Cholesteric order in systems of helical Yukawa rods

Abstract

We consider the interaction potential between two chiral rod-like colloids which consist of athin cylindrical backbone decorated with a helical charge distribution on the cylindersurface. For sufficiently slender helical rods a simple scaling expression is derived whichrelates the chiral ‘twisting’ potential to the microscopic properties of the particles, such asthe internal helical pitch, charge density and electrostatic screening parameter. To predictthe behaviour of the macroscopic cholesteric pitch of the fluid bulk phase we invoke asimple second-virial theory generalized to treat anisotropic states with weakly twisteddirector fields. It is shown that, while particles with weakly coiled helices always form acholesteric phase whose helical sense is commensurate with that of the internalhelix, more strongly coiled rods lead to the formation of a cholesteric state ofopposite sense. The correlation between the helical symmetry at the microscopic andmacroscopic scale is found to be very sensitive to the pitch of the Yukawa helix. Mixinghelical particles of sufficiently disparate length and internal pitch may give rise toa demixing of the uniform cholesteric phase into two fractions with a differentmacroscopic pitch. Our findings could be relevant to the interpretation of experimentalobservations in systems of cellulose and chitin microfibres, DNA and fd virus rods.