  The growth of laminar-turbulent band patterns in plane Couette flow is
studied in the vicinity of the global stability threshold R_g below which
laminar flow ultimately prevails. Appropriately tailored direct numerical
simulations are performed to manage systems extended enough to accommodate
several bands. The initial state or germ is an oblique turbulent patch of
limited extent. The growth is seen to result from several competing processes:
(i) nucleation of turbulent patches close to or at the extremities of already
formed band segments, with the same obliquity as the germ or the opposite one,
and (ii) turbulence collapse similar to gap formation for band decay. Growth
into a labyrinthine pattern is observed as soon as spanwise expansion is
effective. An ideally aligned pattern is usually obtained at the end of a long
and gradual regularisation stage when R is large enough. Stable isolated bands
can be observed slightly above R_g. When growth rates are not large enough, the
germ decays at the end of a long transient, similar to what was observed in
experiments. Local continuous growth/decay microscopic mechanisms are seen to
compete with large deviations which are the cause of mesoscopic nucleation
events (turbulent patches or laminar gaps) controlling the macroscopic
behaviour of the system (pattern). Implications of these findings are discussed
in the light of Pomeau's proposals based on directed percolation and
first-order phase transitions in statistical physics.
