Macroscopic manifestations of surface crystallization

Two rather dramatic and unusual, visually observable phenomena occurring in molten wax, are reported and explained: Shaking the liquid in a vial will yield a foam, even though no surfactant is present; dust spread on the liquid surface is seen to move laterally with surprising speed. We performed systematic measurements on a single-component wax, the n-alkane C₂₀H₄₂, visualizing the surface under linear temperature gradients and in the presence of gas slowly bubbling from below. These macroscopic effects are studied quantitatively, and shown to be macroscopic manifestations of the existence of a monomolecular crystalline alkane layer at the liquid-vapor interface. A proposed phase diagram for the thin bubble wall is shown to account for the bubbles' coalescence and bursting behavior. These results imply the existence of a new mechanism for foam stability and can be expected to broadly occur in systems containing alkyl-chains.