Article pubs.acs.org/jchemeduc
A Cost-Effective Optical Device for the Characterization of Liquid Crystals Brian Millier† and Gianna Aleman Milán* Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada S Supporting Information *
ABSTRACT: The design and construction of an apparatus to measure the optical birefringence of a liquid crystal is described. The instrument also includes temperature control and monitoring circuitry to allow for the measurement of the nematic-to-isotropic phase transition temperature. An important feature of this design is that the students are able to build the optical bench section of the instrument from a kit of parts supplied. The electronics instrumentation for the illuminator, photometer, and temperature controller functions are all low-cost designs using common components, yet provide more than adequate accuracy for the measurements required. The entire instrument cost is $430, an estimated cost reduction of approximately $3000 compared to the design previously proposed by Waclawik et al. This instrumentation is suitable for second or third-year undergraduate students in materials science or physical chemistry courses. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Laboratory Equipment/Apparatus, Materials Science
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n recent years, liquid-crystal displays (LCDs) have become familiar to the general public. Currently, LCDs can be found in many electronic devices, including digital watches, calculators, computers, TVs, and mobile phones, to mention just a few. Other applications of liquid crystals less known to the general public include optical imaging, recording and switching, wave-retarders, and the visualization of RF in waveguides.1 LCDs have become a large part of the multibillion dollar electronics industry. Despite the fact that most undergraduate students are aware of the widespread use of liquid crystals, they know much less about the chemistry of the liquid crystalline state. The optical device described here was designed for a laboratory experiment that is part of a third-year materials science course in the Department of Chemistry at Dalhousie University, given concurrently as an undergraduate physics course. The experiment, previously published in this Journal,2 involves the fabrication and measurement of a twisted nematic liquid-crystal cell. It introduces students to the principles of the liquid crystalline state and its characterization. The undergraduate lab experience is divided into three parts: (i) construction of a nematic liquid-crystal cell, (ii) measurement of the optical birefringence of the liquid crystal, and (iii) measurement of the nematic-to-isotropic phase transition temperature. The instrumentation described here provides a low-cost alternative to the experimental arrangement initially proposed by Waclawik et al.2 for steps (ii) and (iii). Pedagogically, an important aspect of this design is the implementation of an optical bench (Figure 1), which can be assembled by the students from a supplied kit of parts. A © 2014 American Chemical Society and Division of Chemical Education, Inc.
Figure 1. Optical bench arrangement for the measurement of the birefringence of the liquid-crystal cell. The photometer module is not shown in this picture.
detailed description of the components required to build the optical bench is available in the Supporting Information. This feature enhances the students’ hands-on experience in the lab and fosters a better understanding of the operation of the instrument. From an instrumentation stand point, the present design includes a high brightness cool-white LED module that Published: February 14, 2014 518
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provides an adequate illumination source for the experiment, thereby eliminating the cost and safety hazards of a He−Ne laser or a laser diode module. Furthermore, this design allows for the measurement of the photodiode current using an inexpensive op amp feeding a low-cost digital panel meter module. Students are expected to manually collect transmitted light intensity versus angle data pairs. This is a less expensive alternative to the oscilloscope/NI-DAQ proposed by Waclawik et al.2 Another important feature of the new design is the use of a simpler custom-designed thermal block system for the measurement of the nematic-to-isotropic phase transition temperature shown in Figure 2. The temperature control Figure 3. The complete apparatus: optical bench (right), temperature controller (top left), and photometer−illuminator controller (bottom left).
apparatus for the measurement of the birefringence and the nematic-to-isotropic phase transition temperature. Figure 4 shows the sequence of steps for the assembling of the optical bench for birefringence measurements. The optical bench has a vertical arrangement and is built from bottom to top. All components are labeled and stored in a kit (Figure 4A). The first component is an aluminum base that contains a cool-white LED mounted at the top (Figure 4B). Four brass vertical rods are attached to the base and fastened in place using lateral screws (Figure 4C). The purpose of the rods is to support the rest of the bench components. The first polarizing film is then mounted on top of four metal spacers, which are required to ensure an adequate separation between the light source and the first polarizer (Figure 4D). Once the sample is mounted on the rotation stage (Figure 4E), it is placed above the first polarizing film by a distance defined by four additional metal spacers and O-rings (Figure 4F). The second polarizing film is crossed at 90° with respect to the polarization of the LED (determined by the first polarizing film), and placed above the sample at approximately 7 cm from the top of the aluminum base by using O-rings (Figure 4G). The photodetector is then placed approximately 3 cm above the second polarizing film (Figure 4H) and connected to the photometer−illuminator controller (Figure 4I). Finally, students measure the variation of the transmitted light intensity as the cell is rotated between the crossed polarizers. The assembly of the optical bench gives the students an opportunity to discuss and explore concepts such as polarization of light, and how it can be affected by the order of liquid-crystal molecules.
Figure 2. Custom-designed thermal block: (A) top view showing the cooling water heat exchanger, the two power resistors used as heaters, and the thermocouple, and (B) lateral view showing the thermocouple attached to the bottom surface of the thermal block in close proximity to the sample.
system uses tap water running through a heat exchanger pipe to cool the thermal block below ambient room temperature, and it includes two power resistors heaters to heat it to approximately 40 °C. Because the accuracy of the angle measured by the students from the rotation stage is limited by the scale itself, the overall accuracy of the results was not materially degraded by the inexpensive electronics and optical components chosen for this design. Rather, this low-cost device allowed for the fabrication of six units that accommodate a total of 12 students in a fourhour laboratory session. The total cost for materials for one unit was $430. This corresponds to an estimated cost reduction of approximately $3000 compared to the commercial equipment, including computer and software, described by Waclawik et al.2 This instrumentation is suitable for second- or third-year undergraduate students in materials science or physical chemistry courses.
Photometer−Illuminator Controller
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In the second part of the experiment, students measure the transmission of light through the liquid-crystal cell that they made as the first part of the experiment. This required a stable illumination source and a photometer. The custom-designed photometer−illuminator controllers (four of each unit in our case) were built by one of the coauthors prior to the experiment. Both of the controllers are basic enough to allow for their duplication by electronics staff at most universities. Both circuits were built into a single Hammond 1455T1601 aluminum enclosure. An Endor Star cool-white LED module (which can provide up to 105 lm) was used for the light source. To provide stable illumination, an LED must be fed with a constant current. Figure 5 shows the schematic diagram of the
DESCRIPTION OF THE INSTRUMENTATION As shown in Figure 3, the complete experimental setup consists of three components: the optical bench, the photometer− illuminator controller, and the temperature controller. Assembling of the Optical Bench
Prior to the measurement of the optical birefringence of the liquid crystal, students are provided with a kit that contains all the components required to build the optical bench. A detailed description of the components is available in the Supporting Information. The assembling of the optical bench takes approximately 30 min. Students work in pairs to assemble the 519
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Figure 4. Sequence of steps for the assembling of the apparatus for birefringence measurements. Each frame is described in the text.
Figure 5. Schematic diagram of the photometer and illuminator controller.
photometer and illuminator power source. To supply the LED current, an LM317T regulator IC is used, in a constant current configuration. The actual current supplied is proportional to the combined resistance of the R10 potentiometer (pot) and R9.
This current can be adjusted in the range of 11−120 mA. During the experiment, the student adjusts pot R10 to provide enough light to result in a 100% reading on the digital display. R10 is a 10-turn pot, which provides for very fine adjustment in 520
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Figure 6. Schematic diagram of the heater controller and temperature readout.
crystal cell was controlled via the custom-designed thermal block that was mounted in very close proximity to the sample (Figure 2). The assembly of the optical bench for the measurement of the nematic-to-isotropic phase transition temperature requires the same sequence of steps described above, with the addition of the mounting of a thermocouple and the custom-designed thermal block on top of the sample cell, which is then connected to the temperature controller (Figure 3). Once the unit is properly assembled, students measure the variation of the transmitted light intensity as the temperature of the cell is increased from 25 to 40 °C. The custom-designed temperature controller was built prior to the experiment. The thermal block is made of brass and contains both a heater and a heat exchanger. The heater consists of two MP915-10 power resistors connected in parallel. The heat exchanger is merely a cut out in the brass block through which a 3/16 in. copper pipe is inserted, and then soldered in place. Tap water is run though the copper pipe to bring the block to a temperature below ambient (approximately 15 °C, depending upon the actual tap water temperature). To raise the temperature of the liquid crystal, power is applied to the MP915-10 power resistors by a heater controller. The controller power is manually adjusted by the student, to slowly increase the temperature of the liquid crystal up to about 40 °C. Figure 6 shows a schematic diagram of the heater controller. A commercial 12 V, 2 A switching power supply module is used to supply 12 V of power. By adjusting the bias current of Q1, via pot R7, it is possible to provide for a heating power from zero up to about 10 W. Pot R7 is a 10-turn pot and allows the student to accurately ramp the temperature up to about 45 °C at the desired rate.
the LED light level. With the current range used, and the more than adequate heat-sinking provided by the unit’s base, the LED does not even get warm, ensuring long-term light stability. The photometer uses a BPV11 silicon phototransistor as its detector. An LTC2050HV, zero-drift, rail-to-rail op amp is used as a photocurrent-to-voltage converter. The value of resistor R2 (390 Ω) provides 0.39 V/mA output. Given the power of the illuminator, this is sufficient to provide the necessary signal to the Martell V125 digital panel meter readout, which has a 200.0 mV full scale range. Notice that the op amp uses a 4.5 V reference on its noninverting input. This provides an output signal referenced to 4.5 V. The circuitry comprising R4, R6, and pot R5 provides for a zero adjustment, also centered around 4.5 V. With the phototransistor mounted in the instrument, the liquid crystal in place, and the illuminator turned off, pot R5 is adjusted to provide a zero reading on the V125 digital readout. This nulls out any tiny amount of room light that leaks into the instrument. R5 is a 10-turn pot, allowing very fine zero adjustment. The same power supply that runs the LED is also used for the photometer electronics. However, the voltage is reduced to a regulated 9 V by R7 and Zener diode D2. Temperature Controller
The third part of the experiment involves the measurement of the nematic-to-isotropic phase transition of the liquid crystal. This is accomplished by measuring the optical transmission though the liquid-crystal cell as a function of temperature, using the same optical and electronics setup used for the birefringence measurements. The temperature of the liquid521
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As part of the controller, a digital temperature readout is also included. A T-type thermocouple is attached to the thermal block, which is in close proximity to the sample. The circuit made up of the R3 pot, R4, and TH1 (a BC1489 thermistor) provides the cold junction compensation needed for the thermocouple measurement. Pot R4 is adjusted to provide a 0.992 mV offset to the thermocouple’s negative lead, at 25 °C (this value taken from the T thermocouple tables). An LTC2050HV zero-drift, rail-to-rail op amp amplifies the small signal from the thermocouple by a factor of 24.1. This brings it up to the correct amplitude to be fed directly to a Martell V125 digital panel meter, which has a 200.0 mV fullscale range. This provides a readout with a resolution of 0.1 °C. Over the very limited 15−40 °C range, the nonlinearity of the T thermocouple is small enough to be ignored. The same 12 V power supply that is used for the heater power supply is also used for the thermocouple digital readout. This is accomplished by reducing the 12 to ±5 V, using two 5 V Zener diodes: D1 and D2. As indicated in the diagram, the ground reference for the temperature measurement subsystem is taken at the junction of D1 and D2.
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Figure 7. Average normalized intensity of transmitted light through an aligned sample of the nematic liquid crystal 5CB, placed between crossed polarizers, as a function of the angle of the polarization vector of the incident light and the orientation of the director. Vertical error bars correspond to the standard deviation of three measured values at each angle. Horizontal error bars correspond to an error of ±0.5°. The solid line corresponds to the normalized distribution given by eq 2
TESTING OF THE EXPERIMENTAL APPARATUS
Construction of the Nematic Liquid-Crystal Cell
the transmitted intensity increased to a maximum at 45°, followed by a decrease in intensity as the 90° rotation was completed. Students were able to determine the phase shift using eq 2 and to calculate the birefringence using eq 1.
In the first part of the experiment, students build a nematic liquid-crystal cell containing 4′-pentyl-4-biphenylcarbonitrile (5CB). The experimental procedure for the construction of the liquid-crystal cell has been described in detail by Waclawik et al.2
Determination of the Nematic-to-Isotropic Phase Transition Temperature
Measurement of Birefringence
Unlike solids, where atoms and molecules are bonded together in a regular pattern (i.e., lattice structure), the molecules in liquid-crystal mesophases exhibit a lack of positional order, while maintaining long-range orientational order.3 With increasing temperature, a compound that exists in the liquidcrystal state transforms from the ordered solid state, to the liquid-crystal mesophase(s), to the isotropic liquid state (i.e., state with random molecular orientations). In the third part of the experiment, students determine the nematic-to-isotropic phase transition temperature of 5CB by measuring the change in transmitted light intensity through the sample cell as a function of temperature, using the same arrangement described for birefringence measurements. Details on the experimental procedure for the measurement of the transition temperature can be found in the Supporting Information. The results are shown in Figure 8. The experimental average nematic-to-isotropic transition temperature of 5CB was determined to be 34.9 ± 0.3 °C. (The experimental arrangement described by Waclawik et al.2 yielded a transition temperature of approximately 32 °C.) The results are in good agreement with the literature value of 35 °C.4 To ensure that the nematic-to-isotropic phase transition temperature measurements are as accurate as possible, the apparatus was designed to minimize the distance between the liquid-crystal sample and the thermal block. This is achieved by mounting the thermal block directly above the Thor Laboratories rotation stage, which holds the liquid-crystal sample. Because the sample is mounted on the top surface of the rotation stage, there is virtually no separation between the thermal block and the sample. Furthermore, to ensure good thermal contact between the thermocouple and the thermal block, the thermocouple is fastened to the bottom surface of the thermal block using adhesive tape (Figure 2). This ensures
In the second part of the experiment, students test the functionality of the nematic liquid-crystal cell as a wave-retarder device. Wave retarders are used to change the state of polarized light through a birefringent medium by a specified amount. The resultant phase shift, ΔΦ, is given by
ΔΦ =
2π d|Δn| λ
(1)
The phase shift depends on the sample thickness, d, and its birefringence, Δn, as well as on the wavelength of the incident light, λ. The intensity of transmitted light through the sample, I, relative to the intensity of plane polarized light incident upon the sample, I0, is related to the phase shift, ΔΦ, and the angle of the polarization vector of the incident light and the orientation of the director (molecular direction of preferred orientation in liquid crystalline mesophases), φ0, by, ⎛ ΔΦ ⎞ ⎟ I = I0 sin 2(2φ0) sin 2⎜ ⎝ 2 ⎠
(2)
Details on the experimental procedure for the measurement of the optical birefringence of the 5CB sample cell can be found in the Supporting Information. Figure 7 shows a plot of the average intensity of transmitted light through an aligned sample of the nematic liquid crystal 5CB, placed between crossed polarizers, as a function of the angle of the polarization vector of the incident light and the orientation of the director. The experimental design used here yielded results that are in excellent agreement with the results obtained by Waclawik et al.2 When the 5CB director was aligned with one of the polarizers, a minimum in intensity was observed (0° and 90°). As this alignment was disrupted by rotation of the sample cell, 522
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REFERENCES
(1) Kawamoto, H. The History of Liquid-Crystal Displays. Proc. IEEE 2002, 90 (4), 460−500. (2) Waclawik, E. R.; Ford, M. J.; Hale, P. S.; Shapter, J. G.; Voelcker, N. H. Liquid-Crystal Displays: Fabrication and Measurement of a Twisted Nematic Liquid-Crystal Cell. J. Chem. Educ. 2004, 81 (6), 854−858. (3) White, M. A. Physical Properties of Materials, CRC Press, Boca Raton, FL, 2012. (4) Lebovka, N.; Melnyk, V.; Mamunya, Ye.; Klishevich, G.; Goncharuk, A.; Pivovarova, N. Low-Temperature Phase Transformations in 4-Cyano-4′-pentyl-biphenyl (5CB) Filled by Multiwalled Carbon Nanotubes. Physica E 2013, 52, 65−69.
Figure 8. Average normalized intensity of transmitted light through an aligned sample of the nematic liquid crystal 5CB, placed between crossed polarizers at 45°, as a function of temperature. Vertical bars correspond to the standard deviation of three measured values at each temperature. Horizontal error bars correspond to an error of ±0.5 °C.
that the temperature that the student reads on the digital display is within 0.5 °C of the sample. During a cooling run, a steady and slow flow of water is essential to achieve an accurate measurement of the transition temperature.
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SUMMARY An optical device for the measurement of birefringence and phase transition temperature in the range from 25 to 40 °C of liquid crystals was assembled for $430. The innovative design gives the students an opportunity to assemble the optical bench, which enhances the hands-on experience in the teaching laboratory. The device is suitable for a materials science or a physical chemistry experiment at the third- or fourth-year undergraduate level that intends to provide an exciting introduction to the world of optics and liquid crystals.
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ASSOCIATED CONTENT
S Supporting Information *
A detailed description of the components required to build the optical bench; details of the experimental procedure; notes for the instructor. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address †
Brian Millier: Computer Interface Consultants, 31 Three Brooks Drive, Hubley, Nova Scotia, B3Z 1A3, Canada. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the students of the CHEM3305 Materials Science course for their feedback on the use of this instrument. We acknowledge Mary Anne White for her helpful comments on the manuscript. The financial support of Dalhousie University also is gratefully acknowledged. 523
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