Polar and quadrupolar order in smectic liquid crystals

In several smectic phases the long molecules are tilted towards the layer planes. The molecules in the layers of smectic C phases have a preferred tilt direction with a C2 rotation axis perpendicular to the tilt plane. If the molecules have a brick-like shape, a configuration possessing the D2h symmetry is also possible. For molecules shaped like chevrons or bananas, the existence of a smectic phase with the symmetry Cv was recently reported. We consider different in-plane configurations of smectics using a geometrical approach based on the 'scaled particle theory'. Varying the geometrical parameters of hard rod particles, a phase diagram for several smectic configurations is predicted. Depending on the particle shape, phases with dipolar order (C2,Cv) and quadrupolar order (D2h) can be stable.     

Interaction of colloidal molecules adsorbed on soft solid films

Colloidal molecules adsorbed on soft solid films interact indirectly via elastic substrate deformations. For this substrate mediated attraction force an effective pair potential can be evaluated by using the elasticity theory for solids. The range of the indirect force increases with increasing film thickness. For very soft substrates the interaction is found to be strong enough to influence the structure and the thermodynamic properties of the adsorbate, e.g. enforce aggregation or crystallization.     

Molecular statistical model for adsorbed membrane particles

Indirect membrane-mediated forces influence aggregation, segregation and freezing processes of large particles inserted into a membrane or adsorbed to its surface. Colloidal particles, polymers, peptides and proteins are the source of deformations of the lipid double layer. These perturbations cause membrane-mediated attractive or repulsive forces between the particles. A molecular statistical treatment is useful for investigating the lateral organization of peripheral and trans-membrane peptides. To describe aggregation and freezing phenomena a density functional, which includes conventional short-range forces and membrane-mediated effects, is proposed. The method is applied to an ensemble of adsorbed peptides. It is investigated how the elastic splay-distortion modulus, the surface tension, the compression-expansion modulus of the bilayer and the cross-section of adsorbed particles influence aggregation and freezing. Assuming conventional values for the material parameters, membrane-mediated attractive forces are found to be sufficiently strong to enforce aggregation.