2.5 Heterogeneous Nucleation

Interactive Graph
Makers of rock candy know that sucrose crystals will nucleate preferentially from a supersaturated aqueous solution on irregular surfaces, such as a string or a popsicle stick. If the interfacial energy for the portion of a sucrose crystal nucleus contacting the string is less than the sucrose-solution interfacial energy, then it may be possible to assemble a critical nucleus with fewer atoms. Nucleus growth on the surface of another phase is called heterogenous nucleation. To quantify and help understand heterogenous nucleation, a model of a nucleus with the simple geometry of a spherical cap is provided in Figure 2.07.

The model of Figure 2.07 is similar to the model of Figure 2.06 with additional sliders to set the interfacial energy of the base of a spherical cap and to change the height (H) to base (B) ratio (H/B) for the nucleus. Click on Figure 2.07 to open a larger version and move the |ΔT| and slider to 125°C. The other parameters are preset to match those from the question on the previous page concerning the kyanite to sillimanite reaction . Note that both the hemisphere cap nucleus and the spherical nuclues have a 1 nm critical radius, but only half the number of sillimanite formula units as a spherical nucleus. Now move the slider to add interfacial energy for the base of the spherical cap. And/or move the slider to change the shape (H/B) of the spherical cap.

If the spherical cap shape factor (H/A) is reduced, will the critical volume of the spherical cap increase or decrease? Please make a choice from the following list and press "Enter":

Pick one.
Increase   Decrease   No change