Electron scanning microscopy shows that the wool fi bre is covered by a
thin sheath of overlapping scales like the slates on a roof (Fig. 8.1). These
scales are responsible for the felting and shrinkage that occurs during laundering.
This is due to the differential frictional effect (DFE) caused by the
difference in friction between μa (friction coeffi cient when rubbing in the
against-scale direction) and μw (friction coeffi cient when rubbing in the
with-scale direction).