Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Constructing a Low-Cost Polarized Optical Microscope for Undergraduate Material-Characterization Studies Sunmeng Wang and Derek J. Schipper* Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
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S Supporting Information *
ABSTRACT: The polarized optical microscope (POM) is an invaluable tool for characterizing materials such as liquid crystals and other optically anisotropic materials. Despite its frequent use in research settings, students are rarely introduced to its fundamental operation and importance. Curricula are starting to appear for applications of the POM in undergraduate settings; however, the initial startup cost can be a deterring factor, with each microscope costing thousands of dollars. Herein, we provide a blueprint for the construction of an economical POM with a heating stage and digital connection for facile recording of data, totaling about $150−200. We subsequently demonstrate its effective application in visualizing liquid crystals. KEYWORDS: Second-Year Undergraduate, Upper-Division Undergraduate, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives, Demonstrations, Laboratory Equipment/Apparatus, Materials Science, Physical Chemistry
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BASIC SETUP The microscope setup was constructed using a pluggable USB microscope, purchased from Amazon, capable of 250× magnification (Figure 3a). The microscope came with clear plastic housing (Figure 3b) that protrudes from the end with the lens preventing focus of 250× magnification. To make use of the full magnification, the protruding plastic was first ground off with a dremel tool and then sanded so that the maximum focal plane could be easily accessed (Figure 3c). Next, a piece of wood was processed into a stage using a dremel tool and a band saw. The dimensions and the assembly of the stage can be found in Section S1 of the Supporting Information. Teflon tape can be further used to wrap around the periphery of the block, preventing extraneous light from entering (Figure 3d). The stage can also be machined from aluminum if desired (vide infra) for better conductivity with a heating element. A wooden stand was then built to support the stage using scrap wood. Likewise, the dimensions and assembly for the stand can be found in Section S2 of the Supporting Information. Next, polarized filters were affixed to both the microscope (Figure 3c) and stage. The filters employed were obtained from a pair of movie-theater 3D glasses. The starting orientation of the filters must be orthogonal, and this was ensured by putting the two filters on top of one another first to see which relative position provided maximum destructive wave interference as indicated by darkening of the light passing through. It is important to note that not all 3D movie glasses might work, as some may use circular polarizing filters instead of linear filters. Thus, it is important to perform this task to also determine if the correct type of filter is in the glasses. Upon distinguishing the correct orientation for the filters, one filter was glued onto the end of the microscope and the other was put into the carved 13 mm well in the stand. Finally, an LED
olarized optical microscopy (POM) is used in a wide variety of applications in the scientific community. It can be employed for the exploration of molecular structures in various biological samples,1 for the examination of rocks and minerals,2 and for the characterization of liquid crystals.3 A polarized optical microscope operates by probing a sample with polarized light, which then passes through a subsequent polarized filter prior to entering a microscope lens. The polarizers from the light source and lens can range from being orthogonal to being parallel with respect to one another to extract different optical information from a sample placed in between.3 Figure 1 below illustrates the concept of light interacting with polarized filters that are orthogonal and polarized filters that are parallel with respect to one another. Despite the simple operating principle, POMs can cost upward of thousands of dollars, limiting their use to mainly research laboratories. An introduction to this tool in an undergraduate setting would be an invaluable experience, exposing students to equipment used by a wide variety of disciplines. Updated teaching modules written by Pavlin et al. involving the use of this tool demonstrates a continuous effort to integrate it into various course curricula.4 In order to promote this process, the cost of a POM should be more affordable to adapt to teaching facilities. Precedent has been made in the production of POMs via low-cost modifications of existing microscopes; however, a low-cost build of a POM from common, household, accessible items has yet to be reported.5 Herein, we report the successful construction of a stand-alone, low-cost polarized optical microscope with a wide range of capabilities suitable for undergraduate study or research purposes. The setup uses widely available materials and features, including a heated stage and a digital connection where images and video can easily be captured on a computer. Finally, the operation of the microscope is demonstrated by visual characterization of liquid-crystal samples. Figure 2 below shows the general concept of the low-cost-polarized-optical-microscope design. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: October 29, 2018 Revised: February 12, 2019
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DOI: 10.1021/acs.jchemed.8b00879 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 1. (a) Nonpolarized light becoming laterally polarized after traversing through the first filter. The subsequent vertical gratings on the second filter block light from traveling any further. (b) Nonpolarized light becoming vertically polarized after traversing through the first filter. The subsequent vertical gratings on the second filter allows all the light to pass through.
smectic and nematic phases.6 Liquid crystals are ideal candidates for analysis in a polarized optical microscope as they produce different birefringent characteristics depending on the liquid-crystal phase they are in. As a result, a plethora of different images can be seen to evaluate the quality of our setup. Figure 4 shows an image taken from the simple microscope setup on a sample of 4′-octyl-4-biphenylcarbonitrile (8CB). 8CB can vary between nematic and smectic A liquid-crystalline phases depending on the temperature.7 The image shows 8CB in the more ordered smectic A phase at room temperature, which is the more organized of the two liquid-crystalline states. Visible in the image are randomly orientated grains of liquid crystals giving a “snowy” texture, with each grain representing a group of molecules orientated in a particular direction
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ADDITION OF A HEATING ELEMENT The addition of a heating element allows optical properties of thermotropic liquid crystals to be explored. The setup in Figure 5a uses a milled block of aluminum to replace the wooden stage for better conduction of heat during observation. However, as a result of the heating, the polarized filters from the 3D glasses no longer work as they degrade under high temperatures. Instead, heat-resistant polarized filters were purchased from Amazon as a suitable replacement. At room temperature, 8CB exhibits a smectic A phase, whereas at 35 °C, it transitions to a nematic phase. Figure 5b shows the same 8CB sample from Figure 4 but in the more disordered nematic phase. The nematic state is characterized as having domains with molecules organized in parallel, but it loses the layered configuration seen in the smectic phase shown in Figure 4.6 This increased disorder is observed macroscopically as a colorful canvas. Microscopically, this can be explained by the different wavelengths of polarized light undergoing numerous refractions and interference patterns through the different organized domains of the material before reaching the microscope.8
Figure 2. Schematic outline of a low-cost polarized optical microscope with the following parts: (i) USB microscope, (ii) polarized filters, (iii) stage with sample well, (iv) housing, and (v) LED. Arrows show the path of irradiance.
circuit was built with the following materials: a breadboard, LED, copper wires, and a 10 kΩ resistor. The wiring of the circuit is depicted in Figure 3e with the schematic diagram found in Section S4 of the Supporting Information. The LED was suspended through the 3 mm hole in the stand with the fitting being snug enough for the LED to hold its own weight. If desired, the 10 kΩ resistor can be replaced with a 20 kΩ potentiometer to add the feature of a tunable light source. The fully assembled apparatus, including the computer, microscope, stage, housing, and electronics, is shown in Figure 3f.
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LIQUID-CRYSTAL OBSERVATION Liquid crystals are compounds that exhibit physical states with characteristics of both solids and liquids. For thermotropic examples, these phases can be precipitated by adding or removing heat. Some of these states include the more ordered
LIQUID-CRYSTAL ALIGNMENT The added feature of heating further allows the capacity of performing a proper alignment procedure. Liquid crystals can be aligned by means of a physical-alignment layer,9 magnetic B
DOI: 10.1021/acs.jchemed.8b00879 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 3. (a) Commercial pluggable-digital-microscope kit. (b) Microscope housing that needed to be removed. (c) Microscope after removal of the plastic cover and affixing of the polarized filter. (d) Teflon-wrapped stage. (e) Stand setup. (f) Fully assembled apparatus. A milled aluminum stage is shown in this picture instead of a wooden one.
Figure 5. (a) Polarized optical microscope setup along with the silicon heating pads and aluminum stage. (b) Image of nematic 8CB heated to 35 °C taken directly from the low-cost microscope setup.
Figure 4. 8CB at room temperature in the smectic A phase. This image was directly taken from the low-cost microscope setup.
fields,10 or electric fields.11 The former two were explored on the described low-cost microscope with the heated stage. Figure 6a shows the alignment of 8CB produced under our setup, and a corresponding video recording can be found in the Supporting Information demonstrating the different phase transitions. Additionally, Figure 6b illustrates the seeding of the smectic A phase of pentyl 4′-(decyloxy)-[1,1′-biphenyl]-4carboxylate, which can be seen as large bubbles with bold, black borders. Magnetic fields can also induce alignment in liquid crystals.10 A magnetic field of 0.35 T was achieved by sandwiching a 15 mm wide aluminum stage between eight N52-grade neodymium magnets, four on either side (Figure 7a). Figure 7b was the
Figure 6. (a) Aligned 8CB on a polyimide alignment layer. (b) Transition from the nematic phase to the smectic A phase of pentyl 4′-(decyloxy)-[1,1′-biphenyl]-4-carboxylate. The images were directly generated with the low-cost microscope setup.
result of the alignment of 8CB under the 0.35 T magnetic field. The alignment was not optimal, as one can distinguish a general assembly of structures pointing upward in Figure 7b, a high density of defects illustrated as different colors, and directional misalignment. If a higher degree of alignment, like that found in Figure 5, is desired, a stronger magnetic field or a liquid crystal with a higher magnetic response (susceptibility) C
DOI: 10.1021/acs.jchemed.8b00879 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 7. (a) Magnetic-alignment setup. (b) Smectic A phase of 8CB under a 0.35 T magnetic field. (c) Smectic A phase of 8CB under a 0.6 T magnetic field. The images were directly generated with the low-cost microscope setup.
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should be employed. Figure 7c shows an image of 8CB aligned completely with a field of 0.6 T. It is important to note that employing magnets of this field strength is very dangerous, and one should proceed with caution when using them. Herein, we have described the construction of a low-cost microscope setup and outlined some of the capabilities through examining the properties of liquid crystals. The motivation behind this work is to reduce the financial requisite for undergraduate laboratories to integrate polarized optical microscopy into their repertoires. This low-cost setup could be a great supplement to a liquid-crystal modules such as those written by Tousley,12 Hartley,13 and Liberko.14 Exposure of the students to POM could possibly invigorate interest in one of the many fields that use this piece of equipment as an integral part of research. In addition to the samples that were explored in this paper, this microscope could also be employable in other traditional applications, such as in the analysis of rock2 or biological specimens.1
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00879. Comprehensive list of materials, dimensions for constructing the stage and stand, and procedures (PDF, DOCX) Phase transitions of 8CB (AVI)
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REFERENCES
(1) Koike-Tani, M.; Tani, T.; Mehta, S. B.; Verma, A.; Oldenbourg, R. Polarized Light Microscopy in Reproductive and Developmental Biology. Mol. Reprod. Dev. 2015, 82 (7−8), 548−562. (2) Bloss, F. D. Optical Crystallography; Mineralogical Society of America: Washington, DC, 1999. (3) Scharf, T. Polarized Light in Liquid Crystals and Polymers; John Wiley & Sons, Inc: Hoboken, NJ, 1994. (4) Pavlin, J.; Vaupotič, N.; Č epič, M. Liquid Crystals: A New Topic in Physics for Undergraduates. Eur. J. Phys. 2013, 34 (3), 745−761. (5) Maude, R. J.; Buapetch, W.; Silamut, K. A Simplified, Low-Cost Method for Polarized Light. Am. J. Trop. Med. Hyg. 2009, 81 (5), 782−783. (6) Chester, A. N.; Martellucci, S. Phase Transitions in Liquid Crystals; Springer: New York, NY, 1992. (7) Oweimreen, G. A. On the Nature of the Smectic A-to-Nematic Phase Transition of 8CB. J. Phys. Chem. B 2001, 105 (35), 8417− 8419. (8) Zou, Y.; Namkung, J.; Lin, Y.; Ke, D.; Lindquist, R. Interference Colors of Nematic Liquid Crystal Films at Different Applied Voltages and Surface Anchoring Conditions. Opt. Express 2011, 19 (4), 3297. (9) Stöhr, J.; Samant, M. G. Liquid Crystal Alignment by Rubbed Polymer Surfaces: A Microscopic Bond Orientation Model. J. Electron Spectrosc. Relat. Phenom. 1999, 98−99, 189−207. (10) Koshida, N.; Kikui, S. Magnetic Field Assisted Alignment of Nematic Liquid Crystal on a Polymeric Surface. Appl. Phys. Lett. 1982, 40 (6), 541−542. (11) De Oliveira, B. F.; Avelino, P. P.; Moraes, F.; Oliveira, J. C. R. E. Nematic Liquid Crystal Dynamics under Applied Electric Fields. Phys. Rev. 2010, 82 (4), 1−7. (12) Tousley, M. E. Liquid Crystal Demonstration of Binary Phase Behavior for the Classroom. J. Chem. Educ. 2018, 95 (11), 2000− 2005. (13) Jensen, J.; Grundy, S. C.; Bretz, S. L.; Hartley, C. S. Synthesis and Characterization of Self-Assembled Liquid Crystals: P-Alkoxybenzoic Acids. J. Chem. Educ. 2011, 88 (8), 1133−1136. (14) Liberko, C. A.; Shearer, J. Preparation of a Surface-Oriented Liquid Crystal An Experiment for the Undergraduate Organic Chemistry Laboratory. J. Chem. Educ. 2000, 77 (9), 1204−1205.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Derek J. Schipper: 0000-0003-4854-531X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank Zia Ahmed for fruitful discussions. We thank NSERC, the University of Waterloo, and the Canada Research Chairs Program (CRC-Tier II, D.J.S.) for financial support. D
DOI: 10.1021/acs.jchemed.8b00879 J. Chem. Educ. XXXX, XXX, XXX−XXX
