J. R. Lalanne and F. Hare C. A. P. E. S. Laboratory University of Bordeaux I France
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I
Three Liquid-Crystal Teaching Experiments
It is well known that liquid-crystal substances are currently used to create a new family of devices for the display of numbers and letters by scattering of amhient light. All these devices which do not need any energy for displaying action are characterized by the relative independence of the contrast ratio on brightness and direction and can he easily used for outdoor measurements. These two properties set them far apart from other classical displays and justify the intense growth of interest in this field in the past several years. Many imnortant technoloeical and commercial applications are now .. in their higher phase of development. The physical processes involved in the functims of these devices are all linked to the property of easy orientation by a weak applied voltage. They are diversified and sometimes not clearlv understood. More generally, the physics of liquib-crystals is rather complicated ( I ) and, in spite of a great number of review papers(2-9) where the reader will a s i l y find the general descriptions wanted, teachina of the physical pmpenies of these intermediate states of matter is, i t t h i s time, not very well developed. Since our laboratory is in charge of the training of secondary schod teachers, this year we have proposed a program on the subiect and reoort here on three exoeriments illustratine some tecinological 'applications of these compounds. All th& can he considered as simplified transcri~tionsof research work already performed and quoted in the literature. Using very simnle comonnents thevcan he reoroduced without difficultv in most teaching lahoratories. We shall present them, giving in each case some specific practical applications, sometimes commercially available. But the reader must know that many applied works in this area, remain unpublished at this time.
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Dldactlcal Electro-Optical Filter Without Memory We want to illustrate the physical effect called guest-host interactions in nematics (10). Cooperative alignment of nematic molecules (host) is used to orient a dve " (mest) . whose absort)ance depends upon its orientation with respect to the oolarrzation of the incident lieht. The difficult nrohlem is to find a positive nematogen (Ar = r - el > 0 where q and €1 are dielectric constants, respectively, parallel and perpendicular to the molecular axes at the frequency of the orienting field) at roum temperature, in order to avoid dynamic scattering (2). We propose the ester named Nematel105, which
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Figure 1. mructures and physical properties of compounds.
Figure 2. Electro-optical filter without memory: experimental set-up: f: focal length of t b lens; C diameter of the lens.
!m
Figure 3. Electrwptical cell
was synthesized by the C.S.F.-Thomson Company (see formula in Fig. 1). The dye used is indophenol blue a t aconcentration of about 1%. Figure 2 shows our experimental set-up. White light coming from the iodine lamp (car type halogen lamp) is focused through pinhole P by lens LI on the electro-optical cell C. Lz coniueates P and the screen S laced at some meters. The cell .(see Fig. ,3t is constructed in the form of parallel plate capacitor with transparent electrodes (gold or Sn0, coated alass). These transparent electrodes can he obtained in ~ r a n c from e NU!vlEI.EC, 2 Petite Place, 78MO-Versailles ($6 m e plate 51 X 63 X 3 mm). Typical spacing of about 40 r is simply ohtained with the use of thin polyethylene film. A rectangular frame of polyethylene is placed hetween the two glass plates, under a press, in a drying cupboard, heated a t 140-150°C, for 45 min. In this wav. the cell is convenientlvsealed. I t can then be filled with liquid crystal by a little hole'drilled through one of the olates. Two or three droos of liouid are nut over the hole and the cell is placed inside a vessel connected to a vacuum system. The air is, in this way, driven out. After a few minutes, the vessel is slowly filled with air and the liquid crystal progressively penetrates inside the cell, giving rise to a thin, homogeneous film, The hole is then sealed. Electrical connections are made by two copper f h s fixed by two holders on the ends of the plates. We have used such cells for a whole year without important modifications of their optical properties. Volume 53,Number 12 December 1976 / 793
The cell is driven by an ac supply giving voltages in the range &50 V, 50 H z 1 0 kHz. The variation of the transmission is easily shown on the screen when the voltage is suddenly switched on. The effect disappears when the electric field is switched off (no memory device) and when the temperature of the sample is raised above the nematic-isotropic transition temperature (4Z°C), simply with the use of a hair-dryer. This direct observation can be easily completed by a auantitative measurement of the variation of transmission hy placing the cell inside the sample compartment of a apectrometer working in the visible range. Figure 4 show the result
Figure 6. Dynamic scanering of 632.8 nrn laser light by MBBA. Temperature: 20°C applied field: 5 kV cm-'. Figwe 4. E l e ~ t r ~ p t ifilter ~ s l wimout me-: no field applied and (B) a field applied.
Absorption spectrum wim (A)
obtained with the use of a Caiy 16 spectrophotometer without any special adaptation of this apparatus. The guest-host interaction effect is, a t this time, being investigated in many applied research laboratories in order to develop new devices such as optical shutters and voltage controlled variable intensity filters. The use of both pleochroic and photochromic guests gives rise to "two colors" devices, each color being able to be erased by application of a suitable field. A recent technical publication (11) reports the use of this effect in conjunction with colored indicators. Didactical Electro-Optical Swltch Without Memory
The physical effect used in the experiment is now dynamic scattering (2).Ionic impurities contained in nematic solutions are pulled by the electric field. If the nematogen molecule is a negative one (At = rll - e l < O), the axes of the molecules are perpendicular to the field and the ions cannot pass through such an arrav without creating large disturbances in the sample. ~ i g h i iscattered s wherias thk transmitted intensity decreases. Figure 5 shows our experimental set-up which is the simplest possible. The laser is a Spectra Physics model 133 of 1 mW. The nematogen used ~ S - M H H A(see Fig. 1). Figure 6 illustrates the observed effect for an applied voltagef' about 20 V. This cloudy picture must be connected to diffraction of laser light by large regions of nematogen in the range 1-5 fi. At about 10 kV cm-1, all the laser light is scattered. When the ~
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Figure 5. Eiectrwptical switch without memory: exprimentai set-up
794 / J o u r ~of l Chemical Education
frequency of the applied field increases, ions are progressively trapped and the effect disappears. In another way, when the voltage is removed, the panel returns to its transparent state (no memory device). By replacing the screen by a photocell coupled to a recorder, it becomes possible to perform quantitative measurements. Fieures 7.8, and 9 eive such variations of transmission with applied voitage, frequency, and time. I t is possible to use the optical set-up described in Figure 5, without any important difficulty if the He-Ne laser is not available in the laboratory. The direct observation remains possible in a dark room. In order to use common components, the quantitative measurements reported in Figures 7,8, and 9 can be obtained by inserting the electro-optical cell in the compartment of a spectrophotometer, the working wavelength being arbitrarily chosen a t the beginning of the experiment. Dvnamic scatterine has been used in manv devices. some of tl;em at this time &mmercially available. i ~ can e cite, for examole.aloha-numeric indicators (121:matrix dis~lavs113). and fiyidg spot scanner (14). But, i t must be mentioned that the use of the electro-ootical properties of twisted-nematics seems to be, a t this time, the most promising. Didacllcal Electro-Optical Switch With Memory This experiment is simply performed by a little modification of the chemical composition of the liquid. The physical effect used, also caused by moving ions, is called storage mode (2). We have mixed MBBA with cholesteryl chloride a t a weieht ratio of nine to one and repeated the orecedine exper&ent. We show that, in contrast to dynamic scattering, the milkv. apoearance of the cell and the small transmission .. of the incoming light remain after the electrical field is removed. conferrine properties to the samole. The " memorv .. . dynamics of recovering transparency can be followed by the ohotocell. The observed characteristic time is a few hours. The information can be quickly rubbed out by the application of 50 V a t 1kHz. The storage mode has been used in optical conversion devices (15).In these. an imaee can be held bv the liauid crvstal device bkcause of the "memory" of the disturheb nematiccholesteric mixture. This imaee can be reoroiected with the possibility of varying the waveLngth of the.light. For instance,
F i g m 7. E l e c b q t i c a l switch w i m a n me-: applied voltage.
variations of bansmhsion wRh
the use of photoconductor electrodes allows ultraviolet recording followed by reprojection in the visible range. In conclusion, we propose three experiments in the field of chemical physics of nematic and cholesteric liquid crystals. All are characterized by the choice of molecules showing their cooperative properties a t room temperature and with the use of very simple electro-optical devices. We hope they can hecome the experimental support of theoretical teaching in a field no longer regarded as a laboratory curiosity because of the recent increase of its technological applications.
Figwe 8. E l e c b w p t i c a i switch without m e m a y : variations of bansmission with frequency.
Acknowledgment One of us (J.R.L.) wants to thank Helene Harvey for im-
provement of the manuscript.
.. (2) Heilmeier, G. H.,S&nti/ic American. 1 W (April 19701. (31 Lo Cmupodss C~iataurLiquidea d'orsay, Lo Rachwcha, 12.2.433 119711. 141 Science. 1.20l1971l. 151 ln/orm. Chimis. 33.85 11971). 161 Baltzer. D.H.. "Liquid Crystals: Vari-Light Corporation, Cincinnati, Ohio 45242.
,"*. .3n,.
171 Brom.G. H.,andDoane,J. W.,Appl.Ph~a..4.1 119741. (8) Pelrie, S. E. B., Bilcher. H. K..Klinyhiel, R. T., end Rose. P. I.Eortmon . Orymnic Chamieol Rulldn. 4s. 2 (19731. 19) Ellioft.G.,Cham. Rrif.. 1.213 119731. 110) Hrilmcier, G. H., Castellsno, J. A.. and Zanoni. L A,. Mol. C r m end Liq. C r p l . , 8. 7. 9 1 119M . .~~~.,. (11) Casbllano.J.A.Heilmeisr,G. H.,Paaierb,E.F..and MeCaffrey. M.T.."Electrunieally-Tund Optical Filter." Eleetmnica Resasreh Center. N.A.S.A..eontrsct NAS 12-638,1963. I121 Hcilmeior,G. H.,Appl. En#, 2.21 119681.
Y the i ~ r c
Figure 9. Electrmptical switch w i m a n memory: variations of tansmission wilh time. I131 Leehner, B. J.. Mar1owe.F. J., N ~ t e rE. , 0.. and TTIU,J., RRPPNof the Inbrnatimd Solid State Circuit. Conference Universityaf Pennsyhnii, Philedelphia, 1969. 1141 Soref.R.A.,Appl. Optic& 9.1323 (19701. I151 Malgetum. J . D., Nimoy, J.. and Won& S. Y., Appl Phyr Lett., 17.51 119701.
Volume 53,Number 12, December 1976 / 795
