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Compared to materials experiencing homogeneous nucleation, heterogeneous nucleation requires supercooling to a lesser extent: only a few degrees compared to hundreds of degrees difference. Why is this?
Heterogeneous nucleation occurs at preferential sites.
Callister, fig.10.5
At these preferential sites, the wetting angle (θ, the angle between the interface and the solid-liquid free energy component) is not equal to 0, so the surface area, shape and surface free energy of the growing nucleus will change.
In this situation, the surface free energy will decrease. This means that it is easier for nucleation to occur at interfaces and surfaces as opposed to other sites.
You can derive the equations for critical radius and free activation energy for heterogeneous nucleation in a similar way to homogeneous nucleation.
Critical radius of heterogeneous nucleation:
From the equation for r*, we can see that as long as γSL (the component of γ corresponding to the solid/liquid boundary) is equal to γ (the total surface free energy) from the equation for homogeneous nucleation, the critical radius behaves as it would in a homogeneous case:
so, the critical radius is the same for both homogeneous and heterogeneous nucleation.
However, once the value of γSL changes, so does r*.
Free activation energy of heterogeneous nucleation:
From this equation, it can be shown that the activation energy for heterogeneous nucleation is smaller than the activation energy for the homogeneous case, with a factor S(θ).
This explains why heterogeneous nucleation occurs more readily than homogeneous nucleation: it requires significantly less energy to occur, corresponding to a much lower degree of supercooling.
Now test your knowledge with a short quiz!
S(θ) is a function of the wetting angle, describing the shape of the nucleation site, with a value between 0 and 1.
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