29 research outputs found
Effect of optical purity on phase sequence in antiferroelectric liquid crystals
We use the discrete phenomenological model to study theoretically the phase
diagrams in antiferroelectric liquid crystals (AFLCs) as a function of optical
purity and temperature. Recent experiments have shown that in some systems the
number of phases is reduced if the optical purity is extremely high. In some
materials the SmC phase is the only stable tilted smectic phase
in the pure sample. In the scope of the presented model this high sensitivity
of the phase sequence in the AFLCs to optical purity is attributed to the
piezoelectric coupling which is reduced if optical purity is reduced. We limit
our study to three topologically equal phases - SmC, SmC
and SmC and show that the reduction of optical purity forces the
system from the antiferroelectric to the ferroelectric phase with a possible
SmC between them. The effect of the flexoelectric and
quadrupolar coupling is considered as well. If the phase diagram includes only
two phases, SmC and SmC, the flexoelectric coupling is
very small. The materials which exhibit the SmC in a certain
range of optical purity and temperature, can be expected to have a significant
flexoelectric coupling that is comparable with the piezoelectric coupling. And
finally, when temperature is lowered the phase sequence SmA
SmC SmC SmC is
possible only in materials in which quadrupolar coupling is very strong.Comment: 17 pages including 6 figures, submitted to PR
Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic phase resolved by resonant soft X-ray scattering
Periodic structures of phases with orientational order of molecules, but
homogenous electron density distribution: a short pitch cholesteric, blue phase
and twist-bend nematic phase, were probed by a resonant soft x-ray scattering
(RSoXS) at the carbon K-edge. The theoretical model shows that in case of a
simple heliconical nematic structure two resonant signals corresponding to the
full and half pitch band should be present, while only the full pitch band is
observed in experiment. This suggests that the twist-bend nematic phase has
complex structure with a double-helix, built of two interlocked, shifted
helices. We confirm that the helical pitch in the twist-bend nematic phase is
in a 10 nm range, for both, the chiral and achiral materials. We also show that
the symmetry of a blue phase can unambiguously be determined through a resonant
enhancement of x-ray diffraction signals, by including polarization effects,
which are found to be an important indicator in phase structure determination
Short-range smectic fluctuations and the flexoelectric model of modulated nematic liquid crystals
We show that the flexoelectric model of chiral and achiral modulated nematics predicts the compression modulus that is by orders of magnitude lower than the measured values. The discrepancy is much larger in the chiral modulated nematic phase, in which the measured value of the compression modulus is of the same order of magnitude as in achiral modulated nematics, even though the heliconical pitch is by an order of magnitude larger. The relaxation of a one-constant approximation in the biaxial elastic model used for chiral modulated nematics does not solve the problem. Therefore, we propose a structural model of the modulated nematic phase, which is consistent with the current experimental evidence and can also explain large compression modulus: the structure consists of short-range smectic clusters with a fourfold symmetry and periodicity of two molecular distances. In chiral systems, chiral interactions lead to a helicoidal structure of such clusters
Dielectric response of a ferroelectric nematic liquid crystalline phase in thin cells
We studied dielectric properties of a polar nematic phase (NF) sandwiched
between two gold or ITO electrodes, serving as a cell surfaces. In bulk, NF is
expected to exhibit a Goldstone mode (phason), because polarization can
uniformly rotate with no energy cost. However, because the coupling between the
direction of nematic director and polarization is finite, and the confinement,
even in the absence of the aligning surface layer, induces some energy cost for
a reorientation of polarization, the phason dielectric relaxation frequency is
measured in a kHz regime. The phason mode is easily quenched by a bias electric
field, which enables fluctuations in the magnitude of polarization to be
followed in both, the ferronematic and nematic phases. This amplitude (soft)
mode is also influenced by boundary conditions. A theory describing the phase
and amplitude fluctuations in the NF phase shows that the free energy of the
system and, consequently, the dielectric response are dominated by
polarization-related terms with the flexoelectricity being relevant only at a
very weak surface anchoring. Contributions due to the nematic elastic terms are
always negligible. The model relates the observed low frequency mode to the
director fluctuations weakly coupled to polarization fluctuations
Critical behavior of the optical birefringence at the nematic to twist bend nematic phase transition
This research was supported by the National Science Centre (Poland) under the grant no. 2016/22/A/ST5/00319. NV acknowledges the support of the Slovenian Research Agency (ARRS), through the research programme P1-0055.Peer reviewedPublisher PD
Multi-level chirality in liquid crystals formed by achiral molecules
M.S., D.P., and N.V. acknowledge the support of the National Science Centre (Poland) under the grant no. 2016/22/A/ST5/00319. E.G. acknowledges the funding from the Foundation for Polish Science through the Sabbatical Fellowships Program. N.V. acknowledges the support of the Slovenian Research Agency (ARRS), through the research core funding no. P1-0055. R.W. gratefully acknowledges the Carnegie Trust for the Universities of Scotland for funding the award of a PhD scholarship. The beamline 11.0.1.2 at the Advanced Light Source at the Lawrence Berkeley National Laboratory is supported by the director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.Peer reviewedPublisher PD
THE HISTORY OF AUTOMOBILISM
Zgodovina avtomobilizma ni le zgodovina avtomobila kakrÅ”nega poznamo danes, ampak sega mnogo dlje nazaj v Äas, ko so ljudje za prenos svojih bremen uporabljali voziÄke in vozove, za pogon pa veter, roke in noge. Å ele kasneje so iznaÅ”li oziroma dodali motor z notranjim zgorevanjem.
Prvi pravi zaÄetki avtomobila segajo v leto 1769, ko je bilo predstavljeno prvo samohodno vozilo in je bilo namenjeno za vojaÅ”ke potrebe. Že v tem obdobju in kasneje so bile prav vojaÅ”ke razmere gonilna sila razvoja avtomobilizma, saj so prav te narekovale hitrejÅ”i in uÄinkovitejÅ”i razvoj avtomobila.
NajveÄja Ā»revolucijaĀ« v zgodovini avtomobilizma se je zgodila proti koncu 19. stoletja, ko je Karl Benz izumil bencinski motor in ga kasneje vgradil v vozilo. Takrat avtomobili zaradi previsoke cene niso bili dostopni Å”irÅ”i množici, vendar so bili privilegij premožnejÅ”ega sloja ljudi. Ta privilegij družbe je konÄal Henry Ford, ko je Å”irÅ”i množici ponudil prvi serijski avtomobil - model T. Tako je postal glavni cilj izdelovalcev avtomobilov ustvariti Ā»ljudsko voziloĀ«, ki bo dostopno Å”irÅ”i množici ljudi.The history of automobilism is not only the history of automobile, known in modern times, but it goes back to the time when people used carriages and handcarts saddled with a burden. They used hand, wind and treadle drive.
The first real origin goes back to the year 1769 when the first self-propelled vehicle was presented and used for military purposes. In this period and later on, the military conditions were the ones who dictated the development of automobilism fast and effectivly.
The greatest Ā»revolutionĀ« in the history of automobilism happened at the end of the 19th century when Karl Benz invented the petrol engine which was later built in a vehicle.
At that time the cars weren\u27t accessible to the large crowd of people, however, they were the privilege of the wealthy class. Henry Ford ended this privilege when he offered the first serial car ā T model. Accordingly, the main goal of car makers was to create a Ā»people\u27s vehicleĀ« which would be easy of access to the large crowd of people