1. Crystal Growth in OTP VINCENT MULL and DR. STEPHEN HALL
Department of Physics| Pacific University | 2043 College Way | Forest Grove, OR |
Introduction
We are investigating the crystal growth of O-terphenyl (OTP). This substance is easily supercooled. Supercooling is when a liquid is brought to a temperature that is below its freezing point and does not solidify. OTP also exhibits anomalous growth at temperatures below a critical point, where normal growth is very slow. The anomalous growth is significantly faster than the normal growth below the critical temperature. The relationship between temperature and the rate of crystal growth is shown on Fig. 1. Filamentary growth has been observed at temperatures that are above the critical temperature. Recently, Xi et al have claimed that this filamentary growth uses the same mechanism as the anomalous growth1. Their observations are shown on Fig. 2. Our goal is to determine if the filamentary growth uses the same mechanism as the anomalous growth or is due to impurities or bubbles in the OTP. We observe the crystal growth by using a microscope and a camera to take time lapse photography. We tested a process know as sweeping. Sweeping is when the OTP is melted back to a specific point and is regrown in the same direction a number of times. This process is supposed to sweep bubbles and impurities off to the side and may also reduce the frequency of filamentary growth.
Unpurified
Fig. 7 shows results for 3 trials with unpurified OTP. All trials had at least some filamentary growth before sweeping was done and one had many filaments, but grew slowly. Only one trial showed a reduction in the quantity of filaments after sweeping. The others did not show much change.
FIG. 7: Images from the microscope slides of unpurified OTP. The left column shows crystal growth before sweeping and the columns to the right show growth after 5, 10, 15, and 20 sweeps. Each row represents a separate trial.
Pure
Fig. 8 shows results for 3 trials with purified OTP. One out of the three sets of data started with a high frequency of filamentary growth and grew quickly. Sweeping caused slower growth with fewer filaments on this slide. The other trials showed little filamentary growth to begin with and sweeping did not cause much change.
FIG. 2: The crystal growth of OTP observed by Xi et al. Filamentary growth is shown on the top row.
FIG. 1: Plot of the temperature vs
the log of the rate of crystal growth of OTP
Experimental Methods
In order to view the crystal growth, a small amount of OTP is melted and placed between two microscope slides. The OTP is allowed to completely crystallize and is then melted back from one side of the slide, leaving a small amount of crystallized OTP for the crystal to grow from. The microscope slide is placed inside of a brass cell with windows in it, shown in Fig. 3. The temperature of the OTP is regulated with a heater/cooler that pumps silicon oil through the brass cell. The crystals are illuminated with a light source that is connected to a fiber optic cable and are placed between crossed polarizers in order to make them more visible. OTP crystals are birefringent, so they appear colorful while the liquid OTP appears dark. The OTP is viewed through a camera that is connected to a microscope. The camera is used for time lapse photography. The brass cell is enclosed by a humidity free acrylic box, which contains desiccant, shown in Fig. 4. Nitrogen is pumped through the box to help prevent water from condensing on the microscope slide.
FIG. 8: Images from the microscope slides of recrystallized OTP. The left column shows crystal growth before sweeping and the columns to the right show growth after 5, 10, 15, and 20 sweeps. Each row represents a separate trial.
Pure +
Fig. 9 shows results for 3 trials with purified OTP that was placed in a desiccator immediately after being melted. All trials started with many filaments and only one sample did not grow very fast. Sweeping did not appear to affect the slower growing slide, but it did decrease the speed and the quantity of filaments of the other trials.
FIG. 3: Brass cell
FIG. 4: Apparatus with cooler
Sweeping
We tested the effect of sweeping on OTP with different treatments. We tested unpurified OTP from a bottle (Unpurified), OTP that was recrystallized three times (Pure), and OTP that was recrystallized, melted, and stored in a desiccator with desiccant (Pure+). Figures 5 and 6 show the GC/MS results for recrystallized and unpurified OTP, which suggest that both are very pure and have about the same purity. The crystal growth of slides prepared from the three categories of OTP were observed before any sweeps were done, and after 5, 10, 15 and 20 sweeps. The pictures of the crystal growth were observed for the presence of filaments and the relative rate of growth was determined. Time lapse video was recorded at 1 frame per 10 seconds.
FIG. 9: Images from the microscope slides of purified OTP that was placed in a desiccator immediately after being melted. The left column shows crystal growth before sweeping and the columns to the right show growth after 5, 10, 15, and 20 sweeps. Each row represents a separate trial.
Conclusion
It appears that sweeping does not affect the frequency of filamentary growth very much. For the most part, the presence of filaments and the rate of growth only decreased when there was a large amount of filamentary growth prior to sweeping. It is unknown why the pure + category had significantly more filamentary growth than the other categories. There was also significant variation between individual slides and there was some variations in the appearance of the growth at different locations within the same slide. More tests will need to be done in order to get more conclusive data and ultimately determine the cause and nature of the filamentary growth of OTP.
FIG. 5: Gas Chromatography of unpurified OTP
FIG. 6: Gas Chromatography of recrystallized OTP
[1] Xi et al., J. Chem. Phys.130, (2009).
Acknowledgements: Research Corp., Pacific University