The first demonstration of converse piezoelectricity in 3D fluids is presented by measuring a linear electromechanical effect in ferroelectric nematic liquid crystals. The observed piezoelectric ...coupling constant below 6 kHz electric field is larger than 1 nC/N, comparable to, or better than, values for the strongest solid piezoelectric materials. Symmetry considerations indicate that the alignment of the ferroelectric nematic liquid crystal in the experimental study is not optimized, so the observed signal is likely only a fraction of the theoretically achievable signal. Understanding the electromechanical response of ferroelectric nematics will enable mechanical energy harvesting and open up a new avenue for developing fluid actuators, micro positioners, and electrically tunable optical lenses.
The first observations of piezoelectricity is reported in a 3D fluid. For which, linear electromechanical effects, corresponding to conserve piezoelectricity, are observed and analyzed, in two room‐temperature liquid ferroelectric nematic liquid crystals. The observed piezoelectric coupling constant is found to be comparable to or exceed that of the strongest solid piezoelectric materials.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The present article entails the generation of flexoelectricity during cantilever bending of a solid polymer electrolyte membrane (PEM), composed of poly(ethylene glycol) diacrylate (PEGDA) precursor ...and ionic liquid (hexylmethylimidazolium hexafluorophosphate). The effects of thiosiloxane modification of PEGDA precursor on glass transition, ionic conductivity, and flexoelectric performance have been explored as a function of PEM composition. The glass transition temperature (T g) of the PEM declines with increasing thiosiloxane amount in the PEGDA co-network, while the ionic conductivity improves. The PEM/compliant carbonaceous electrodes assemblies were assembled to determine the flexoelectric coefficients by monitoring electrical voltage/current outputs for various PEM compositions under the intermittent square-wave and dynamic oscillatory sine-wave deformation modes. Of particular interest is that the room temperature flexoelectric coefficient exhibits strong frequency dependence in the vicinity of 0.01–10 Hz, suggesting that ion polarization and ion transport through the ion-dipole complexed networks can still be affected by the mobile side chain branches even in the elastic regime of the covalently bonded PEGDA network. The in-depth understanding of the effect of thiosiloxane side chain on flexoelectricity generation is anticipated to have impact on the development of mechanoelectrical energy conversion devices for energy harvesting applications from natural and dynamical environment.
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Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold, the interface of the ...fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in connection to the Rayleigh and circular drop-type instabilities.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Understanding the origin of structural ordering in supercooled liquid gallium (Ga) has been a great scientific quest in the past decades. Here, reflective polarized optical microscopy on Ga ...sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection at 120 °C that increases on cooling and persists down to room temperature or below. The polymer rubbing usually aligns the director of thermotropic liquid crystals (LCs) parallel to the rubbing direction. On the other hand, when Ga is sandwiched between substrates that align conventional LC molecules normal to the surface, the reflection is isotropic, but mechanical shear force induces anisotropic reflection that relaxes in seconds. Such alignment effects and shear‐induced realignment are typical to conventional thermotropic LCs and indicate a LC structure of liquid Ga. Specifically, Ga textures obtained by atomic force and scanning electron microscopy reveal the existence of a lamellar structure corresponding to a smectic LC phase, while the nanometer‐thin lamellar structure is transparent under transmission polarized optical microscopy. Such spatial molecular arrangements may be attributed to dimer molecular entities in the supercooled liquid Ga. The LC structure observation of electrically conductive liquid Ga can provide new opportunities in materials science and LC applications.
The existence of a liquid crystal (LC) mesophase in metallic materials is counterintuitive. Typical LCs are made of covalently bonded organic molecules. This study reports on the observation of LC mesophases in supercooled liquid gallium and eutectic gallium–indium alloy. An LC texture of the lamellar structure similar to that of the smectic phase is unveiled at room temperature.
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Aspheric lenses reduce aberration and provide sharper images with improved spot size compared to spherical lenses. This paper demonstrates that applying shear flow can produce plano‐concave liquid ...crystal (LC) lens arrays with paraboloid aspheric profiles. The focal length of individual lenses, with a 0.2 mm aperture, decreases from 0.67 to 0.45 mm as the chiral dopant increases from 0 to 6 wt%. The focal length is also sensitive to the polarization state of the incoming light. The lenses are stabilized by photopolymerizing with 6 wt% of reactive monomer added to the LC. A qualitative explanation for the flow‐induced lens formation and the optical properties of the lenses is provided. The potential tunability of the lenses in various fields and their use as paraboloid reflectors are discussed.
This work shows that by applying shear flow, plano‐concave paraboloid aspheric liquid crystal (LC) lens arrays can be made. These lenses show unique optical properties, such as sensitivity of focal length to chirality and the polarization state of the incoming light.
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The isotropic to ferroelectric nematic liquid transition was theoretically studied over one hundred years ago, but its experimental studies are rare. Here we present experimental results and ...theoretical considerations of novel electromechanical effects of ferroelectric nematic liquid crystal droplets coexisting with the isotropic melt. We find that the droplets have flat pancake-like shapes that are thinner than the sample thickness as long as there is room to increase the lateral droplet size. In the center of the droplets a wing-shaped defect with low birefringence is present that moves perpendicular to a weak in-plane electric field, and then extends and splits in two at higher fields. Parallel to the defect motion and extension, the entire droplet drifts along the electric field with a speed that is independent of the size of the droplet and is proportional to the amplitude of the electric field. After the field is increased above 1 mV μm
−1
the entire droplet gets deformed and oscillates with the field. These observations led us to determine the polarization field and revealed the presence of a pair of positive and negative bound electric charges due to divergences of polarization around the defect volume.
Pancake-like ferroelectric nematic droplets in their isotropic melt in horizontal electric fields showing defect motion and shape deformation of the droplets. White arrows show the polarization field.
This paper elucidates the means to control precisely the morphology of electrospun liquid crystal/polymer fibers formed by phase separation. The relative humidity, solution parameters (concentration, ...solvent), and the process parameter (feed rate) were varied systematically. We show that the morphology of the phase‐separated liquid crystal can be continuously tuned from capsules to uniform fibers with systematic formation of beads‐on‐a‐string structured fibers in the intermediate ranges. In all cases, the polymer forms a sheath around a liquid‐crystal (LC) core. The width of the polymer sheath and the diameter of the LC core increase with increasing feed rates. This is similar to the results obtained by coaxial electrospinning. Because these fibers retain the responsive properties of liquid crystals and because of their large surface area, they have potential applications as thermo‐, chemo‐, and biosensors. Because the size and shape of the liquid‐crystal domains will have a profound effect on the performance of the fibers, our ability to precisely control morphology will be crucial in developing these applications.
You spin me round: The morphologies of liquid crystal/polymer composites can be tuned from capsules through a beads‐on‐a‐string structure to uniform fibers by varying the working parameters during the electrospinning process. In all three cases, the polymer forms a sheath around the liquid‐crystal core.
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The recently discovered ferroelectric nematic (NF) liquid crystals (LCs) with over 0.04 C m−2 ferroelectric polarization and 104 relative dielectric constants, coupled with sub‐millisecond switching, ...offer potential applications in high‐power super capacitors and low voltage driven fast electro‐optical devices. This paper presents electrical, optical, and electro‐optical studies of a ferroelectric nematic LC material doped with commercially available chiral dopants. While the NF phase of the undoped LC is only monotropic, the chiral NF phase is enantiotropic, indicating a chirality induced stabilization of the polar nematic order. Compared to undoped NF material, a remarkable improvement of the electro‐optical switching time is demonstrated in the chiral doped materials. The color of the chiral mixtures that exhibit a selective reflection of visible light in the chiral NF phase, can be reversibly tuned by 0.02–0.1 V µm−1 in‐plane electric fields, which are much smaller than typically required in full‐color cholesteric LC displays and do not require complicated driving scheme. The fast switchable reflection color at low fields has potential applications for LC displays without backlight, smart windows, shutters, and e‐papers.
The newly discovered ferroelectric nematic (NF) phase becomes enantiotropic when doped with two commercially available chiral dopants. The selective reflection of visible light of the NF∗ phase can be reversibly tuned in sub‐milliseconds by only 0.02–0.1 V µm−1 in‐plane electric fields, which are much smaller than full‐color displays need, and do not need complicated sequential driving scheme.
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19.
Electroresponsive Ionic Liquid Crystal Elastomers Feng, Chenrun; Rajapaksha, Chathuranga P. Hemantha; Cedillo, Jesus M. ...
Macromolecular rapid communications.,
October 2019, Volume:
40, Issue:
19
Journal Article
Peer reviewed
This paper describes the preparation, physical properties, and electric bending actuation of a new class of active materials—ionic liquid crystal elastomers (iLCEs). It is demonstrated that iLCEs can ...be actuated by low‐frequency AC or DC voltages of less than 1 V. The bending strains of the unoptimized first iLCEs are already comparable to the well‐developed ionic electroactive polymers. Additionally, iLCEs exhibit several novel and superior features, such as the alignment that increases the performance of actuation, the possibility of preprogrammed actuation patterns at the level of the cross‐linking process, and dual (thermal and electric) actuations in hybrid samples. Since liquid crystal elastomers are also sensitive to magnetic fields and can also be light sensitive, iLCEs have far‐reaching potentials toward multiresponsive actuations that may have so far unmatched properties in soft robotics, sensing, and biomedical applications.
The preparation and electric bending actuation of the first ionic liquid crystal elastomers are presented. They can be actuated by low (≈1 V) voltages, providing bending strains comparable to ionic electroactive polymers. The actuation is sensitive to alignment of the LCE director, offering surface‐pattern‐regulated bending shapes. Samples with hybrid alignment can combine thermal and electric actuation.
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In the past decades, ion conductive polymers and elastomers have drawn worldwide attention for their advanced functions in batteries, electroactive soft robotics, and sensors. Stretchable ionic ...elastomers with dispersed soft ionic moieties such as ionic liquids have gained remarkable attention as soft sensors, in applications such as the wearable devices that are often called electric skins. A considerable amount of research has been done on ionic-elastomer-based strain, pressure, and shear sensors; however, to the best of our knowledge, this research has not yet been reviewed. In this review, we summarize the materials and performance properties of engineered ionic elastomer actuators and sensors. First, we review three classes of ionic elastomer actuators—namely, ionic polymer metal composites, ionic conducting polymers, and ionic polymer/carbon nanocomposites—and provide perspectives for future actuators, such as adaptive four-dimensional (4D) printed systems and ionic liquid crystal elastomers (iLCEs). Next, we review the state of the art of ionic elastomeric strain and pressure sensors. We also discuss future wearable strain sensors for biomechanical applications and sports performance tracking. Finally, we present the preliminary results of iLCE sensors based on flexoelectric signals and their amplification by integrating them with organic electrochemical transistors.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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