  The effective surface dilatational viscosity is calculated of a flat
interface separating two immiscible fluids laden with half-immersed
monodisperse rigid spherical non-Brownian particles in the limit of high
particle concentration. The derivation is based upon the facts that (i)
highly-concentrated particle arrays in a plane form hexagonal structure, and
(ii) the dominant contribution to the viscous dissipation rate arises in the
thin gaps between neighboring particles.
