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Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets
Chapters
Summary May 15th, 2017
Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.
Transcript
The overall goal of this procedure is to trigger an orientational transition of a liquid crystal in response to temperature. And to characterize the transition. This method can help answer key questions in the liquid crystal field, such as thermodynamic surface anchoring behaviors.
And the advantage of this protocol is that the orientational transition can be clearly visualized in this continuous manner. And if the involved physical properties can be systematically probed. To begin this procedure, prepare one milliliter of perfluoropolymer solution by dissolving the desired perfluoropolymer in a commercial solvent at a ratio of one to two to ensure uniform films 0.5 to 1 micrometer thick for spin coating.
Next, wash glass substrates by sonication at 42 kilohertz in an alkaline detergent. Then, repeatedly rinse each substrate with distilled water. After rinsing and drying the substrates, subject them to UV ozone cleaner for 10 minutes.
Following this, drip 20 microliters of the perfluoropolymer solution onto each cleaned glass substrate. Immediately spin coat the solution at 3, 500 rpm and room temperature for 70 seconds. Then, bake the film at 80 degrees Celsius for 60 minutes to remove the solvent, and at 200 degrees Celsius for 60 minutes for curing.
Apply a photo-curable resin pre-mixed with micrometer sized glass particles to one of the film-coated glass substrates. Then, place a second film coated glass substrate on top of the first substrate And use an LED lamp with a wavelength of 365 nanometers to glue the two substrates together. Next introduce 0.2 to 10 microliters of CCN47 to each of the prepared LC cells using a spatula under capillary force at a temperature higher than the isotropic nematic phase transition temperature.
Now, observe the LC cells under polarizing optical microscopy with four to 100X objective lenses in conjunction with a hot stage to control the sample temperature with plus or minus 0.1 Kelvin accuracy. Using a digital color camera, record the textures in more than five frames, at even intervals per Kelvin, both upon cooling and heating in the range of 291 to 343 Kelvin. For dielectric spectroscopy, prepare an LC cell with ITO electrodes by soldering lead wire to each substrate.
To measure the capacitance of the LC cells, wire the cells to the terminals of a commercial impedance gain phase analyzer. Then, measure the time dependence of capacitance of the LC cells every five minutes with the analyzer. For HR-DSC analysis place the LC cells into a homemade HR-DSC to be examined.
Perform measurements with scan rates of 0.05 to 0.10 Kelvin per minute to enhance the minimum temperature resolving power. The evolution of the texture made by polarizing optical microscopy and dielectric spectroscopy measurements during the orientational transition from the planar to vertical orientational state during cooling and heating is depicted here. Upon cooling, the planar orientation is right below the isotropic nematic transition temperature.
With the appearance of two and four brush Schlieren textures. By annealing or further cooling, the field of vision becomes completely dark, suggesting the completion of the orientational transition. HR-DSC data representing the heat flow through the sample as a function of temperature and time is shown here.
The variation of heat flow with time data was used to analyze the Avrami exponent after the orientational transition. Grazing incidents x-ray diffraction patterns in droplet geometry and in C2 LC cell geometry at various temperatures. Demonstrate short range ordering of quasi-smectic a-wetting sheets.
With layer structures formed near the surface. Even in the temperature range of the planar orientational states, quasi-smectic a-wetting sheets persist, indicating that the surface orientational condition is frustrated. The hysteresis range is consistent with the hysteresis range confirmed by polarizing optical microscopy and dielectric spectroscopy, which suggests that the orientational transition is triggered by quasi-smectic a-wetting sheet growth.
So once mastered, the observation of the orientational transition can be done in four hours if the preparation procedures are performed properly. Following this procedure, other methods like fluorescent confocal microscopy and non-linear optical analysis can be performed in order to answer additional questions, like the special distribution of orientations of the liquid crystal director. After its development, this technique paved the way for researchers in the field of surface science of liquid crystals to explore links between the microscopic structures near surfaces and the microscopic structures and properties.
After watching this video you should have a good understanding of how to prepare a sample to observe the orientational transition and how to characterize the important physical properties.
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