3.8: Observing Phase Transitions
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As mentioned in the introduction, the liquid crystalline phase usually occurs in a small temperature range between the solid and liquid phases. In the following section we are going to observe this phase transition using MBBA, which is a nematic liquid crystal between 21°C and 48°C.
In each of the following experiments a microscope slide containing MBBA is heated until it becomes an isotropic liquid. It is then observed between crossed polarisers as it is allowed to cool down to room temperature.
Experiment 1 uses regular MBBA on a regular glass slide;
Experiment 2 uses regular MBBA on a slide with parallel scratches on its surface;
Experiment 3 uses MBBA mixed with Canada balsam (a chiral glue) on a regular glass slide.
Experiment 1: Isotropic Liquid to Nematic Liquid Crystal
- Note that as the temperature decreases the coloured liquid crystalline phase begins to nucleate at various random points across the slide (remember that the colours seen during the transition occur due to birefringence).
- As described in previous section, darker regions are visible where the director is aligned with either the polariser or the analyser .
- The video above is at too low a magnification to confidently identify any schlieren brushes. In the video sequence below, another sample is recorded forming at a higher magnification:
View Part 1 of the phase transformation in another sample
- Finally, towards the end of the video we can see that the different regions of the liquid crystal are still drifting around, albeit slowly. This shows how fluid the MBBA is, despite having entered the liquid crystalline phase (high fluidity is another characteristic of nematics).
Experiment 2: Isotropic Liquid to Nematic Liquid Crystal (On Grooved Surface)
- The elongated liquid crystal molecules tend to orientate along the scratches – this is why we now see uniform extinctions rather than various ‘blotches’ of darkness.
- Nucleation of the liquid crystalline phase is also guided by the scratches, with the liquid crystal sweeping in from the side rather than appearing at random points.
Experiment 3: Isotropic Liquid to Chiral Nematic Liquid Crystal
- Note how different the growth and final appearance of the liquid crystalline phase is, even when the only change to the sample is the addition of a chiral liquid.
- Although nucleation begins in a similar fashion to the regular nematic, we can see the different regions merge with one another to form the final ‘fingerprint structure’ that is characteristic of chiral nematics with their helical axis parallel to the surface of the slide.
- Circular birefringence only occurs when the light is travelling up the helical axis; therefore in this case the lines we are seeing are actually turns of the helix.
Note that there also exist phase transitions between different degrees of ordering (e.g. a smectic à nematic phase transition). Whilst often thermally activated as well, these can also be induced by factors such as the application of an external electric field or the addition of a particular type of solvent.