  We report numerical simulations of membrane tubulation driven by large
colloidal particles. Using Monte Carlo simulations we study how the process
depends on particle size, concentration and binding strength, and present
accurate free energy calculations to sort out how tube formation compares with
the competing budding process. We find that tube formation is a result of the
collective behavior of the particles adhering on the surface, and it occurs for
binding strengths that are smaller than those required for budding. We also
find that long linear aggregates of particles forming on the membrane surface
act as nucleation seeds for tubulation by lowering the free energy barrier
associated to the process.
