  Despite the extensive research that has been conducted for decades on the
behavior of confined liquids, detailed knowledge of this phenomenon,
particularly in the mixed/boundary lubrication regime, remains limited. This
can be attributed to several factors including the difficulty of direct
experimental observations of the behavior of lubricant molecules under
non-equilibrium conditions, the high computational cost of molecular
simulations to reach steady state, and the low signal-to-noise ratio at
extremely low shear rates corresponding to actual operating conditions. To this
end, we studied the correlation between the structure formation and shear
viscosity of octamethylcyclotetrasiloxane confined between two mica surfaces in
a mixed/boundary lubrication regime. Three different surface separations
corresponding to two-, three-, and five-layered structures were considered to
analyze the effect of confinement. The orientational distributions with one
specific peak for $n=2$ and two distributions, including a parallel orientation
with the surface normal for $n>2$, were observed at rest. The confined liquids
exhibited a distinct shear-thinning behavior independent of surface separations
for a relatively low sliding velocity, $V_{\rm x}\lesssim 10^{-1}\,{\rm m/s}$.
However, the shear viscosities at $V_{\rm x}\lesssim 10^{-1}\,{\rm m/s}$
depended on the number of layered structures. Newtonian behavior was observed
with a further increase in the sliding velocity. Furthermore, we found a strong
correlation between the degree of molecular orientation and the shear viscosity
of the confined liquids. The magnitude of the shear viscosity of the confined
liquids can primarily be determined by the degree of molecular orientation, and
shear-thinning originates from the vanishing of specific orientational
distributions with increasing sliding velocity.
