Command Surfaces Research Articles

Effect asymmetry of diffraction efficiency in LC cells with different command surfaces

We report the new results on the measurements of diffraction efficiency in liquid crystal (LC) cells with different orienting surfaces in the presence of a DC electric field. Diffraction gratings were recorded in combined cells with E7 liquid crystal. We used LC cells with thickness of ∼10 µm. Gold nanoparticles, chalcogenide and ITO nano-surfaces were used as command surfaces. Diffraction gratings were formed during irradiation of LC cells by 2-beam spatially modulated laser light (λ = 532 nm) with linear p-polarization in experiments of two-wave mixing. As a result, we found an asymmetry in the diffraction efficiency of the recorded gratings in different cells, which depends on the type of command surface and the polarity of applied voltage. The obtained results are appropriately described in the terms of surface-induced photorefractive mechanism, when a diffraction grating arises due to the formation of an unstable charge in pure liquid crystal. We believe that unstable charge depends on a surface on which different types of charge carriers are excited by light.

Bioinspired Energy Harvesting from Atmospheric Phenomena for Small Unmanned Aerial Vehicles

This Paper discusses energy harvesting from atmospheric phenomena for small unmanned aerial vehicles, theoretically through simulations and practically through experimental flights. A comparison between different scenarios for flight within the sinusoidal wind profile is presented. A significant improvement in performance with active control of command surfaces has been found for an energy-harvesting mode when compared to autostabilization or fixed-stick flight. Moreover, a detailed decomposition of the stochastic wind profile generated from the Kaimal spectrum has shown which frequencies and magnitudes of wind time series have the highest contribution to the energy-transfer process. It is found that wind profiles with higher turbulence intensity potentially provide more energy for transfer to the aircraft. Furthermore, the Paper reveals a biologically inspired sensory system for wind field estimation. It describes the necessary equipment and control algorithms for the exploitation of atmospheric energy. Initial flight tests were performed to determine the average power consumption of the motor for altitude hold tasks and to evaluate the performance of sensors. Moreover, additional flights for autonomous exploitation of several atmospheric phenomena are presented and analyzed.

Numeričko modeliranje kompozitnih ploča sa NOMEH saćastom ispunom na mezo nivou

Honeycomb core composite plates are becoming more important in the construction of primary aerospace structures. Nowadays, these types of materials are used for construction of fuselage skins, central and outer wing boxes, engine tail cones, landing gear doors, command surfaces like spoilers and ailerons etc. To determine the stress strain field in loaded honeycomb plates elastic coefficients are required. In the present work, a method for determining all required elastic coefficients for the core and plates is presented. Using experimentally obtained values for Nomex paper (type 410) and phenolic resin material model is presented and FEA model of composite plate with honeycomb core is created and three point bend test is simulated. Numerically obtained stress and strain values are compared to the experiment. Good agreement between proposed material model and experimentally obtained values is observed.

Simulation and Test of Discrete Mobile Surfaces for a RC-Aircraft

Morphing structures suitable for unmanned aerial vehicles (UAVs) have been investigated for several years. This paper presents a novel lightweight, morphing concept based on the exploitation of the “lever effect” of a bistable composite plate that can be integrated in an UAV horizontal tail. Flight dynamics equations are solved in Simulink environment, thus being able to simulate and compare different flight conditions with conventional and bistable command surfaces. Subsequently, bistable plates are built by using composite materials, paying particular attention to dimensions, asymmetric stacking sequence and total thickness needed to achieve bistability. NACA0011 airfoil is chosen for proving this concept. Wind tunnel tests demonstrate that the discrete surface is capable of withstanding the aerodynamic pressure. A remotely piloted vehicle is employed to test the discrete horizontal tail command during the take-off. The results show that, choosing a proper configuration of constraints, stacking sequence and aspect ratio for the bistable laminate, it is possible to tailor the snap-through mechanism. The proposed concept appears lighter and increases aerodynamic efficiency when compared to conventional UAV command surfaces.

A High‐Density Azobenzene Side Chain Polymer Brush for Azimuthal and Zenithal Orientational Photoswitching of a Nematic Liquid Crystal

AbstractThe azimuthal and zenithal orientational switching of nematic liquid crystals (NLCs) can be performed by photoresponsive azobenzene alignment layers called command surfaces. Such photoresponsive layers have been prepared by molecular assembly methods or by spincasting of polymer materials. This work reports on the photoswitching orientation behavior of an NLC on a high‐density brush of an azobenzene‐containing side chain liquid crystalline polymer. On the high‐density polymer brush surface, the zenithal homeotropic/planar photoalignment switching modes triggered by UV/visible light irradiation are reversed from those observed for traditional command surface systems, reflecting the azobenzene side chain orientation directing parallel to the substrate surface. Because the thermally stable trans‐azobenzene provides the in‐plane orientation of the NLC, irradiation with linearly polarized visible light readily switches the azimuthal direction to an arbitrary direction, and the resulting orientation is retained persistently. This paper discusses the features of the architecture of a high‐density polymer brush as the command surface for NLC orientation.

Effect of dynamically changing the substrate’s easy axis on the response time of nematic samples

Recent discoveries of advanced photocontrolled materials have kindled a great deal of interest on their use as command surfaces that switch easy axis under light radiation. One noticeable point when using switchable surfaces on any application is how the dynamical process propagates to the bulk directors. In this paper, we theoretically study the effect of a relaxing easy axis over time on a nematic sample when finite anchoring energy and surface viscosity are included. We first consider the case where just one of the substrates decay over time in an initially distorted director organization. Next, we assume that both substrates can be switched simultaneously. From the calculated director we obtained the optical profile and finally the molecular response time of the material. The response time depends on both the materials and the surfaces properties including its decay time. Our results might be used for understanding and engineering liquid crystal displays and other electro-optical devices with photocontrolled alignment layers.

Light- and electro- induced Fredericks transition in liquid crystal cells with photosensitive chalcogenide glasses surfaces As2S3

In this paper we represent light- and electro- induced Fredericks transition in liquid crystal cells with 5 CB on thin photosensitive chalcogenide glassy films As2S3 as command surfaces. Easy axis reorientation in a cell with nematic liquid crystals under the low intensity illumination with non-polarized beam (0.7…2mWt; 473nm) and decreasing of the Fredericks transition in dc field were observed. We suggest that easy axis reorientation with the optical illumination in the presence of a dc electric field is due to a light-induced desorption process on the photosensitive surface As2S3. We believe that the decreasing of the Fredericks transition voltage is caused by light-induced free charges in chalcogenide layer that discharge electrical double layer on the LC - chalcogenide interface. As a consequence, the energy of interaction between the liquid crystal molecules and the constant electric field near the surface overcome the anchoring energy, and, as a result, the reorientation of the director of LC in polar plane takes place in this case. From this point of view the utilization of the effects of light-induced anisotropy in inorganic materials, in particular, in chalcogenide glasses for photoalignment of LCs looks very promising.

Command Surface of Self-Organizing Structures by Radical Polymers with Cooperative Redox Reactivity.

Robust radical-substituted polymers with ideal redox capability were used as "command surfaces" for liquid crystal orientation. The alignment of the smectic liquid crystal electrolytes with low-dimensional ion conduction pathways was reversible and readily switched in response to the redox states of the polymers. In one example, a charge storage device with a cooperative redox effect was fabricated. The bulk ionic conductivity of the cell was significantly decreased only after the electrode was fully charged, due to the anisotropic ionic conductivity of the electrolytes (ratio >103). The switching enabled both a rapid cell response and long charge retention. Such a cooperative command surface of self-assembled structures will give rise to new highly energy efficient supramolecular-based devices including batteries, charge carriers, and actuators.

2B05 コマンドサーフェスを用いた液晶分子のプレチルト角光制御 : 斜め非偏光照射によるプレチルト角の発現

2B05 コマンドサーフェスを用いた液晶分子のプレチルト角光制御 : 斜め非偏光照射によるプレチルト角の発現

A versatile photochemical procedure to introduce a photoreactive molecular layer onto a polyimide film for liquid crystal alignment

A polyimide film containing a cholesteryl moiety spin-coated on a solid substrate is treated with vacuum ultraviolet light from a Xe2* excimer lamp at 20 Pa. This procedure generates oxidized chemically functional groups, such as hydroxyl groups, selectively on the top-most surface without changing the nature of the bulk polymer. Onto the chemically functionalized surface, photofunctional units such as azobenzene and cinnamate units are attached at a monolayer or sub-monolayer level. When nematic liquid crystal (LC) cells are fabricated, such photoresponsive surfaces work as effective ‘command surfaces’ for photoswitching the zenithal LC orientation and photoaligment layers to achieve the homogeneous azimuthal orientation applicable for vertical alignment mode LC devices. The proposed photochemical treatment provides a simple but versatile alternative process for the preparation of surface molecular films comparable to self-assembled monolayers or Langmuir–Blodgett films.

Liquid crystals photoalignment by films of side-chain azobenzene-containing polymers with different molecular structure

Liquid crystals photoalignment on the films of several side-chain azobenzene-containing polymers with various molecular structures was studied. Kinetics of the alignment and realignment of the liquid crystals director under irradiation by the polarized light was investigated and a comparison of the rates of polymers’ azobenzene moieties reorientation and director realignment were performed. The degree of the orientation, stability of the alignment, the possibility of multiple reorientation and other parameters of the photoinduced processes occurring in polymers films and liquid crystal cells were studied in detail. A rubbing technique was applied as alternative method for LC-alignment using these azobenzene-containing polymers. It was shown that the studied azobenzene-containing polymers can be considered as a photocontrollable command surfaces for the application in display technology and optoelectronics.

Azo-Containing Polymer Brushes: Photoalignment and Application as Command Surfaces

A brush polymer containing photosensitive mesogenic side chains has been directly synthesized on glass substrates by a grafting method involving polymerization of a metacrylic azobenzenic monomer under Atomic Transfer Radical Polymerization (ATRP) conditions. Brush polymer surfaces, with respect to more traditional films, are resistant to solvents and to high temperatures due to the covalent bonds between chain and substrate, which stabilizes the film morphology. The peculiar characteristics of the present polymeric brushes, namely the intrinsic in-plane orientation of the mesogenic units located in the side chains, together with the possibility of a fine control on the structure, make them very attractive for application as command surfaces. We then employed such brushes as photo-controllable command surfaces in twisted/planar switchable cells filled with a low molecular weight liquid crystal and we characterized the molecular order at interface by means of confocal polarized Raman microscopy.

Stimuli-responsive command polymer surface for generation of protein gradients

Mixed polyelectrolyte brushes with a composition gradient were used as a platform for fabrication of stimuli-responsive command surfaces to control the generation of concentration gradients of adsorbed protein molecules. Switching between homogeneously adsorbed protein layers and adsorbed layers with protein concentration gradients was achieved by changing the pH of protein aqueous solutions. Protein adsorption and the direction of the adsorption gradient were tuned and also turned off and on or reversed by tuning the proton concentration in the pH range 4.0-8.6.

Fast Dynamics in the Optical Storage with Langmuir–Blodgett Films of a Diazocrown Ether Molecule

Azobenzene-containing molecules have been studied for a number of applications exploiting the efficient trans-cis-trans photoisomerization cycles, which lead to molecular alignment and even micrometer-scale mass transport. One of the limitations of these materials, though, is the slow dynamics of the alignment process that requires significant molecular rearrangement. In this paper, we report on Langmuir-Blodgett (LB) films of a derivative of azocrown ether, referred to as 29-membered diazocrown ether, specially synthesized for the photoisomerization to involve only distortion of the shape of the crown. As a result, the dynamics of the writing process was extremely fast, with maximum birefringence being reached within less than 1 s, to be contrasted with tens or even hundreds of seconds for other azobenzene materials. A disadvantage is this strategy, however, is that the resulting birefringence was low, of the order of 10(-3), because the closely packed arrangement prevented the whole diazocrown molecule to be fully reoriented. From a comparison with results obtained in guest-host, cast films where the 29-membered diazocrown ether was incorporated into a polystyrene matrix, we confirmed that both fast dynamics and small birefringence features are due to a combination of effects from the nanostructured nature of the LB films and the molecular structure of the specially-designed diazocrown ether. Significantly, the fast dynamics opens the way for various applications, as in command surfaces for liquid crystal devices.

Azobenzene-Containing Polyamic Acid with Excellent Langmuir−Blodgett−Kuhn Film Formation Behavior Suitable for All-Optical Switching

To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-Blodgett-Kuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 degrees C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths.

Azo-pendant polyamides which have the potential to photoalign chromonic lyotropic liquid crystals

A new series of polyamides (ph-azo-PAMs) was synthesized by the polycondensation of isophthalic acids having a photoisomerizable phenyl azo unit with 4,4′-diamino-3,5,3′,5′-tetraethyldiphenylmethane or by the esterification of poly(imino-5-oxyisophthaloylimino-1,4-(3,5-diethyl)phenylenemethylene-1,4-(3,5-diethyl)phenylene) with 4-(phenylazo)- or 4-(4′-substituted phenylazo)phenoxyalkanoic acid chlorides in the presence of triphenyl phosphate and pyridine in N-methylpyrrolidone. It has been confirmed that the spin-coated films of ph-azo-PAMs thus produced show reversible trans–cis photoisomerization of the azobenzene moieties. To examine the ability of the polymer films to act as command surfaces for a chromonic lyotropic liquid crystal (LLC), they were exposed to linearly polarized light and coated with aqueous LLC solutions of a chromonic dye compound. After drying the LLC solution, polarizing microscopic measurements were carried out to see whether the phototreated polymer films could bring about the in-plane anisotropic alignment of the dye molecule. The result was that 5h having an azo moiety with a dimethylamino substituent as a side group was among the most promising candidates.

Improvement of LPS-based command surfaces: effect of inserting a flexible disiloxane segment into the azo side chain on photo-driven response

A new synthetic route for azo grafted ladder-like polysilsesquioxanes (LPS) used for fabricating command surfaces has been developed. In comparison with previous work, the grafting ratio of azobenzene on the LPS backbone could be controlled more accurately and the low molar mass impurity, which might influence the concerted action of the azobenzene units as command moieties on the surrounding LC molecules, could be readily removed by repeated precipitation. The results of our measurements indicate that inserting a flexible disiloxane segment into the spacer of the side group can improve the photo-driven response. The response steepness of a LC cell is 20 times that of our previous result. However, the response speed also depends on the spacer length and the content of azo side groups. Using azo grafted LPS with a shorter spacer as the material for the command surface, the response speed is faster than with a longer spacer. Optimum response can be achieved at an azo content of 10 mol %.

4-2b コマンドサーフェスの側鎖導入率依存性と赤外吸収スペクトル

4-2b コマンドサーフェスの側鎖導入率依存性と赤外吸収スペクトル

Photoswitchable biomaterials as grounds for optobioelectronic devices

Optobioelectronic systems provide a means for the electronic transduction of recorded optical signals. The methodologies to assemble optobioelectronic devices are reviewed. One method involves the chemical modification of redox enzymes with photoisomerizable units and their integration with electrode surfaces. In one photoisomer state the protein structure is perturbed and its bioelectrocatalytic activities are blocked. In the second photoisomer state, the tertiary structure of the protein is retained and the enzyme exhibits bioelectrocatalytic functions. This method is exemplified by the chemical modification of glucose oxidase (GOx) with photoisomerizable nitrospiropyran units and with the reconstitution of apo-GOx with the photoisomerizable nitrospiropyran-FAD (flavin adenine dinucleotide phosphate) cofactor. The two systems enable the cyclic amplified amperometric transduction of optical signals recorded by the photoactive proteins. The second approach to assemble optobioelectronic systems involves the organization of photoisomerizable monolayers on electrode surfaces acting as ‘optical command surfaces’ for the control of the electrical contact between redox proteins (or redox enzymes) and the electrode. This method is exemplified by the control of the electrical contact of cytochrome c (Cyt. c) with a functionalized electrode consisting of a mixed monolayer of pyridine/nitrospiropyran assembled onto an Au-electrode. The ON-OFF photostimulated electrical contact of Cyt. c with the electrode is coupled to mediated electron transfer cascades, i.e. bioelectrocatalyzed reduction of oxygen by cytochrome oxidase (COx). The systems allow the amplified amperometric transduction of optical signals recorded by the photoisomerizable monolayer-electrode. The photochemically-triggered activation or deactivation of the electrical communication between Cyt. c and the electrode, originate from the association or repulsion of the protein from the photoisomerizable monolayer-electrode interface. This enables the microgravimetric transduction of the optical signals recorded by the monolayer using a quartz crystal microbalance (QCM). The photostimulated electrical contact of Cyt. c and the electrode, and the subsequent activation of an electron transfer cascade via the electrobiocatalyzed reduction of oxygen by COx, allow the amplified amperometric transduction of optical signals recorded by the monolayer-functionalized-electrode. Reversible immunosensors based on photoisomerizable antigen monolayers assembled onto electrodes represent another configuration of an optobioelectronic device. Assembly of an antigen monolayer on electrodes yields an active interface for the amperometric transduction of the association of the complementary antibody (Ab) to the monolayer. Binding of the Ab to the monolayer blocks the electrical contact of a redox enzyme, i.e. ferrocene-modified glucose oxidase (Fc-GOx), with the electrode, and inhibits the electrobiocatalyzed oxidation of glucose. A photoisomerizable antigen assembled as a monolayer on an electrode allows the tailoring of reversible immunosensing interfaces. In one photoisomer state, the monolayer acts as an active interface for amperometric detection of the antibody. The complementary photoisomer state lacks affinity for the Ab and allows the washing-off of the antibody and the regeneration of the active antigen monolayer by a second illumination cycle. This approach is exemplified by the application of a dinitrospiropyran monolayer assembled onto an Au-electrode as a reversible sensing interface for the dinitrophenyl antibody (DNP-Ab). The dinitrospiropyran monolayer, SP-state, acts as an active interface for the association of the DNP-Ab. Photoisomerization of the monolayer to the dinitromerocyanine configuration, MRH +-state, allows the washing-off of the DNP-Ab and regeneration of the active SP-monolayer electrode by a secondary photoinduced isomerization. The reversible photostimulated binding and dissociation of DNP-Ab to and from the electrode is transduced by the application of Fc-GOx as redox probe and is further supported by microgravimetric QCM analyses.

Command surfaces, 20. Fixation of surface‐assisted homogeneous alignment of nematic liquid crystals by cationic photopolymerization

AbstractThe surface‐assisted alignment of a nematic liquid crystal yields a persistent ordered structure upon cationic photopolymerization of liquid‐crystalline epoxides. Linearly polarized light irradiation of a photocrosslinkable polymer containing azobenzene moieties played a crucial role in this procedure since the photoinduced alignment is not deteriorated by heating for polymerization.