Showing posts with label liquid crystals. Show all posts
Showing posts with label liquid crystals. Show all posts
June 12, 2018
December 19, 2015
What is an order parameter?
For those of us working with liquid crystals, the answer tends to be fairly automatic: "the average of the second Legendre polynomial over the orientation distribution". Only in a second step do we think to qualify the definition: it concerns the quadrupolar order parameter (call it S) in a three-dimensional system. S = 0 for isotropic orientation, S = 1 when the molecules are perfectly oriented along an axis (the director) and S = –1/2 when they are all perpendicular to the director.
Whether it is the appropriate one depends on the problem at hand: if the particles constituting the system do not have inversion symmetry we should probably use the dipolar order parameter. It is less obvious that, although S describes well the tendency of molecules to align along an axis or perpendicular to it, it is not appropriate for any preferred angle: if all molecules make the "magic angle" θm = arccos(1/√3) ≅ 54.7° with the director, S is again zero so it cannot help distinguish between this situation and a purely isotropic distribution. One should then resort to the octupolar order parameter (or possibly a combination of quadrupolar and octupolar terms, for other angles?)
It may be useful to look at order parameters pragmatically, as Landau did for his theory of phase transitions: they are constructed such as to be zero in one phase and finite in another one. It is up to us to identify the phases and to define the most convenient parameter with regard for the particular system but also for the information we want to extract.
It may be useful to look at order parameters pragmatically, as Landau did for his theory of phase transitions: they are constructed such as to be zero in one phase and finite in another one. It is up to us to identify the phases and to define the most convenient parameter with regard for the particular system but also for the information we want to extract.
July 6, 2014
ILCC2014: days four and five
The plenary lectures of these last two days were given by chemists (Carsten Tschierske and Tadashi Kato) and they were both impressive for the results but also for the huge amount of work these clearly required.
Overall, I think the scientific level was higher than two years ago, maybe because of the smaller number of oral presentations: there were only three parallel sessions (four on Tuesday) compared to five in Mainz. Finally, the next (26th) edition of the ILCC will take place at Kent State!
July 1, 2014
ILCC 2014: day two
Not too many events today (at least in the sessions I attended). Two highlights:
- Very nice talk by Sin-Doo Lee about surface patterning using micron-sized nanoparticles
- I wasn't there, but it seems that Ivan Dozov's talk on twist-bend nematics was very appreciated (and led to lively discussion afterwards.)
June 30, 2014
ILCC 2014: day one
The 25th International Liquid Crystal Conference (ILCC 2014) opened today at the Trinity College Dublin. There are over 600 participants (only 24 of them from France). The organization seems a bit less meticulous than two years ago in Mainz, but the food is definitely better!
On the scientific side, modulated nematics (such as the twist-bend phase) are clearly the hot topic. There are also many talks on nanocomposites. Geographically, the Ljubljana groups (both at the University and the Jožef Stefan Institute) are very strong, and their collaborations with the Boulder teams of Noel Clark and Ivan Smalyukh look very fruitful, after Slobodan Žumer's talk this afternoon.
March 12, 2014
February 27, 2014
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