A combination of thin- and thick-plate theories, and finite element
models is used to systematically analyze folding in multilayer stacks.
We show that if the interlayer spacing is large, individual layers fold
as single layers, if the spacing is small the entire stack folds as one
effective single layer. In between, a third folding mode exists that is
characterised by a dominant wavelength that scales with n(1/3),
irrespective of total number of layers, n. The maximum growth rates in
the true multilayer-folding mode are higher than the corresponding
single layer growth rates, increase with n and are bounded by a
saturation value that is directly proportional to the viscosity
contrast. This growth rate saturation as well as the applicability of
the true multilayer-folding mode with respect to interlayer spacing can
be explained by the normal and inverse contact strain theory. The true
multilayer-folding mode is expected to be the most frequent mode in
nature, because it exhibits the highest growth rates and has a
relatively large applicability range with respect to interlayer spacing.
The increased growth rates in multilayer folding are especially
important for systems where the corresponding single layer values are
not sufficient to drive the folding instability, such as folding in
low-viscosity contrast layers and detachment folding.