Wolfgang Helfrich

Ferroelectrically switchable columnar liquid crystal

Abstract The synthesis of a new disc-like mesogenic compound permitted, for the first time, ferroelectric electrooptical switching in a tilted columnar liquid crystal. The spontaneous molecular dipole moment is induced by the bend between the dibenzopyrene core and part of the eight O-hexyllactic acid chains attached to it. Two similar new compounds displayed a weak electrooptical effect in the columnar phase.

Two ferroelectric phases of a columnar dibenzopyrene

Abstract The ferroelectric switching of columnar 1,2,5,6,8,9,12,13-octakis-((S)-2-heptyloxy-propanoyloxy)dibenzo[e,l]pyr-2-heptyloxy-propanoyloxy)dibenzo[e,1]pyrene was studied in detail between 60°C and the clearing point 115°C. The switching angle (optical tilt angle) is ±24.5° up to 10V μm−1 and ±37° at higher field strengths. The electric polarization is 60nC cm−2 in the low field phase and 180nC cm−2 in the high field phase. The switching rate has apparent activation energies that increase with voltage from 3 × 10−19 to 10−18 J. It varies roughly with a power of the voltage, the exponent increasing with decreasing temperature from 2 to 5. At equal voltages, switching is faster in the low field than in the high field phase. We tentatively infer the structures of the two columnar lattices from the ratio of polarizations and other data. From the switching angles, we calculate a tilt angle of 44° for the aromatic cores of the disc-like molecules stacked in the columns. Finally, we point out possible adva...

Invited Lecture. Hats and saddles in lipid membranes

Abstract Extrinsic or intrinsic defects in membranes may produce deformations resembling hats or saddles. It is shown that hats cannot absorb much area without reducing the bending rigidity of the membrane below 10− erg. Estimates indicate that intrinsic saddles are promoted by the stress profile of electrically neutral biological model membranes. The saddle structures might warp the membrane if they cooperate.

Field dependent switching angle of a columnar pyrene

Abstract The switching behaviour of 1,3,6,8-tetrakis-((S)-2-(heptyloxy)propanoyloxy)pyrene was studied near room temperature in a monotropic columnar phase. The switching angle (optical tilt angle) increases linearly with the applied electric field from 0° to c. ± 10° at ± 0.35°/(V μm −1), then rapidly increases to its upper limit of 22°. The electric polarization is roughly proportional to the switching angle with a maximum value of 110nC cm−2 at high field strengths. At field strengths up to 10 V μm−1, a voltage and temperature independent 0 to 90 per cent switching time of c. 650 μs was measured. The observations are explained in terms of deformation and unwinding of an intracolumnar helix.

Inherent Bounds to the Elasticity and Flexoelectricity of Liquid Crystals

Abstract Phenomenological and molecular considerations indicate certain limitations to the elastic moduli and flexoelectric coefficients of liquid crystals.

Spontaneous and Induced Adhesion of Fluid Membranes

Soft membranes have recently been shown to possess many novel properties due to their out-of-plane fluctuations. In particular, spontaneous adhesion including a temperature-driven unbinding transition and adhesion induced by lateral tension have been observed with electrically neutral biological model membranes. We review existing theories of adhesion in terms of direct forces and undulation forces. An outline of the experimental results is also given. Some contradictions between theory and experiment suggest an unknown submicroscopic roughness of these membranes. It should far exceed the contribution of undulations, i.e. of the out-of-plane fluctuations controlled by bending rigidity and lateral tension.

Electrooptically Switchable Columnar Liquid Crystals: Modeling and Conjectures

Elements of a theory of electrooptically switchable columnar liquid crystals are developed. Assuming spontaneously polarized columns of circular cross section, we consider their dipolar interaction, a special van der Waals interaction of the aromatic molecular cores, and a mean-field theory of a ferroelectric phase transition. Available X-ray data are tentatively assigned to column lattices of low dipolar energy. The agreement between theory and experiments seems satisfactory, especially when the columnar dipole moments per unit length are split and placed at a distance from the column center. However, a dependence of molecular side-chain packing energies on dipolar orientations, so far unmanageable, may intervene and determine which of the low-energy states is the ground state.