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Mixtures of Positive and Negative Anisotropy Nematics

IP.com Disclosure Number: IPCOM000082220D
Original Publication Date: 1974-Oct-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 4 page(s) / 47K

Publishing Venue

IBM

Related People

Freiser, MJ: AUTHOR [+2]

Abstract

Since the unforced relaxation of nematic devices is ordinarily slow, there have been made devices wherein, with a suitable choice of materials, a single set of electrodes can both drive a cell ON and force it back to its OFF state.

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Mixtures of Positive and Negative Anisotropy Nematics

Since the unforced relaxation of nematic devices is ordinarily slow, there have been made devices wherein, with a suitable choice of materials, a single set of electrodes can both drive a cell ON and force it back to its OFF state.

In Fig. 1 there is illustrated a type of nematic device, which is based upon the variation of the orientation and the index of refraction of the nematic fluid by an applied electric field, wherein a nematic fluid film 10 is contained between two transparent electrodes 12 and 14. The surfaces of electrodes 12 and 14 have been treated to cause film 10 to be optically uniaxial with the optic axis parallel to the surface, when the voltage V applied across the film = 0.

If the nematic fluid constituting film 10 is chosen such that it has positive dielectric anisotropy, then when a sufficient voltage V is applied across film 10, the optical axis of the bulk of the fluid constituting film 10 will align itself in the direction of the applied field. In this case, when the cell constituted by electrodes 12 and 14 and film 10 is disposed between cross-polarizers 16 and 18, there will be an extinction of light. In the quiescent, parallel aligned state, the cell will be transmissive.

A disadvantage presented in the use of the type of device depicted in Fig. 1 is the slow relaxation from the driven to the quiescent state, a relaxation which is driven by internal elastic forces and slowed by viscous forces in the liquid medium. For example, the characteristic time for relaxation is: Tau = Gamma 1/2/ Over k Pi /2/. where Gamma is a viscosity, k is an elastic constant and l is the thickness of the cell. Some typical values for room temperature nematic fluids are Gamma Approximately 0.5 poise, k Approximately 10/16/ dyne. Thus, in a cell 20 Mu thick, there is obtained a Tau Approximately 200 msec. For many purposes, it would be desirable to shorten the latter time. One way of so doing is to employ auxiliary electrodes which provide a field orthogonal to the driving field. This is difficult in the flat-type film configuration. Alternatively, there is employed a liquid whose dielectric anisotropy changes sign at some convenient frequency, f(R). Thus, with the same set of electrodes, the cell is driven ON (with DC or AC of a frequency f < f(R)) or OFF (at frequency f > f(R)). The effect on the relaxation time is given by the expression: Tau = Gamma 1/2/ Over [(Absolute Value of Triangle Epsilon) Over 4 Pi] V/2/(rms) + k Pi /2/. where here, at a frequency greater than f(R), Triangle Epsilon = Epsilon (11) - Epsilon is negative. Thus, if (Absolute Value of Triangle Epsilon) = 0.5 and V(rms) = 120 volts = 0.4 statvolts, there is obtained a reduction in the relaxation time from 200 msec to
0.3 msec. In other words, with a given set of electrodes, the optic axis can be driven into either one of two orthogonal directions.

Fig. 2 illustrates the case using the same device as sh...