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What is heterogenous nucleation?
Contrary to homogeneous nucleation, during heterogeneous nucleation clusters of atoms begin to form at the interfaces of a phase, not inside a bulk fluid. The process generally begins at lower temperatures than homogenous nucleation.
Why does it occur more easily?
The activation energy (ΔG*) for heterogeneous nucleation will always be the same or lower than that of homogeneous nucleation in the same medium.
(Red= Heterogenous, Blue=Homogenous)
As atoms begin to form a cluster on an interface, they will take up either a more flattened or more bulbous characteristic. So their volume to surface area ratios will vary. This is dependent on the properties of the materials being used and so the surface energies of these phase boundaries.
The angle, θ, between the solid-liquid, and the solid-interface boundaries is known as the contact angle. This is also the angle between the acting planes of the interfacial energy.
The activation energy for growth under this nucleation mode is the same as for homogenous but it’s equation includes an extra function, S(θ). This is a function of the contact angle and varies between zero and one, meaning the energy required for heterogeneous nucleation will always be the same or lower as for growth in the bulk fluid.
The critical radius, r*, does not change between the nucleation modes so a cluster of atoms must still reach a given size to become a meta-stable nucleus.
In summary, the energy required for nucleation to proceed is lower for heterogeneous nucleation due to the lower surface free energy and so, the degree of undercooling required is lower.
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