After coated ammonium nitrate by stearic acid there be particles stuck with other particles. in filtration process break down this sticking particles and be uncoated by stearic acid, and this leads to the absorption of water droplets in certain parts of the surface and not all of the surface.

Dear Baha : 'this leads to the absorption of water droplets in certain parts of the surface and not all of the surface'.  There is no logic in above statement  unless  your presumption that all the sticking particles becomes free and uncoated by  stearic acid is falls. Don't forget there is no black magic in natural sciences there is always some logical reasoning  behind in any thing looks like us strange!!! or our ignorance!!! BEST REGARD  

After coating ammonium nitrate by stearic acid, surface properties of the ammonium nitrate are modified. Most likely, the surface has changed from hydrophilic to hydrophobic therefore, it starts to repel water and the water beads up on the surface. If you want to confirm this hypothesis/conclusion, try wetting the ammonium nitrate with water without adding stearic acid and then with stearic acid. If ammonium nitrate absorbs or wetted by water without stearic acid then the above hypothesis will definitely apply. 

I completely agree with Muhammad arguments,  since the physico-chemical properties of surface layers (2D phases) are strongly influence by their chemical compositions as well  as their strain state. The Surface specific free energy whether its Helmholtz or Gibbs will change upon the dilution of its  stearic acid content by the absorption of water during wetting process. That means the interfacial free energy FInter between the nitrate particle and the water substrate will change during the wetting process or reaaction, which immediately effects  dihedral angle:

  Theta = Arch COs[ Fwater -  FInter /  FParticle  

 Any  increase in  FInter  increases the wetting angle from zero to Pi.  Zero is the perfect wetting, and  Pi  corresponds to   the particle just at the detachment configuration or 'non-wetting' , which is an  unstable state..