Heterogeneous Nucleation

In practice heterogeneous nucleation occurs more readily than homogeneous nucleation. It requires less energy than homogeneous nucleation. In case of heterogeneous nucleation, vapors condense on phase boundaries (solid surfaces). These solid surfaces are generally found as impurity within the vapors. Due to lower surface energy, the free energy barrier reduces and facilitates nucleation at these preferential sites. Surfaces having contact angles between phases greater than zero encourage particles to nucleate.

Fig. 5.5: Schematic diagram of heterogeneous nucleation on a solid surface

The physics of the problem remains same as that of homogeneous nucleation except that we have to account for surface tension of solid phase for calculation of free energy change .

Assumptions:

• Size of impurities present is larger than the critical radius of the nucleus. So we can assume solid surfaces as flat.
• The nucleus is part of a sphere. This assumption is valid since the effect of gravity is negligible for small nuclei (diameter around 10 to 100 nm). Along with this we also neglect the effect of van der Waals interactions which in reality will be significant for such small dimensions.