  The thin-shell instability has been named as one process, which can generate
entangled structures in astrophysical plasma on collisional (fluid) scales. It
is driven by a spatially varying imbalance between the ram pressure of the
inflowing upstream plasma and the downstream's thermal pressure at a non-planar
shock. Here we show by means of a particle-in-cell (PIC) simulation that an
analogue process can destabilize a thin shell formed by two interpenetrating,
unmagnetized and collisionless plasma clouds. The amplitude of the shell's
spatial modulation grows and saturates after about ten inverse proton plasma
frequencies, when the shell consists of connected piecewise linear patches.
