A simple hydrodynamic model has been developed to explain the experimentally observed chirality selection in stirred solutions of self-assembling achiral dyes. Selection depends on the stirring direction: the dichroic signal reverses its shape in clockwise or anti-clockwise rotations. Our model investigates the possible role of the liquid-solid interface in nucleating, growing, and transferring to the bulk of chiral seeds. The nucleation step requires a double modulation of the hydrodynamic field exhibiting different velocity along two orthogonal axes. Under a series of restrictions, such a condition is easily met at the solid-liquid interface and it is dictated by the boundary conditions and geometry of stirring. In stagnant conditions, growing helices made-up of self-assembled achiral dyes have no chiral preference forming a racemic mixture that contains identical amount of right-handed (R) and left-handed (L) configurations. The application of a hydrodynamic torque (related to the velocity gradient and width of the helix) breaks down the original symmetry, a further velocity gradient perpendicular to the first one ensures, after averaging, a slightly different population of R and L conformations. The yields of the hydrodynamic-induced chirality excess are extremely tiny, hence the suggested mechanism is significant only if next chirality amplification processes are efficient. Again, hydrodynamics provides a tool for the detachment of weakly bound aggregates once they have reached a critical length. Aggregates are transported in the bulk where the ripening process goes to completion. The efficiency of the surface catalytic effect strongly depends on the aggregate-surface sticking energy, reaching a maximum at intermediate sticking energies (of order of 10 kT). Numerical estimates show that the proposed mechanism is rather efficient, giving rise to entatiomeric excesses near (but smaller than) those experimentally found.
Coloring Agents Hydrodynamics