When two individual islands coalesce, they form one island as shown in the figure. If the islands are movable, they may collide with each other while moving and coalesce. This process is known as dynamic coalescence or Smoluchowski ripening, named after Polish physicist M. Smoluchowski, who devised the kinetic theory of colloid particles coarsening in accordance with this process in 1916. Static coalescence occurs when fixed adjacent islands coalesce due to an increase in their size as a result of sputtering.

Ripening of islands is sometimes also called Ostwald ripening after the name of W.F. Ostwald, who described a change in the size of pellets in a solid solution due to the diffusion of particles among pellets in 1900. In the case of two-dimensional single-atom islands, ripening occurs due to the system’s thermodynamic tendency to reduce its energy through a decrease in the total length of the edges of the islands (or decrease in the total surface area in the case of 3D islands). Smaller islands create a higher fractional pressure of their components around them, which results in diffusion currents from smaller to larger islands.