A Simple Experiment to Study Liquid Crystal Phase Transitions*

Transcription

1 A Simple Experiment to Study Liquid Crystal Phase Transitions* S.M. KHENED AND V.T. DESHPANDE Department of Physics, L.V.D. College, Raichur ABSTRACT Liquid Crystals are the intermediate phases between a crystalline solid and an isotropic liquid. They are self-organizing fluids and their properties can be changed by the application of electric or magnetic fields. This allows them to be used in display and many other devices. The Subject of Liquid Crystals has become a part of the curriculum at intermediate and degree levels. In this paper, we report a simple experimental method to observe the liquid crystal textures and to study the liquid crystal phase transitions utilizing the equipment that are usually available in an undergraduate physics laboratory. *This work was presented at the National Seminar held at the G.T.B. Khalsa College, Jabalpur, November 2-4, Introduction In addition to the three well-known forms of matter, viz., solid, liquid and gas, certain organic materials show intermediate phases between crystalline solids and isotropic liquids, which are thermodynamically stable. These phases exhibit rheological behavior similar to liquids but have anisotropic properties of crystalline solids and hence are termed as liquid crystal (LC) phases. 1-3 Liquid crystallanity can be realized either by the action of heat (thermotropic LC) or by the action of solvent (Lyotropic LC). Here we limit Physics Education September October

2 ourselves to the former variety. A thermotropic liquid crystal material exhibits the LC phase upon melting the solid material and on further heating, the LC phase goes over to an isotropic liquid at a temperature called the clearing point. Between the melting and clearing points the material exhibits LC phase. For a material to exhibit LC phase, shape of the molecule should be anisotropic. For example rod like, disc like and banana shaped molecules exhibit LC phases. However, vast majority of thermotropic LCs are composed of rod-like molecules. The LC phases exhibited by them are broadly classified into three typesnematic, cholesteric and smectic. Optical polarizing microscopy, differential scanning calorimetry and X-diffraction are some of the important techniques employed to identify as many as 50 different types of LC phases. The unusual combination of anisotropic behavior and fluidity has made liquid crystals exhibit characteristic optical properties that, particularly in the case of nematic, can be modified by applying a low magnitude external (especially electric) field. Cholesteric LCs are employed as temperature monitoring devices, which are used for monitoring neurological and vascular pathways, detection of tumors, stress and mood sensors and detection of skin temperature. Amenability to modification by external fields lies at the heart of a number of practical applications of liquid crystals 4. Liquid Crystal Displays (LCDs) are ubiquitous today appearing as displays in wristwatches, pocket calculators, mobile phones, video games, pocket diary, portable computers, camera view finder, projection TV, etc. Low power consumption, sleeker appearance and nonemission of the electromagnetic radiation are some of the main advantages of LCD over the traditional CRT displays. Owing to their large viscosity, certain types of liquid crystals are used in hydraulic and aerodynamic equipments. The field of liquid crystals is a highly interdisciplinary one, with synthesis by organic chemists, characterization by physicists and device applications by technologists. This fascinating soft condensed matter material has attracted the attention of active workers in research and technology. The study of LC at the college level is confined to only reading about the LC, without undertaking any experimental observations on their textures or phase transitions. This is because the study of liquid crystal needs often expensive equipments like polarizing microscope, temperature controlled hot-stage etc. Thus it is rather difficult to study these systems in colleges. Here we report a simple experimental method to observe the liquid crystal textures and to study the liquid crystal phase transitions utilizing the equipment available in the physics laboratory of any college along with a home-built low cost heater. # 2. Experimental The aim of the experiment is to study the phase transitions by observing the textures while heating or cooling a liquid crystal sample. The apparatus consists of a slide with a known liquid crystal sample, travelling microscope, a pair of polaroids, a source of light and a specially designed and constructed heater which enables varying the temperature of the liquid crystal sample to bring about the phase transitions of interest. # Note: Such of the teachers, who are willing to conduct this experiment in their Colleges, may contact the authors for any help in the construction of the heater or conducting the experiment. Further the authors are willing to demonstrate the experiment in their colleges if requested. The teachers may feel free to contact authors at the address given above or call on for any help. 182 Physics Education September October 2007

3 2a. Sample slide At one end of a clean dry glass slide a small speck of liquid crystal material (in our case 4- n-octyloxycyanobiphenyl, a compound available commercially from Sigma Aldrich Co. Ltd., which exhibits Nematic and Smectic A LC phases) is placed and on top of it kept another piece of thin glass (usually called cover slip) is placed. When the slide is heated, at a particular temperature the sample melts and spreads uniformly between the bottom glass plate and the top cover slip and held in place due to surface tension forces. This slide can be used for future texture observations and hence to determine the phase transition temperatures. 2b. Liquid crystal textures Unlike ordinary liquids, liquid crystals exhibit the property of birefringence, i.e., the phenomenon of double refraction that is found in anisotropic crystals like calcite or quartz. Along any general direction of observation, the optics of the material can be described in terms of two refractive indices. The LC studied here is optically uniaxial with the rod like molecules preferentially oriented along a direction which is the optic axis of the material. When the optic axis is perpendicular to the glass slides the refractive index will be equal to the ordinary refractive index. Consequently, for this orientation the field of view under the microscope will be dark. In contrast, if the optic axis is lying in the plane of the glass plate then one could observe extinction in two orthogonal settings of the polaroids. In a general setting the field of view will be bright. Depending on the local orientation of the molecules, a thin layer of such a liquid crystal film can give rise to beautiful defect patterns (textures) that are unique to the particular type of LC phase. For example, the nematic phase generally exhibits a thread-like (nema means thread in Greek) texture. The other liquid crystalline phase, namely smectic A, exhibited by our sample exhibits the focal conic fan shaped texture, characteristic of layered smectic LC structures. The observation of the texture thus provides a simple yet powerful way to determine the phases and phase transitions. 5,6 2c. Construction of the sample heater As mentioned in the earlier section, the observation of phase transitions needs variation in the sample temperature; to achieve this, a temperature controlled heating block is essential. The heater is made up of a rectangular copper block of dimension 10cm 6cm 3cm. At the center of the block a slot parallel to its length is made to hold the glass slide that has the liquid crystal sample. (A schematic diagram of the heater is given in Figure 1.) The block is wrapped with nichrome wire strip (which serves as the heating element) by taking care that the wires do not get electrically shorted and that the thermal power is uniformly spread out ensuring a uniform heating of the sample cell placed inside the block. The copper block is covered on all sides by thick PTFE (Teflon) sheets, to provide proper thermal insulation. A small circular hole (diameter ~ 5 mm) running through the body of the heater served as an optical window to observe the liquid crystal sample. When the heating element leads are connected to an electrical power supply, preferably a DC power supply, current flows through the resistive heating element and heats the copper block, in turn the sample inside the block. The temperature is measured using a two-junction thermocouple whose hot junction is embedded inside the block and the cold junction in a copper rod placed inside a flask filled with melting ice. The thermo emf produced in the thermo couple is measured using a microvoltmeter. To calculate the temperature from the thermo-emf data, the thermo couple is Physics Education September October

4 calibrated using several liquid crystalline materials of high purity and known transition temperatures. Such a plot of the transition temperatures and the thermo emf is given in Figure 2. The solid line represents a fit to the data to a linear relation T = A 1 X + A 0 where T and X are respectively the temperature and the thermo-emf. The values of constants A 0 and A 1, determined from a least-square fit of the experimental data using the Microsoft Excel software are also given. Using these constants the thermo-emf value measured experimentally can be easily converted into temperature. (It is also possible to use certain type of commercial hand-held low cost digital multi meters; for example MASTECH M890G with the facility of directly measuring the temperature in degree Celsius). 2d. Working A beam of light from a conventional optical source S, falls on a plane mirror M inclined at 45 o to the incident beam (see Figure 3). The reflected light passes through the window before entering a traveling microscope. The incident light is polarized by keeping a polaroid P between the source and the mirror and a second polaroid acting as the analyzer is placed on top of the heater (more conveniently attached to the eyepiece of the microscope). The analyzer is rotated and set for extinction of the light. Now the slide with the sample is inserted inside the heater and viewed through the microscope. The sample in the crystalline state, due to its birefringence, appears with sparkling colours between the crossed Polaroids exhibiting the optical anisotropic nature of the crystal (see Figure 4(a)). The sample is heated by applying a DC voltage to the heating coil and the temperature of the heater is monitored. At a particular temperature, specific to the material, the sample melts and goes to the liquid crystalline state (Smectic A phase in this case) this is confirmed by the appearance of the characteristic focal conic texture of the smectic A liquid crystalline phase, shown in Figure 4(b) on further heating the sample goes from Smectic A to Nematic phase at 65 0 C. This is confirmed by the schlieren texture, of the nematic liquid crystal, shown in Figure 4(c). The fluid nature of the phase can be confirmed by poking a needle on the cover slip. On further heating, the sample goes to the isotropic liquid state at a clearing temperature of 80 C, which is marked by the disappearance of the liquid crystal texture and the field of view becoming dark (Figures 4(d) and 4(e)). On cooling from the isotropic state, the nematic texture reappears as the material enters the liquid crystalline state at nearly the same transition temperature as obtained in the heating cycle. On further cooling the sample goes to smectic A phase and then it crystallizes at lower temperature. The disappearance of the nematic texture while heating and its reappearance on cooling, at nearly the same temperature, convinces the students of the existence of stable liquid crystal state between the solid crystalline and isotropic liquid states. 3. Conclusions We have shown that, the facilities available in an undergraduate physics laboratory is sufficient to demonstrate the solid to liquid crystal and liquid crystal to isotropic liquid, phase transitions by observing their textures under a travelling microscope fitted with a polaroid. Further this experiment can also be used to study the textures of different LC phases with associated phase transitions. The main drawback of the technique is the limitation in the optical magnification arising from the inability to use very short focal length objectives owing to the thickness of the sample heater. However, this apparatus provides the magnification, sufficient enough to observe and distinguish different liquid crystal textures. 184 Physics Education September October 2007

5 Figure 1. Sample heate Figure 1. Sample heater (sectional view). TS Teflon shield, FLC Sample cell, CB Copper block, TC Thermocouple, H Heater, S Screw to fix the sample cell, W Window for optical observation y = x R 2 = Figure 2. Temperature calibration plot, solid line fit to the equation T = A 1 X + A 0. Physics Education September October

6 EXPERIMENTAL SET-UP Traveling Microscope Sample Heater A S P M Figure 3. Experimental set-up. S- Source of light, P-Polarizer, M- Mirror, A- Analyzer. Figure 4(a). Crystal at room temperature. Figure 4(b). Focal conic fan texture of smectic A 186 Physics Education September October 2007

7 Figure 4(c). Thread like texture of nematic. Figure 4(d) At the transition from Nematic to Isotropic (see the brushes) Figure 4(e) Isotropic. Acknowledgements The work presented here was conducted as a part of the Minor Research Project sanctioned by the University Grants Commission, New Delhi. The authors are grateful to the Commission for the same. The authors are thankful to Dr. S. Krishna Prasad and Dr. C.V. Yelamaggad of the Centre for Liquid Crystal Research, Bangalore, for kindly providing the sample 4-n-octyloxycyanobiphenyl and the slides used for calibration and for useful discussions. Physics Education September October

8 References 1. S. Chandrasekhar, Liquid Crystals, 2nd Ed. Cambridge University Press, P.J. Collings and M. Hird, Introduction to Liquid Crystals: Chemistry and Physics, Taylor and Francis, P.G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2nd Ed. Oxford University Press, B. Bahadur, Liquid Crystals Applications and Uses, Vol I & II World Scientific, I. Dierking, Textures of Liquid Crystals, John Wiley & Sons, G.W. Gray and J.W. Goodby, Smectic Liquid Crystals: Textures and Structures, Leonord Hill (London), Physics Education September October 2007