  In experiments on model membranes, a formation of large domains of different
lipid composition is readily observed. However, no such phase separation is
observed in the membranes of intact cells. Instead, a structure of small
transient inhomogeneities called lipid rafts are expected in these systems. One
of the numerous attempts to explain small domains refers to the coupling of the
membrane to its surroundings, which leads to the immobilization of some of the
membrane molecules. These immobilized molecules then act as static obstacles
for the remaining mobile ones. We present detailed Molecular Dynamics
simulations demonstrating that this can indeed account for small domains. This
confirms previous Monte Carlo studies based on simplified models. Furthermore,
by directly comparing domain structures obtained using Molecular Dynamics to
Monte Carlo simulations of the Ising model, we demonstrate that domain
formation in the presence of obstacles is remarkably insensitive to the details
of the molecular interactions.
