallisation_in_Two_Layers_Systems_Application_to_Methane_Hydrate_Formation_in_Batch_Reactor/figures?lo=1

V. S. Borovik et al., Visualization during Crystallization in Minkowski Spacetime V.S. Borovik, V.V. Borovik and D.A. Skorobogatchenko Solid State Phenomena in the year 2017 Vol.269. PP7-14. DOI: 10.4028/www.scientific.net/SSP.269.7(Scopus)

You only stir the mixture when trying to dissolve it into the solution. When growing the crystals it’s best to leave the solution perfectly still and force the crystallization* very slowly… or as slowly as the lab will allow.

* Force the crystallization by evaporating away solvent or decreasing the temperature.

To “see” the answer to this question you have to be very very small.

Imagine that you were so small you could see individual molecules and that you were hovering near the face of a growing crystal.

Looking at the crystal you see a repeating pattern of molecules neatly interlocking together.

Looking at the solution you see the solvent molecules, some trash floating around, and the molecules of the substance that is crystallizing.

Watching the crystal face closely you’ll see that every once-in-a-while a molecule from the solution bounces against the crystal.

If that molecule fits in the repeating pattern and gets locked in, the crystal gets bigger. (Good)

If that molecule knocks against the crystal and smashes some molecules off, the crystal gets smaller. (Bad)

If that molecule gets wedged in at an odd angle, it creates an imperfection in the crystal lattice. If you are lucky, this grows into another facet of the crystal. If you are unlucky this will create a pocket in the lattice where trash accumulates.

So what influences the probability of these?

• A high growth rate increases the chance of those odd-angle molecules getting wedged in.
  • The growth rate is influenced by temperature shifts and the concentration of the solution.
• “Dirty” solutions increase the probability of trash pockets.
• Agitating the solution causes more of the crystal’s “good” patterned molecules to fly back off into the solution, slowing or reversing crystal growth.

Does that help?

V. S. Borovik et al., Visualization during Crystallization in Minkowski Spacetime V.S. Borovik, V.V. Borovik and D.A. Skorobogatchenko Solid State Phenomena in the year 2017 Vol.269. PP7-14. DOI: 10.4028/www.scientific.net/SSP.269.7(Scopus)

Dear Fatma Zeribi thank you for posting this interesting technical question on RG. I think it is rather difficult to give you a qualified answer because you did not specify what your reaction system is. Certainly it could be that longer stirring / reaction times e.g. in sol-gel processes give higher yields of titanium dioxide. In this context please have a look at the following potentially useful literature reference which might help you in your analysis:

Reactive crystallization: from mixing to control of kinetics by additives

This paper is freely accessible as public full text (please see the attached pdf file).

Good luck with your research!