Experimental Methods with Permanent Director Rotation

2 Experimental Methods with Permanent Director Rotation 

Mechanical methods utilize Eq. (37) for the determination of the rotational viscosity Yi-If a rotating magnetic field H is applied to a nematic sample, the director will follow the rotation up to the critical velocity 

Nevertheless there will always be a layer at the vessel wall where the director does not follow the rotating field. The thickness 

Larger fields and larger sample volume are possible if the sample rotates instead of the magnetic field, and electromagnets can then be used [62]. The torque on the suspension wire must then be measured in a rotating system, which requires some effort. The rotation of the sample can be monitored by means of a laser beam, which is reflected from a mirror attached to the sample vessel. The reflected laser beam is detected twice in every revolution by a photodiode. If the suspension is rotated by a computer-controlled stepper motor and the photodiode interrupts a timer on the computer, the phase lag between motor and sample vessel can be determined without and with field, and the rotational viscosity can be calculated from the difference between these phase lags. The uncertainty in the determination of the rotational viscosity with this method can be as small as 1%. 

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S.3.2 Experimental Methods with Permanent Director Rotation... 

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