  Binary mixtures of the twist-bend nematic-forming liquid crystal CB7CB with
the prototypical rod-like liquid crystal 5CB exhibit a twist-bend nematic phase
with properties similar to those reported for neat CB7CB. The mixtures appear
homogeneous, with no micron- or nano-scale segregation evident at any
concentration. The linear dependence of the phase transition temperature on
concentration indicates that these binary mixtures are nearly ideal. However, a
decrease in the viscosity with the addition of 5CB allows the characteristic
twist-bend stripe textures to relax into a state of uniform birefringence. We
confirm the presence of nanoscale modulations of the molecular orientation in
the mixtures by freeze-fracture transmission electron microscopy (FFTEM),
further evidence of their twist-bend nature. We devise and implement a
statistical approach to quantitatively measure the ground state pitch of the
twist-bend phase and its mixtures using FFTEM. The addition of 5CB generally
shifts the measured ground-state pitch distributions towards larger pitch.
Interestingly, the pitch appears to increase discontinuously by ~10 nm at the
50 wt% concentration of 5CB, indicating that the twist-bend phase undergoes a
structural transition at higher 5CB concentrations.
