Capacitance method, elastic constants

The theoretical and experimental results obtained above will be compared and discussed in this section. First of all using Eq. (27), the theoretical curves were best fitted to the observed curves by adjusting A and a. The order parameters and elastic constants k i used in this computation were measured by optical and capacitive methods, respectively. Parameters A=10 and a=1.50 were selected by a least sguares method. Here, the quantity A... 

Like in the magneto-optical method, optical detection may be applied in the electrically driven case as well. As an alternative, the cell capacitance C is a convenient measure of the transition threshold and the tilt deformation of the director field. The capac-itanee versus voltage curves can be used to determine the dielectric and elastic constants and the surface tilt angles [ 19]. A", is given by the critical field and 33 is obtained from the slope of the C U) curve. In the... 

Before giving analytical expressions for the director deformations in Freedericksz cells, we will summarize the magnetic and electrical methods. The advantage of electro-optical measurements is that the cell thickness does not enter the equations and is therefore ruled out as an error source. Furthermore, the electric field can always be considered strictly perpendicular to the sample plane. On the other hand, in the electric method conductivity effects can influence the measurements and exact knowledge of and is required to extract the second elastic constant from the birefringence or capacitance characteristics. Moreover, the electric measurement is restricted... 

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