  A novel approach to identify internal interfacial layers, or IILs, in
wall-bounded turbulent flows is proposed. Using a Fuzzy Cluster Method (FCM) on
the streamwise velocity component, a unique and unambiguous grouping of the
Uniform Momentum Zones is achieved, thus allowing the identification of the
IILs. The approach overcomes some of the key limitations of the histogram-based
IIL identification methods. The method is insensitive to the streamwise domain
length, can be used on inhomogeneous grids, uses all the available flow field
data, is trivially extended to three dimensions, and does not need user-defined
parameters (e.g. number of bins) other than the number of zones\add{. The
number of zones can be automatically determined by an \emph{a priori} algorithm
based on a Kernel Density Estimation algorithm, or KDE.} The clustering
approach is applied to the turbulent boundary layer (experimental, planar PIV)
and channel flow (numerical, DNS) \add{at varying Reynolds numbers}.
\modify{The interfacial layers are characterized by a strong concentration of
spanwise vorticity, with the outer-most layer located at the upper edge of the
log-layer. The three-dimensional interface identification reveals a streak-like
organization; we show that the organization of the IILs is correlated to the
underlying wall-bounded turbulent structures. }
