Zur Geschwindigkeit der Ausrichtung nematischer Flüssigkeiten durch Magnetfelder / Kinetics Of Alignment Of A Nematic Liquid Crystal In Magnetic Fields /

Abstract

The rotation of the molecules in a nematic liquid crystal is studied after rotating the axis of the aligning magnetic field. The process of realignment is observed by doping the liquid crystal with an electrolyte and measuring the cross voltage U* which is induced perpendicular to the applied electric field. The resulting cross voltages are compared with the stationary values and it can be shown that the degree of order with respect to the rotating director remains almost constant. Therefore, the time dependence of the angle 0 between director and magnetic field can be calculated from the measured cross voltage using the steady state equation U* = U*(Φ) . Φ(t) satisfies the formula ln | tan Φ | = ln | tan Φ₀ | - (χ_a H²/γ₁) t which is derived from the differential equation for the rotation of the director. This formula allows the calculation of the viscosity coefficient γ₁, giving a value of 1.08 P for MBBA at 25 °C. An activation energy of 11.5 kcal mol ^-1 is determined from the temperature dependence of the viscosity coefficient.