  The propagation of chirality across scales is a common but poorly understood
phenomenon in soft matter. In this work, we use computer simulations to study
chiral monolayer assemblies formed by hard rod-like colloidal particles in the
presence of non-adsorbing polymer and characterize the thermodynamic driving
forces responsible for the twisting. Simulations show that straight (achiral)
rods assemble into monolayers with a spontaneous twist that is either left- or
right-handed, while helical (chiral) rods lead to assemblies with preferential
chiral features that depend on their handedness and curliness. The onset of
chirality in these monolayers can be traced back to small clusters formed at
the initial stage of the self-assembly. In these microscopic monolayers,
entropy drives twisting in ways that differ from the assumptions on which
existing continuum theory is built. Depending on the geometry of the
constituent rods, the preferred chiral twist can be driven by entropy gain of
the polymers, or of the rods, or both. In addition, the variation of the
polymer entropy with twist depends on changes in both the surface area and the
volume of the monolayer. Rod fluctuations perpendicular to the monolayer also
play an important role in stabilising the twisting.
