  The twist-bend nematic liquid crystal phase is a three-dimensional fluid in
which achiral bent molecules spontaneously form an orientationally ordered
macroscopically chiral heliconical winding of molecular scale pitch, in absence
of positional ordering. Here we characterize the structure of the ground state
of the twist-bend phase of the bent dimer CB7CB and its mixtures with 5CB over
a wide range of concentrations and temperatures, showing that the contour
length along the molecular direction for a single turn of the helix is
approximately equal to 2{\pi}Rmol, where Rmol is the radius of bend curvature
of a single all-trans CB7CB molecule. This relation emerges from a model which
simply relates the macroscopic characteristics of the helical structure, which
is mostly biaxial twist and has little bend, to the bent molecular shape. This
connection comes about through the presence in the fluid of self-assembled
oligomer-like correlations of interlocking molecules, arising from the
nanosegregation of rigid and flexible molecular subcomponents, forming a
brickwork tiling of pairs of molecular strands into a duplex double-helical
chain.
