Harmonic strain-optical response revealed in the isotropic (liquid) phase of liquid crystals

International audienceA strong optical birefringence is observed when applying a small amplitude oscillatory strain to theliquid phase of a liquid crystal. This unpredicted birefringence is found to oscillate at the samefrequency as the driving frequency, with frequencies down to 0.01 Hz. This birefringence is visibleup to 15 C above the liquid crystal transition. This opto-dynamic property is interpreted as a resultof a coupling of the orientational pretransitional fluctuations existing in the isotropic phase andlong range elastic interactions recently identified in liquids. The conversion of the mechanicalwave in an optical response is shapeable. Two examples of synchronized periodic signals areshown: the sine and the square waves. The optimization of the signal is analyzed using aHeaviside-step shear test. This optical property is immediately exploitable to design low energyon/off switching material

Hidden solidlike properties in the isotropic phase of the 8CB liquid crystal

International audienceNovel dynamic experiments have enabled the identification of a macroscopic solidlike response in the isotropic phase of a low molecular weight liquid crystal, 4,4'-n-octylcyanobiphenyl (8CB). This unknown property indicates that the low frequency shear elasticity identified in the isotropic phase of liquid crystal polymers is not reminiscent from the glass transition but reveals likely a generic property of the liquid state. The comparison to high molecular weight liquid crystals indicates, however, that the shear modulus is much enhanced when the liquid crystal moieties are attached to a polymer chain. The macroscopic length scales probed (0.050–0.100 mm) exclude wall-induced effects. DOI: 10.1103/PhysRevE.88.050501 PACS number(s): 61.30.Hn, 68.08.−p, 87.15.hg, 83.85.Vb The knowledge of the timescales involved in liquid crystalline systems is of outmost importance to understand, control, and improve their characteristics. The submillimeter scales properties attract a tremendous research interest [1–4]. However, few studies concern the isotropic phase away from pretransitional effects. Assimilated to ordinary viscous liquids, the isotropic phase is not supposed to exhibit solidlike properties, or at very high frequency only (mega-or gigahertz) as ordinary liquids. For this reason, the low frequency behavior of the isotropic phase remains mistakenly unexplored. Experimentally, the viscous or solidlike nature of a material is deduced from its response to a low frequency mechanical solicitation. A couple of years ago, careful dynamic experiments carried out in the isotropic phase of high molecular weight liquid crystals [side-chain liquid crystalline polymers (SCLCPs)] have revealed an as-yet unknown property: the isotropic melt does not flow but exhibits a finite shear elasticity of about several thousand Pascals at low frequency (0.1–10 Hz) [5–9]. The identification of low frequency shear elasticity in the isotropic phase of SCLCPs away from the isotropic-nematic transition opens numerous questions on the origin of this new property. It neither seems to result from the contribution of the liquid crystal moieties nor from surface anchoring effects, but likely from a generic property of the liquid state. Measurable in SCLCPs at macroscopic length scales as far as 100 • away from the glass transition temperature [5,7,9], the shear elasticity of SCLCPs still raises the debated question of reminiscent glass transition effects. In this Rapid Communication, we probe the dynamic properties of the low molecular counterpart: the 4,4'-n-octylcyanobiphenyl (8CB). The widely studied molecule can be considered as a representative liquid crystal molecule. 8CB exhibits a crystalline phase at low temperatures that enables one to rule on the question of pretransitional glass transition effects. We reveal a low frequency, solidlike response at several tens of micrometers sample thickness in the isotropic phase of 8CB, meaning that long range correlations are preserved when the orientational order is lost. This shear elasticity is detectable if special attention is paid to boundary conditions between the substrate and the sample. Under these conditions, the shear stress is optimally transmitted between the sampl

Unexpected giant elasticity in side-chain liquid-crystal polymer melts: A new approach for the understanding of shear-induced phase transitions

International audienceRecent studies have revealed that high molecular weight liquid crystals typically exhibit shear induced phases within the isotropic melt. From rheo-birefringence measurements, we demonstrate that those shear induced phases are not coupled with conventional orientational-order fluctuations. From rheo-SANS experiments, we show that the polymer chain is deformed at time scales longer than the viscoelastic relaxation time. Finally, careful visco-elastic measurements reveal a strong elastic behaviour; the melt is not a viscoelastic liquid but solid-like at equilibrium one hundred degrees over the glass transition temperature and up to 15°C over the Isotropic-Nematic phase transition. This supra molecular cohesion and its extra long relaxation times may explain the emergence of non-linear phenomena

Pressure-induced reduction of the Landau-Peierls instabilities in a side-chain polymer liquid crystal with reentrant polymorphism

International audienceCyanobiphenyl mesogens are known to exhibit partially bilayered smectic A (S Ad) and also reentrant nem-atic (N re) phases. Nematic and smectic orders are coupled parameters which depend both on temperature and pressure. We report the first structural study of the influence of a hydrostatic pressure on the smectic phase. This study was carried out on a side-chain liquid crystalline polymer, by neutron diffraction using two specifically designed pressure cells. These results concluded first that the pressure acts on the phase elastic constants via a reduction of the layer fluctuations giving rise to a hardening of the phase together with an extension of the smectic domain towards higher temperatures. Second, the S Ad-N re phase transition temperature remains unchanged in the studied pressure range revealing that the polymer component plays an important role which allows us to subtract the associated packing interactions from the pressure-induced volume reduction

The frozen state in the liquid phase of side-chain liquid-crystal polymers

International audienceQuenched isotropic melts of side-chain liquid-crystal polymers reveal surprisingly an anisotropic polymer conformation. This small-angle neutron-scattering (SANS) result is consistent with the identification of a macroscopic, solidlike response in the isotropic phase. Both experiments (rheology and SANS) indicate that the polymer system appears frozen on millimeter length scales and at the time scales of the observation. This result implies that the flow behavior is not the terminal behavior and that cross-links or entanglements are not a necessary condition to provide elasticity in melts

Mendil et al Reply

International audienceIn the preceding Comment [1], Collin and Martinoty claim that a conventional flow behavior (G' ~ f2 , G'' ~ f) is obtained at 20 $\mu$m gap thickness, for a LC-polymer (LCP105) in the nematic phase, using a filling with a capillary, whereas a solidlike behavior (G' ~ Cst, G" ~ Cst) is observed by approaching two parallel surfaces to the sample (as we apply in our experiment). Our observations of a terminal solidlike behavior are thus interpreted by Collin and Martinoty as an anomaly induced by compression effects. We clearly refute this interpretation. We note that, on Fig. 13 and 14 of a previous paper written by Collin and Martinoty [2], they report no more a conventional flow but an elastic behavior at lower temperature using the capillary filling and the same polymer (LCP105). Their previous observations are thus in contradiction with their present purpose; i.e., a terminal flow behavior

Innovations to Improve Lung Isolation Training for Thoracic Anesthesia: A Narrative Review.

A double-lumen tube or bronchial blocker positioning using flexible bronchoscopy for lung isolation and one-lung ventilation requires specific technical competencies. Training to acquire and retain such skills remains a challenge in thoracic anesthesia. Recent technological and innovative developments in the field of simulation have opened up exciting new horizons and possibilities. In this narrative review, we examine the latest development of existing training modalities while investigating, in particular, the use of emergent techniques such as virtual reality bronchoscopy simulation, virtual airway endoscopy, or the preoperative 3D printing of airways. The goal of this article is, therefore, to summarize the role of existing and future applications of training models/simulators and virtual reality simulators for training flexible bronchoscopy and lung isolation for thoracic anesthesia

Shear induced instabilities in layered liquids

Motivated by the experimentally observed shear-induced destabilization and reorientation of smectic A like systems, we consider an extended formulation of smectic A hydrodynamics. We include both, the smectic layering (via the layer displacement u and the layer normal p) and the director n of the underlying nematic order in our macroscopic hydrodynamic description and allow both directions to differ in non equilibrium situations. In an homeotropically aligned sample the nematic director does couple to an applied simple shear, whereas the smectic layering stays unchanged. This difference leads to a finite (but usually small) angle between n and p, which we find to be equivalent to an effective dilatation of the layers. This effective dilatation leads, above a certain threshold, to an undulation instability of the layers. We generalize our earlier approach [Rheol. Acta, vol.39(3), 15] and include the cross couplings with the velocity field and the order parameters for orientational and positional order and show how the order parameters interact with the undulation instability. We explore the influence of various material parameters on the instability. Comparing our results to recent experiments and molecular dynamic simulations, we find a good qualitative agreement.Comment: 15 pages, 12 figures, accepted for publication in PR

Rich polymorphism of a rod-like liquid crystal (8CB) confined in two types of unidirectional nanopores

We present a neutron and X-rays scattering study of the phase transitions of 4-n-octyl-4'-cyanobiphenyl (8CB) confined in unidirectional nanopores of porous alumina and porous silicon (PSi) membranes with an average diameter of 30 nm. Spatial confinement reveals a rich polymorphism, with at least four different low temperature phases in addition to the smectic A phase. The structural study as a function of thermal treatments and conditions of spatial confinement allows us to get insights into the formation of these phases and their relative stability. It gives the first description of the complete phase behavior of 8CB confined in PSi and provides a direct comparison with results obtained in bulk conditions and in similar geometric conditions of confinement but with reduced quenched disorder effects using alumina anopore membranesComment: Accepted in EPJ E - Soft Matte