  The collision of winds from massive stars in binaries results in the
formation of a double-shock structure with observed signatures from radio to
X-rays. We study the structure and stability of the colliding wind region as it
turns into a spiral due to orbital motion. We focus on adiabatic winds, where
mixing between the two winds is expected to be restricted to the
Kelvin-Helmholtz instability (KHI). Mixing of the Wolf-Rayet wind with
hydrogen-rich material is important for dust formation in pinwheel nebulae such
as WR 104, where the spiral structure has been resolved in infrared. We use the
hydrodynamical code RAMSES with an adaptive grid. A wide range of binary
systems with different wind velocities and mass loss rates are studied with 2D
simulations. A specific 3D simulation is performed to model WR 104. Orbital
motion leads to the formation of two distinct spiral arms where the KHI
develops differently. We find that the spiral structure is destroyed when there
is a large velocity gradient between the winds, unless the collimated wind is
much faster. We argue that the KHI plays a major role in maintaining or not the
structure. We discuss the consequences for various colliding wind binaries.
When stable, there is no straightforward relationship between the spatial step
of the spiral, the wind velocities, and the orbital period. Our 3D simulation
of WR 104 indicates that the colder, well-mixed trailing arm has more
favourable conditions for dust formation than the leading arm. The single-arm
infrared spiral follows more closely the mixing map than the density map,
suggesting the dust-to-gas ratio may vary between the leading and trailing
density spirals. However, the density is much lower than what dust formation
models require. Including radiative cooling would lead to higher densities, and
also to thin shell instabilities whose impact on the large structure remains
unknown.
