  The mechanical properties of single fibres are highly important in the paper
production process to produce and adjust properties for the favoured fields of
application. The description of mechanical properties is usually characterised
via linearized assumptions and is not resolved locally or spatially in three
dimensions. In tensile tests or nanoindentation experiments on cellulosic
fibres, only one mechanical parameter, such as elastic modulus or hardness, is
usually obtained. To obtain a more detailed mechanical picture of the fibre, it
is crucial to determine mechanical properties in depth. To this end, we discuss
an atomic force microscopy-based approach to examine the local stiffness as a
function of indentation depth via static force-distance curves. This method has
been applied to linter fibres (extracted from a finished paper sheet) as well
as to natural raw cotton fibres to better understand the influence of the pulp
treatment process in paper production on the mechanical properties. Both types
of fibres were characterised in dry and wet conditions with respect to
alterations in their mechanical properties. Subsurface imaging revealed which
wall in the fibre structure protects the fibre against mechanical loading. Via
a combined 3D display, a spatially resolved mechanical map of the fibre
interior near the surface can be established. Additionally, we labelled fibres
with carbohydrate binding modules tagged with fluorescent proteins to compare
the AFM results with fluorescence confocal laser scanning microscopy imaging.
Nanomechanical subsurface imaging is thus a tool to better understand the
mechanical behaviour of cellulosic fibres, which have a complex, hierarchical
structure.
