Molecular dynamics simulations of selected members of the cyano-biphenyl (CB) series of dimers (CBnCB) have been set up using atomistic detail for the description of the alkyl spacer and ...coarse-grained representation of the CB end-segments. Detailed results are presented for the CB7CB dimers, showing an isotropic fluid phase and two nematic phases. The positional and orientational correlation functions extracted from the simulations are used to elucidate the structure of the low-temperature nematic phase. Polar molecular ordering is clearly identified along a direction undergoing helical twisting at right angles to the helical axis, with a constant pitch of about of 8 nm. The local ordering of the various molecular segments is calculated and found to be in excellent agreement with experimental NMR measurements. Key findings of the simulation are shown to be correctly predicted by the theoretical model of the polar-twisted nematic (N
PT
) phase A.G. Vanakaras, D.J. Photinos, Soft Matter. 12 (2016) 2208-2220. The complete failure of the usual twist bend model (N
TB
) to account for these findings is demonstrated.
Properties of the theoretical model of the twist-bend nematic phase (N
TB
) proposed by Barbero et al. were taken into account. In particular, the role of material parameters that determine the free ...energy density of the phase was investigated. The regions in parameter space ensuring stability of the N
TB
were found. Relationships between the parameters, tilt angle and pitch were presented. The ranges of parameters that lead to long (short) pitch and large (small) tilt angle were distinguished. The results allow to obtain useful sets of parameters suitable for studies of stable twist-bend nematic phase.
Dry Aligning Dilute Active Matter Chaté, Hugues
Annual review of condensed matter physics,
03/2020, Letnik:
11, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Active matter physics is about systems in which energy is dissipated at some local level to produce work. This is a generic situation, particularly in the living world but not only. What is at stake ...is the understanding of the fascinating, sometimes counterintuitive, emerging phenomena observed, from collective motion in animal groups to in vitro dynamical self-organization of motor proteins and biofilaments.
Dry aligning dilute active matter (DADAM) is a corner of the multidimensional, fast-growing domain of active matter that has both historical and theoretical importance for the entire field. This restrictive setting only involves self-propulsion/activity, alignment, and noise, yet unexpected collective properties can emerge from it.
This review provides a personal but synthetic and coherent overview of DADAM, focusing on the collective-level phenomenology of simple active particle models representing basic classes of systems and on the solutions of the continuous hydrodynamic theories that can be derived from them. The obvious fact that orientational order is advected by the aligning active particles at play is shown to be at the root of the most striking properties of DADAM systems: (
a
) direct transitions to orientational order are not observed; (
b
) instead generic phase separation occurs with a coexistence phase involving inhomogeneous nonlinear structures; (
c
) orientational order, which can be long range even in two dimensions, is accompanied by long-range correlations and anomalous fluctuations; (
d
) defects are not point-like, topologically bound objects.
We have studied a polar, biaxial nematic liquid crystal formed from bent-core molecules using molecular field theory. The model includes a simple Heisenberg-form dipolar intermolecular interaction in ...addition to the usual quadrupolar nematic interaction, and mimics a system consisting of nematogenic bent-core molecules with a large transverse dipole along the bisector of the two molecular arms. Such systems are regarded as good candidates for biaxial nematic liquid crystals. In principle, the molecular dipoles can align, thus stabilising the ordering of the minor axes. Our calculations predict that, for suitable values of the bent-core interarm angle, the biaxial nematic phase can be stabilised at higher temperatures than in the absence of the transverse dipole. In general, the transverse macroscopic polar order stabilises the biaxial nematic phase. In particular, for a large enough dipolar interaction, the Landau point in the pure biaxial nematic develops into a line of first-order polar biaxial nematic-to-isotropic phase transitions.
When dispersed in thermotropic nematic liquid crystal oils, surfactant-ladden aqueous droplets often lead to the formation of a equatorial ring disclination in the nearby nematic matrix as a result ...of a balance between elasticity and interfacial energy. In this experimental work, the aqueous phase contains an extract of cytoskeletal proteins that self-assemble into an active quasi-two-dimensional shell featuring self-sustained periodic flows. The ensuing hydrodynamic coupling drives the surrounding liquid crystal and triggers oscillations in the disclinations. We describe the dynamic modes of the disclinations under different driving conditions, and explore their pathway to collapse under flow conditions.
Dissipative solitons in liquid crystals (LCs) are represented by three-dimensional solitary waves of director deformation called directrons. The only one exception on the quasi-static counterparts of ...directrons has ever been observed in achiral nematics. In this work, quasi-static solitons and their fission are identified in chiral nematics. The structure, distribution and fission of quasi-static solitons are closely related to the pitch of samples. The critical pitch is about 7.0 µm for LC cells with thickness of ∼10.0 µm. Quasi-static solitons are transformed from directrons by stepping down voltage to facilitate locating solitons. Successive two-soliton fission with increasing fission time occurs for all quasi-static solitons in samples of relatively larger pitches. Multi-soliton fission is also found in some quasi-static solitons when the voltage is stepped up back to the directron domain, leaving behind a region that can modify the trajectories of surrounding directrons. The fission of quasi-static solitons in chiral nematics has predictable fission location, adjustable fission time, and controllable fission number, may acting as an excellent model system for studying general principles of soliton fission in nonlinear systems.
Display omitted
In microtubule-based active nematics, motor-driven extensile motion of microtubule bundles powers chaotic large-scale dynamics. We quantify the interfilament sliding motion both in isolated bundles ...and in a dense active nematic. The extension speed of an isolated microtubule pair is comparable to the molecular motor stepping speed. In contrast, the net extension in dense 2D active nematics is significantly slower; the interfilament sliding speeds are widely distributed about the average and the filaments exhibit both contractile and extensile relative motion. These measurements highlight the challenge of connecting the extension rate of isolated bundles to the multimotor and multifilament interactions present in a dense 2D active nematic. They also provide quantitative data that is essential for building multiscale models.
Electric field-induced deformations in hybrid aligned layers of nematic liquid crystals possessing flexoelectric properties were investigated numerically. The thresholdless one-dimensional ...deformations occurred at low voltages whereas the spatially periodic deformations, taking the form of parallel stripes, were found above sufficiently high threshold voltage. The threshold voltage decreased when the flexoelectric properties get stronger. The stripes were parallel to the direction of planar alignment. The director distributions in the stripes induced by positive voltages differed qualitatively from those arising at negative voltage. The evolution of deformations during increase of bias voltage was investigated. The role of flexoelectric properties and of dielectric anisotropy was studied. The spatial period of deformations shortened with increasing voltage and with increasing flexoelectric coefficients.
Nematic–nematic transitions in a highly polar nematic compound are studied, in thick cells in which the molecules are aligned parallel to the substrates but perpendicular to the applied electric ...field, using dielectric spectroscopy in the frequency range 1 Hz to 10 MHz over a wide temperature range. The studied compound displays three nematic phases under cooling from the isotropic phase: ubiquitous nematic N; high polarizability NX; and ferroelectric nematic NF. Two collective processes were observed. The dielectric strength and relaxation frequency of one of the processes P2 showed a dependence on the thickness of the cell. The process P1 is the amplitude mode, while the process P2 is the phason mode.
PDF
