Chirality under confinement – multidimensional constraints in liquid crystalline materials
Promotion date: December 11.
Promotor: Prof. dr. Jeroen Cornelissen
Assistant Promotor: Dr. Nathalie Katsonis
Liquid crystals are a special type of materials which retain, to some extent, the order of molecular organization and also gain their dynamic properties. One of the main issues is the study of interfaces with liquid crystals. Such interfaces, including the organic/inorganic ones, are particularly important for the development of devices, like e.g. light-emitting diodes or photovoltaic cells. Here the active layers are composed of liquid crystal molecules.
The understanding of the behavior of mesogenic molecules under the influence of a metallic substrate (i.e. 2D confinement), is crucial for the design and evaluation of more developed systems which, due to the presence of specific functional groups, may be controllable, e.g. by light. This could lead to so-called smart materials. Another issue, investigated here, is the influence of 3D confinement on the behavior of cholesteric liquid crystals (CLCs), in particular the dynamic behavior of the photo-responsive CLC mixtures under UV irradiation.
Due to the photo-responsive character of a chiral dopant, we were able to modify the cholesteric pitch of a CLC mixture and thus map the expression of chirality, for varying geometrical confinement parameters: radius-to-pitch ratio. The 3D confinement of a photo-responsive CLC mixture was shown to positively support the helix inversion within cholesteric droplets: the unwinding of the cholesteric droplets being faster for smaller radius-to-pitch ratios.
The cholesteric droplets were already shown to play a promising role as the spherical Bragg microcavities which, thanks to the property of the nematic host, remain tunable. Very interesting would be the observation of the response of the system for the action of more than one stimulus, i.e. temperature and light. The successful demonstration of the variation of cholesteric droplets under UV-irradiation appears particularly promising for future applications such as controllable omnidirectional lasers, based on the spherical onion microresonators.
What were the main issues you worked on during your PhD period?
The main part of my research activities took place in Paris, at the Université Pierre et Marie Curie. The last year I have worked at the Mesa+ Institute in Twente. On both locations liquid crystals were at the heart of my research.
In short, two main topics were leading. First I studied some interesting features on monolayers and interface behavior of liquid crystals. Chirality within the monolayers formed by liquid crystal molecules may occur in surprising ways, as we were able to observe. Causal explanations on these phenomena can be divided into two groups mainly. First of all the ‘generic’ interactions such as the Van der Waals interactions may play a decisive role. On the other hand ‘specific’ chemical relations are found to be crucial, such as hydrogen bonding mediated by the substrate.
The 3D confinement behavior I studied, represented a completely different area of research. Here, the effects of chiral dopants added liquid crystals which were then dispersed into droplets seem very promising. The amount of chiral dopant is a determining factor of the value of parameter called the cholesteric pitch. The latter one may be controlled by temperature or even electric field. We were able to show, by using a special molecule as a chiral dopant, that it is possible to control the cholesteric pitch by UV-light. Moreover, for the first time we showed that it is possible to provoke a helix inversion, which is in other words the unwinding of a helical structure of one handedness and rewinding it with the handedness of the other sign.
Why are these observations interesting, for example in future applications?
The liquid crystals doped with the molecules of the so-called molecular motor, is a very exciting area. Besides the photo-induced phenomenon of helix inversion the possibility of modification of structure of liquid crystalline material by light gives plenty of room for new research.
Spontaneous formation of CLC droplets and their spherical symmetric character gives an opportunity to use them as Bragg microresonators, being at the origin of omnidirectional laser sources. Moreover, the demonstrated tunability of the cholesteric pitch within the droplets, offers a very suitable way of modification of the wavelength of the light emitted. The proof of principle is shown in my thesis. I believe that this relatively new subject is worthwhile studying further.
What are your future plans?
For the next few years I would like to stay in academics, as I like the way scientists work, showing great flexibility and actively searching collaboration networks in an open minded way, willing to change their fields of interest. I am sure some industrial partners are offering great opportunities for their research professionals as well. Nevertheless, the research atmosphere at the university environments seems more vibrant to me. Therefore a position as a post-doc appeals to me. Perhaps a little change of subject is favorable, in order to extent my knowledge base and research skills even further. A change towards organic electronics and, associated therewith, semiconducting properties would suit my field of interest.
In what way did you develop as a researcher and scientist during your PhD project?
In studying literature I am much more cautious now, judging the work of colleagues as well as my own work. I am reading carefully between the lines. I learned to develop a more patient attitude as to trust fully the things I read. This attitude towards the trustworthiness of information is an important virtue, as a lot of information is taken for granted nowadays.
At the same time I am more open to start new projects. During my PhD work I started new lines of research which I had never planned before. Apart from new insights, new collaborations started from this and I gained new sets of skills. I no longer restrict myself to one pre-set goal now, as I am more open to the unexpected.
How did you experience your short stay at the Mesa+ Institute?
The characteristics of the Mesa+ Group were very different from the French Group I worked in before. As the French colleagues work more or less independently on their own projects, here in Twente colleagues are effectively starting collaborations, in a structural way, right from the start of a new project. This attitude results in working more as one group towards collective results, pushing the work in one direction.
Though I didn’t work physically in the Nanolab, I got to know the Mesa+ expertise very well by attending the guide tours and the COMS Conference in August 2013. Here I came acquainted with key technologies such as lab-on-a-chip technology and microfluidics, which impressed me very much. So, my perception of Mesa+ is a very positive one.