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Chemically frozen phase separation in an adsorbed layer.

A mechanism for the formation of adsorbate islands on a surface is proposed. It is based on the freezing of the phase separation in the adsorbed fluid, controlled by adsorption-desorption kinetics. The description relies on the Cahn-Hilliard equation complemented by source terms of chemical origin. In the weak segregation regime the model produces harmonic hexagonal or striped structures. Amplitude equations resulting from a weakly nonlinear approach give excellent agreement with the numerical simulations to explain the pattern competition. The high segregation regime is studied numerically producing structures of analog symmetries.