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Pawel Pieranski – crystallographer of liquids and Alfred-Saupe-prize laureate 2019

Commemorating Alfred Saupe, an outstanding pioneer of liquid crystal research, the Alfred-Saupe-Foundation and the German Liquid Crystal Society bestow the Alfred Saupe Prize to honour outstanding work in the field of liquid crystals. On 28 March 2019, Prof. Dr Pawel Pieranski received this prize, which includes the Alfred SaupeMedal (Figure 1), in recognition of his outstanding works in the field of liquid crystal research and its application. Pawel Pieranski has worked extensively in many different areas of liquid crystal (LC) research, such as the static and dynamic behaviour of nematic LCs [1–8], chiral smectic and especially ferroelectric LCs [9–11], colloidal crystals [12– 19], cholesteric and blue phases [20–32], wetting and anchoring [33–36], application of atomic force microscopy (AFM) to LCs [37–41], freely suspended LC films [42–52], shapes of single crystals [53–61], cellulose derivatives [62– 65], and various aspects of order, structure, symmetry, defects and topology in certain mesophases or special geometries [66–74]. Many times, he raised new scientific questions, developed experimental methods, started and established a topical research area, explored and reviewed the basic physics. Often, he pointed out that the method used, the subject studied or the cognition achieved can be applied to other systems, exhibits an analogy to a different problem in science or has an impact on another research field, so that the liquid crystal may serve as a model system that facilitates studying and visualising more general phenomena at comparably moderate conditions. Born in Liskow (Poland), Pawel Pieranski studied Physics in Poland and France. He received the degree Magister of Physics at the University Adam Mickiewicz in Poznan. In Paris, he encountered the spirit by Pierre-Gilles de Gennes [74] and received the Diplôme d’Études Approfondies de Physique des Solides at the Université Paris-Sud in Orsay. Based on his extensive research with Etienne Guyon at the Laboratoire de Physique des Solides in Orsay, he finished his Thèse de 3éme cycle (1972), became Chargé des Recherches at CNRS (1973) and finished his Thèse d’Etat (1976). After post-doc research with R.B. Meyer at Brandeis University (USA, 1979) and independent research at the Laboratoire de Physique des Solides in Orsay, he became Directeur des Recherches at CNRS (1984). Apart from his early work published together with E. Guyon [3–6], Pawel Pieranski hitherto collaborated extensively with Patricia Cladis [1,2,21–26] and Maria H. Godinho [62–65,71–73] and occasionally published, for example (in chronological order) together with Maurice Kléman [1], with his brother Piotr Pieranski [10,17,18], with Pierre-Gilles de Gennes [7], Jaques Friedel [12], Robert B. Meyer [8], Gerd Heppke [29–31], Richard Hornreich [35], John Goodby [42], Patrick Oswald [51,75,76], Slobodan Žumer [62] and other distinguished researchers.


Liquid single crystals
Hitherto, Pawel Pieranski's most long-lasting continuous research interest over many decades was devoted to the growth and thorough investigation of liquid single crystals, i. e. droplets of liquid crystals, which do not show a spherical shape but exhibit facets, thereby resembling the shape of solid crystals ( Figure 3). His studies included colloidal crystals [15], thermotropic liquid crystalslike blue phases [20][21][22][23] and thermotropic bicontinuous cubic phases [40,41] as well as lyotropic cubic liquid crystals [53][54][55][56][57][58][59][60][61]. To grow liquid single crystals, Pieranski developed special containers for the specimen, where either the temperature and a temperature gradient [21,27,61] or the humidity of the environment [53][54][55][56][57][58][59][60][61] can be controlled very precisely. The latter type of setups facilitates not only the growth and stabilisation, but also changes of the composition of the LC droplet and thus phase transitions in lyotropic LCs at constant temperature [57]. Pieranski pointed out the importance of dislocations for crystal growth rates and studied the motions of dislocations as well as the symmetry and topology of the liquid crystal structure in the vicinity of a dislocation [23,27,59,70]. In solid crystals, dislocations are very important, because they can enhance the growth rate by many orders of magnitude owing to surface steps appearing in the vicinity of a dislocation [70]. Due to stronger interactions, the deposition of a further atom to a surface step is much more likely than its deposition on a flat surface,  which is free of dislocations. Pawel Pieranski observed the shape of surface steps [for example, a spiral-shaped step surrounding a screw dislocation, Figure 3 and their role in crystal growth also for liquid single crystals [23,27,59,70]. According to his observations, liquid single crystals show many similarities, but also striking differences from solid single crystals, which can be explained by the differences in lattice constants, stiffness and surface tension. A very powerful tool for space group analysing is the application of the Friedel-Donnay-Harker (FDH) rules [61,70]. These rules describe, which facets will dominate the shape of a single crystal for a given space group. They result from the observations that those facets will dominate the final shape of a crystal which are perpendicular to the crystallographic directions with smallest growth rate, and that the growth ratein turndecreases with the height of steps on a facet. Pawel Pieranski and his coworkers demonstrated in many cases that the priority of facets predicted by the FDH rules appears also in liquid single crystals. For blue phase 1 (BP1), thorough studies on the shape of single crystals [ Figure 3(e)] facilitated the determination of the space group [20][21][22][23]. In the case of a lyotropic bicontinuous phase [61], the crystal shape [ Figure 3 (f)] was found to be incompatible with FDH rules for the space group Pn3m, which was known to describe the phase in the bulk. However, calculation of the surface topology revealed that edges of the sponge-like membrane constituting this phase would appear [ Figure 3(g)]. Since closing the membrane at the surface is favourable [ Figure 3(h)], Pieranski concluded that the resulting broken symmetry at the surface symmetry will be described by the space group Fd3m (a sub-group of Pn3m) and was able to show that the FDH rules for this sub-group describe the experimentally observed facets [ Figure 3(f)], correctly [61]. Another fascinating observation was made, when liquid single crystals [ Figure 3(i-j)] or soft single crystals [ Figure 3(k-l)] were grown in a gas environment. In this case, a huge number (up to 60) of different kinds of facets were observed [40,53,54], including facets corresponding to large Miller indices, which are rarely found in solid crystals. This devil's staircase scenario [53] was actually expected to appear in solid crystals close to 0 K. This very unusual observation could be explained by striking differences in the roughening temperature T r , above which the smaller facets disappear. The value of T r is very small for crystals and for liquid crystals grown in a liquid environment, but well above room temperature for liquid crystal droplets in a gas environment [53].
In addition to these extras, Pieranski often discusses how quite different fields in science are related to each other, for example, smectic films to quantum billiards [46] and localisations in fractal drums [50] or LCs in general to problems in physics and mathematics [66]. He emphasises, how the recognition gained from LC experiments can be transposed to other fields through universal aspects of LCs [67].

Pawel Pieranski's teaching
Pawel Pieranski is a brilliant teacher, both on the level of classroom experiments and on the academic level [75][76][77]. In his lectures and publications, he invites us to perceive nature with all of our senses. Most evidently, his presentations are a feast for the eyes, owing to their beautiful images and drawings (Figure 4). Not surprisingly, Pieranski received the Luckhurst-Samulsky-Prize 2014, recognising the best publication published in the journal Liquid Crystals in 2013 [68]. However, he also encourages us to use our sense of hearing through his acoustical experiments, for example, when he raises the question 'Can one hear the shape of a drum?' [47,49]. He also inspirits our tactile sense, when he compares the cantilever of an AFM to the fingertip probing a surface, when describing its function in a popular scientific context. Last, but not least, Pawel Pieranski discloses a delightful sense of humour, for example, when he describes the unexpected visit of a longhorn (reduvius personatus) in his lab [69]. Often, reading his articles is like enjoying a thrilling novel about magical creatures and fantastic beasts, when we learn about frustrated liquids [25], diabolical points [45], self-tuning behaviour [44], the magic spiral [48], the devil's staircase [53] or peculiar effects like thermopermeation [60], centrophobic behaviour [52], hygrophilic behaviour [52], the chirogyral effect [62], or rheotropism [72].

Alfred Saupe and Pawel Pieranskisome mutualities
Summarising, Pawel Pieranskilike Alfred Saupeperformed outstanding pioneering work on many subjects within the field of liquid crystal research. However, there are additional mutualities between Alfred Saupe and Pawel Pieranski. These two researchers shared joint interest in several areas, for example, an interest in LC order, defects and topology, in lyotropic liquid crystals, in atomic force microscopy and also in blue phases. As a matter of fact, Alfred Saupe was the first who suggested a cubic superstructure in order to explain the coincidence of cholesteric-like selective reflection and missing birefringence in blue phases [78]. Later, Pieranski and his coworkers confirmed this hypothesis and determined the space groups describing BP1 and BP2 [20,28]. Both Saupe and Pieranski are liquid crystal experts of international renown, spending a major period of their academic career outside of their respective country of birth. Also, both of them spent the academic year 1989 as invited fellows at the Wissenschaftskolleg zu Berlin [79], thereby cultivating a vicinity to liquid crystal research in Germany.

Alfred Saupe ceremony 2019
The Alfred Saupe Medal was handed over to Pawel Pieranski at the 46th German Liquid Crystal Conference (Arbeitstagung Flüssigkristalle) held in Paderborn, 27-29 March 2019. In his lecture 'Physics of the i-Land', Pawel Pieranski ( Figure 5) discussed the generation, motion, interaction of defects in a system,  Figure 4 from Ref. [52]. (© MDPI (Basel, Switzerland)); (c), (d) Figure 22 from Ref. [68]. (© Taylor&Francis); (e) Figure 12 from Ref. [72]. (© EDP Sciences). where the order is given by a phase φ and described by an order parameter exp(iφ). In extension of recent works [71][72][73], he showed beautiful microscopic pictures of defects in the dowser texture, pointed out their analogy with vortices of the vector potential in type II superconductors and emphasised that the liquid crystal facilitates the generation of defects with opposite topological charge, which can annihilate each other ('dowson collider'). In principle, the mutual annihilation of defects in superconductors is not forbidden, but the generation of vortices of the vector potential with opposite handedness, i. e. magnetic flux lines with antiparallel magnetisation, may be very hard to achieve experimentally. In his speech, Pawel Pieranski recognised the work by Alfred Saupe and memorised the friendship with some of his research companions.