Many people gets confused between the two terms diffusion and osmosis. In this section we will focus on these two related terms and study briefly diffusion in solids.

Diffusion occurs when atoms and molecules gain kinetic energy and crosses the threshold limit as a result of which it randomly moves to other location and occupies other new place. This movement of atoms and molecules resulting due to gain of kinetic energy is called diffusion. This can be understood by example shown below. Due to kinetic energy gain the atoms and molecules move to other interstitial sites or location.  Diffusion occurs in solid, liquids as well as gases. Various solid diffusion is explained in this context.
It is very important to know about diffusion in solids when we are learning material science. The diffusion effect various parameters like surface hardening, edge hardening etc which are very important in industrial application.

Osmosis is phenomenon which depends on the concentration of two fluid. The fluids are separated by a semi permeable membrane in such a manner that solvent can pass through the permeable membrane being smaller in size as compared to solute. The solvent concentration is very less as compared to solute. Osmosis occurs from lower concentration to higher concentration. 

Consider case when on the both side of semi permeable membrane there is same fluid ( say water). In such case there is no osmosis phenomenon occurrence. I t occurs when there is pressure differential. Osmosis is of great biological importance. Pictorial representation of osmosis can be observed below. 

Diffusion in solids

Diffusion in solid occurs to rise in thermal energy of atoms or molecules which allows them to move at different location in a solid crystal. We all know well that atoms and molecules are closely packed in a solid crystal. Hence it is practical to say that diffusion will be zero in case of perfect solid crystal. But in actual case such perfect crystal does not imperfections in it which becomes predominant at high temperature as diffusion rate increases at high temperature.
Effects of diffusion:-
  • Phase change in alloy.
  • Molecular bonding.
  • Surface hardening/characteristics.
  • Grain propagation.
  • Edge hardening.
Type of diffusion:-
  • Surface diffusion 
  • Volume diffusion
  • Inter diffusion
  • Self diffusion
Surface diffusion:-

When the migration or movement of atoms takes place at the surface of the crystal, then it is called surface diffusion. The atoms or molecule moves from one interstitial site to another. The figure depicts the surface  atomic movement. It is the vapor-solid interface. 

Volume diffusion:-

It is movement of atom in a matter in bulk amount throughout the material. One atom can move anywhere inside the crystal and occupy interstitial position. This type of diffusion plays an important role in change of property of entire crystal. The distribution of diffused atom may be uniform or may not be uniform.

Self diffusion:-

In this case atoms does not remain in equilibrium at on interstitial site and hence move here and there abruptly. The above figure depicts self diffusion of a crystal.
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