A Workshop entitled ‘Twist-Bend Nematics and Beyond’ was held in the University of Southampton, Southampton, England, from 7 and 8 April 2016. This was organised by Professor Geoffrey R. Luckhurst of Chemistry, Dr. Giampaolo D’Alessandro and Professor Timothy J Sluckin both of Mathematical Sciences at this University.
The total number of participants was 48. A total of 11 countries were represented at the Workshop. Out of a total of these 48 participants, 8 were invited speakers, 16 participants were invited to make oral presentations and 13 made poster presentations.
The Workshop was opened by Professor Geoffrey Luckhurst just after lunch at 13.30 p. m. on Thursday 7 April 2016 with remarks that it was an opportune time to hold a Workshop on the fascinating, recent discovery of the twist-bend nematic (NTB) phase, a new nematic phase, made 125 years after the discovery of the standard nematic and chiral nematic phases. The NTB phase represents a missing link between the conventional nematic where no helicoid is present and the cholesteric phase where the helicoidal axis is directed normal to the molecular director, in which the molecules are distributed between the classical nematic phase and the cholesteric phase. In a way, it is a generalised version of the chiral nematic.
The first invited talk was given by Ivan Dozov on the ‘Analogy between the twist-bend nematic and the smectic A phase and the coarse-grained description of the macroscopic NTB properties’. It is appropriate to mention that Ivan Dozov predicted the NTB phase for an achiral bent-core liquid crystal in a seminal paper entitled ‘On the spontaneous symmetry breaking in the mesophases of achiral banana-shaped molecules’ published in Euro Physics Letters 56 (2), 247–253 (2001). He had suggested that banana-shaped achiral molecules with a negative-bend elastic constant would give rise to this phase. He introduced the twist to stabilise the bend deformations where the twist was twofold degenerate (left and right) because of the achirality of the molecules. In this talk, he showed that on a large-scale dimension >> p (heliconical pitch), the NTB phase has the same symmetry as SmA*. He suggested the existence of an NTB analogue of the twist grain boundary phase with penetration of the twist field in the bulk through a network of screw dislocations of the NTB-pseudo layers.
It was followed by an invited talk given by Antal Jakli on the extraordinary properties of the N and N* phases above the NTB phase. The measurements of the Is-N-NTB transition temperatures as a function of the applied magnetic field showed an unprecedented magnetic field enhancement of the nematic order. With a 22 T magnetic field, the transition temperature was shown to shift by up to 15°C. He explained these observations by the molecular theory given recently by Vanakaras and Photinos on the basis that the magnetic field would induce a decrease in the bend angle to an extent depending on the field.
This talk was followed by Alexey Eremin on mesophase structure and the behaviour of a dimer exhibiting an NTB phase in both bulk and restricted geometries. He showed that the bent-core material in the smectic phases can form freely suspended films and fibres which remained stable until the transition to the NTB phase occurred. This was followed by an oral presentation by Xiangbing Zeng from Sheffield University on the observation of an NTB phase in a seleno-ether compound using resonant X-rays. His result proved to be negative in that no such resonant diffraction peak was observed either in the NTB phase or in other mesophases in the binary mixture of the two compounds that were investigated; however, work on these systems was ongoing. The last talk of this session was by Vitaly P. Panov from Trinity College Dublin on photopolymerisation studies of self-assembly patterns in the NTB phase. He showed that a hierarchy of conical helices exists both in the bulk and in the bimesogens mixed with a photopolymer, polymerised by light. The polymer, after photopolymerisation, was washed away. It left a number of helices with different pitches and complex structure behind and these were seen by electron microscopy.
In the afternoon session, two invited speakers spoke. These were Alberto Ferrarini, of Padova University, who gave a molecular view of the NTB phase and in particular she discussed the relationship between the molecular structure and the phase behaviour. Another invited speaker Corrie Imrie discussed the structure–property relationships in twist-bend nematogens and he pointed out that the paper by Cestari et al. entitled ‘Phase behavior and properties of the liquid-crystal dimer 1″,7″-bis(4-cyanobiphenyl-4′-yl) heptane: A twist-bend nematic liquid crystal’, PRE 84, 031704 (2011) is the first experimental paper in the literature to suggest the existence of NTB phase. This paper involved collaboration between groups in Italy, UK (D.A. Dunmur and G.R. Luckhurst), Spain and Germany. This was followed by three oral presentations on structure and property relationships: Andreja Lesac on the structural variations of the existence of the NTB phase; Craig Archbold on the relationship between the molecular architecture and Wojciech Tomczyk on the occurrence of the NTB phase in liquid-crystal bimesogens; interplay between the arm biaxiality and bend angle of bent-shaped molecules forming NTB phases. Feng Ye in his presentation explained that the existence of various nematic phases including the NTB can be explained in terms of the Onsager tensor model. This was the last talk of the session on 7 April.
This was followed by a well-organised conference dinner in the Hartley Suite of the University of Southampton with wines from Italy and Chile. The menu reflected the quality of the English hospitality in a splendid hall of the University.
The next session was held on Friday 8 April 2016. This started with an invited talk by Oleg Lavrentovich on ‘Electro-optics of heliconical nematics and cholesterics’. He spoke about the NTB phase and the cholesteric phase formed by adding chiral dopants to the dimeric mesogens. He concluded his talk by stating that both structures lead to unusual electro-optics: the first-order structural transition in the NTB and electrically controlled selective reflection of light tuneable over an extraordinarily broad range of wavelengths (from UV to visible and to infrared).
The next talk was given by another invited speaker Jagdish K. Vij on a flexoelectrically driven electroclinic effect and breaking of the chiral symmetry. He gave an overview of the discovery in the context of the joint research efforts of the EU-funded project biaxial nematic device. He stated that the proof of NTB phase was given by a number of research groups almost simultaneously. He pointed out, however, that the first step in the discovery was the proof that the lower temperature phase in bimesogens with an odd number of methylene units was nematic phase with special properties (PRL 105,167801, 2010), rather than a smectic as previously thought. Large chiral domains of opposite handedness were observed and the discovery of a fast electroclinic effect for this phase followed (APL 99, 261903, 2011). Surprisingly, it was found that single-handedness domain got converted into two domains of opposite handedness lying side by side with a periodicity dependent on the amplitude of the applied field and its frequency (APL101, 234106, 2012). These observations pointed to a new nematic phase with interesting properties not all of which could be explained at the time but eventually have followed from the model of Ivan Dozov. He pointed out that published work of the Dublin group in collaboration with Hull (Georg Mehl) referred to above and presentation at International Liquid Crystal conference 2012 persuaded a large number of scientists to become interested in pursuing the work on bimesogens/dimers. He also discussed their recent results on the elastic constants; especially the bend elastic constant K33 which, close to the NTB-N transition temperature, increased whereas the splay elastic constant K11 decreased.
The next talk was an oral invited one given by Satyendra Kumar on the oblique chiral nematic phase in calamitic and bent-core mesogens. He gave results for three orientational order parameters <P2>, <P4> and <P6> found using X-rays. <P4> becomes negative in the NTB phase and this gives rise to the conical distribution of molecules as they follow a helical trajectory around the macroscopic director or helix axis. He showed that the Saupe orientational distribution function was valid for the high-temperature nematic phase but was not valid for the NTB phase. This was followed by an oral presentation by J. Cliff Jones on a comparison of the <P2> and <P4> order parameters between a symmetric and an asymmetric liquid-crystalline dimer with NTB phases. They used Raman spectroscopy to determine <P2> and <P4> and found large values of the order parameters for the even spacers as opposed to those for the odd dimers. He discussed their results in terms of the simple uniaxial theory of biaxial molecules given by Luckhurst et al. which dated back to 1975.
The second morning session started with an invited talk by Epifanio Virga on elastic coarse-graining theories for NTB phases. He answered the basic question: can a nematic coarse-grained elastic energy explain focal conics at equilibrium in NTB liquid crystals? In summary, his answer was a clear Yes.
Another major talk of this session was an invited talk given by Jonathan Selinger. He discussed the various theories and in particular showed that a coarse-grained theory identifies the long-wavelength fluctuation modes of the NTB phase and this theory explains well the results of dynamic light-scattering experiments.
There was an interesting presentation given by Jim Emsley on what can nuclear magnetic resonance reveal? He obtained the order parameter, Szz, for the para axis of the mesogen and showed that the order parameter starts to decrease even in the high-temperature nematic phase as the low-temperature phase is approached.
One of the interesting presentations given was by Nerea Sebastian, who presented distinctive dielectric properties of nematic liquid-crystal dimers and described the role of the conformational distribution. She showed how information on the conformational distribution can be obtained from the experimental dielectric spectroscopic data by means of a simple model.
There were 15 contributed posters. It is hard to summarise all of these but the reader is referred to the book on the programme for this Workshop at the website https://tbnw.wordpress.com. This demonstrates in greater detail than is possible here the diversity of the Workshop and this new nematic phase. Some of the studies presented at the Workshop are to be published as a Special Issue of Liquid Crystals.
With this presentation, the Workshop was concluded and the closing remark was given by Tim Sluckin. He ended the Workshop on a pleasant and humorous note making every one laugh while departing with a soft landing of the technical details. He also thanked Geoffrey Luckhurst and Giampaolo D’Alessandro for the organisation and the choice of speakers.
