  The nonlinear thin-shell instability (NTSI) may explain some of the turbulent
hydrodynamic structures that are observed close to the collision boundary of
energetic astrophysical outflows. It develops in nonplanar shells that are
bounded on either side by a hydrodynamic shock, provided that the amplitude of
the seed oscillations is sufficiently large. The hydrodynamic NTSI has a
microscopic counterpart in collisionless plasma. A sinusoidal displacement of a
thin shell, which is formed by the collision of two clouds of unmagnetized
electrons and protons, grows and saturates on timescales of the order of the
inverse proton plasma frequency. Here we increase the wavelength of the seed
perturbation by a factor 4 compared to that in a previous study. Like in the
case of the hydrodynamic NTSI, the increase in the wavelength reduces the
growth rate of the microscopic NTSI. The prolonged growth time of the
microscopic NTSI allows the waves, which are driven by the competing ion
acoustic instability, to grow to a large amplitude before the NTSI saturates
and they disrupt the latter. The ion acoustic instability thus imposes a limit
on the largest wavelength that can be destabilized by the NTSI in collisionless
plasma. The limit can be overcome by binary collisions. We bring forward
evidence for an overstability of the collisionless NTSI.
