research

Electro-Active Ionic Elastomers Motivated by the low voltage driven actuation of ionic Electroactive Polymers (iEAPs) [1] [2], recently we began investigating ionic elastomers. In this talk I will discuss the preparation, physical characterization and electric bending actuation properties of two novel ionic elastomers; ionic polymer electrolyte membranes (iPEM), and ionic liquid crystal elastomers (iLCE).[3] Both materials can be actuated by low frequency AC or DC voltages of less than 1 V. The bending actuation properties of the iPEMs are outperforming most of the well-developed iEAPs, and the not optimized first iLCEs are already comparable to them. Ionic liquid crystal elastomers also exhibit superior features, such as the alignment dependent actuation, which offers the possibility of pre-programed actuation pattern at the level of cross-linking process. Additionally, multiple (thermal, optical and electric) actuations are also possible. We also study issues with compliant electrodes and possible soft robotic applications. [1] Y. Bar-Cohen, Electroactive Polyer Actuators as Artficial Muscles: Reality, Potential and Challenges, SPIE Press, Bellingham, 2004. [2] O. Kim, S. J. Kim, M. J. Park, Chem. Commun. 2018, 54, 4895. [3] C. Feng, C. P. H. Rajapaksha, J. M. Cedillo, C. Piedrahita, J. Cao,V. Kaphle, B. Lussem, T. Kyu, A. I. Jákli, Macromol. Rapid Commun. 2019, 1900299.

Current Research

Responsive Liquid Crystal/Polymer Fibers Airbrushing of a homogeneous LC and polymer solution to make LC/polymer as well electro- spinning and force spinning are techniques via which piezoelectric and responsive fibers are obtained. Quantitative measurements of the optical response of liquid crystal (LC)/polymer composite fiber mats to toluene and acetone vapors have been. Reported Our analyses in comparison with control measurements of pure LC film and polymer fiber mats show that the chemicals can pass through the polymer sheath of the fibers and be absorbed by the LC in the core. This absorption changes the optical properties of the fiber mats which can be used to produce sensitive and reversible detection. The sensitive response at low concentrations of both acetone and toluene demonstrates the feasibility of using these fibers for highly sensitive and specific sensors for volatile organic compound detection. A simple process is demonstrated to clad conventional monofilament fibers with low molecular weight liquid crystals stabilized by an outer polymer sheath. The fibers retain the responsive properties of the LCs in a highly flexible/drapable format. The monofilament core makes these fibers much more rugged with a magnified response to external stimuli when compared to previously reported LC core fibers produced by electrospinning or airbrushing. The microscopic structure and the optical properties of round and flattened fibers are reported. The sensitivity of the response of individual fibers can be tuned over a broad range by varying the composition of the liquid crystals. Complex fabrics can be easily woven from fibers that respond to different external stimuli, such as temperature variation, chemicals and pressure. The fabrics can be fashioned into garments that can sense and report the state of health or the environment. [1] Yu Guan, Dena Mae Agra-Kooijman, Shaohai Fu, Antal Jákli, and John L. West, “Responsive Liquid-Crystal-Clad Fibers for Advanced Textiles and Wearable Sensors”, Advanced Materials, 1902168 (2019); DOI: 10.1002/adma.201902168 [2] Md Mostafa, Deña Mae Agra-Kooijman, Kelum Perera, Alex Adaka,John L. West and Antal Jákli, “Colloidal Latex / Liquid Crystal Coatings for Thermochromic Textiles”, Colloid and Interface Science Communications, 52, 100693, 2023, https://doi.org/10.1016/j.colcom.2022.100693,

Biological Sensing With Liquid Crystal Liquid-crystal–based biosensors utilize the high sensitivity of liquid-crystal alignment to the presence of amphiphiles adsorbed to one of the liquid-crystal surfaces from water. They offer inexpensive, easy optical detection of biologically relevant molecules such as lipids, proteins, and cells. The techniques use linear or circular polarizers to analyze the alignment of the liquid crystal. [1] Kelum Perera, Thiloka M. Dassanayake, Mahesha Jeewanthi, H. Nilanthi Padmini, Songping D. Huang, Edgar Kooijman, Elizabeth Mann, and Antal Jákli, “Liquid Crystal-based Detection of Antigens with ELISA Sensitivity”, Advanced Materials Interfaces, 2200891 1-8 (2022) Liquid Crystal Spherical Caps in Magnetic & Electric Fields We carry out both experimental and theoretical studies of the director structure in magnetic and electric fields of nematic liquid crystal drops forming spherical cap shape, with initial director structure perpendicular to the air and glass boundaries. At sufficiently high magnetic or electric fields a metastable inversion wall forms in the middle of the drop perpendicular to the fields and then moves outward. In DC magnetic and low-frequency electric fields, the wall direction is unchanged during the motion, while under high frequency (>120Hz) electric fields the defect wall rotates parallel to the field. The linear displacement of the electric field-induced defect wall combined with the threshold for director deformation enables to determine both the bend elastic constant and the rotational viscosity. Drops with uniform director structure can be used as tunable optical lenses, where the focal length can be controlled by light polarization, viewing angle, magnetic, or electric fields. Uniform electric field-induced generation, rotation, and linear movement of defect walls is a unique phenomenon in soft matters. Articles Published: [1] P. Salamon, Z. Karaszi, V. Kenderesi, Á. Buka, A. Jákli, Liquid crystal spherical caps in magnetic fields, Phys. Rev. Res. 2 (2020), https://doi.org/10.1103/physrevresearch.2.023261 023261. [2] Z. Karaszi, P. Salamon, Á. Buka, A. Jákli, Lens shape liquid crystals in electric fields, Journal of Molecular Liquids 334 (2021) 116085, https://doi.org/10.1016/j.molliq.2021.116085 0167- 7322. [3] Zoltán Karaszi, Marcell Máthé, Péter Salamon, Ágnes Buka, Antal Jákli; “Electric field induced buckling of inversion walls in lens-shape liquid crystal droplets”, Journal of Molecular Liquids 365, 120177-1-7 (2022) [4] Zoltán Karaszi, Marcell Máthé, Péter Salamon, Ágnes Buka, Antal Jákli; “Lens-shaped nematic liquid crystal droplets with negative dielectric anisotropy in electric and magnetic fields”, Liquid Crystals, DOI: 10.1080/02678292.2022.2134594, published online (October 24, 2022) [5] Ágnes Buka, Péter Salamon, Marcell Máthé, Zoltán Karaszi, Antal Jákli, “Nematic liquid crystals in lens shape geometry”, Liquid Crystals (Invited paper) (January 30, 2023), 10.1080/02678292.2023.2168307

Flexo-ionic Effect of Ionic Liquid Crystal Elastomers (iLCE) Flexoelectricity that couples electric polarization with strain gradient has been studied independently in both crystals and liquid crystals since the 1960s[1,2]. In crystalline solids the electric polarization is attributed to the electron displacenet of centrosymmetric crystals. In liquid crystals it arises when the director of anisometric (pear- or bent-shape) molecules have splay or bend deformations resulting in a net polarization. Recently, a new class of electromechanical coupling is introduced which is very similar to the flexoelectric effect except for that the induced polarization is due to displacement of differently sized of cations and anions [3–7]. These materials possess very large flexo-ionic coefficients (29 – 323 μC/m), Such a large effect may be useful in soft robotics, sensors, and micro power generation. Motivated by those work, we study flexo-ionic effects of a new class of materials, ionic liquid crystal elastomers. We investigate the material properties (elastic modulus, phase transition, ionic conductivity, morphology) and the alignment dependence of flexo-ionic coefficients. [1] Mayer, R.M. Piezoelectric Effects in Liquid Crystals. Phys. Rev. Lett. 1969, 22, 25–29. [2] Kogan, S.M. Piezoelectric effect during inhomogeneous deformation and acoustic scattering of carriers in crystals. Sov. Phys. Solid State 1964, 5, 2069. [3] Rajapaksha, C.P.H.; Feng, C.; Piedrahita, C.; Cao, J.; Kaphle, V.; Lüssem, B.; Kyu, T.; Jákli, A. Poly(ethylene glycol) Diacrylate Based Electro‐Active Ionic Elastomer. Macromol. Rapid Commun. 2020, 41, 1900636, doi:10.1002/marc.201900636. [4] Feng, C.; Rajapaksha, C.P.H.; Cedillo, J.M.; Piedrahita, C.; Cao, J.; Kaphle, V.; Lussem, B.; Kyu, T.; Jákli, A.I. Electro-responsive Ionic Liquid Crystal Elastomers. Macromol. Rapid Commun. 2019, 1900299, doi:DOI: 10.1002/marc.201900299. [5] Piedrahita, C.R.; Yue, P.; Cao, J.; Lee, H.; Rajapaksha, C.P.; Feng, C.; Jákli, A.; Kyu, T. Flexoelectricity in Flexoionic Polymer Electrolyte Membranes: Effect of Thiosiloxane Modification on Poly(ethylene glycol) Diacrylate and Ionic Liquid Electrolyte Composites. ACS Appl. Mater. Interfaces 2020, 12, 16978–16986, doi:10.1021/acsami.0c02328. [6] Albehaijan, H.A.; Piedrahita, C.R.; Cao, J.; Soliman, M.; Mitra, S.; Kyu, T. Mechanoelectrical Transduction of Polymer Electrolyte Membranes: Effect of Branched Networks. ACS Appl. Mater. Interfaces 2020, 12, 7518–7528, doi:10.1021/acsami.9b15599. [7] A. Albehaijan, H.; Cao, J.; R. Piedrahita, C.; Jákli, A.; Kyu, T. Role of Cationic Size and Valency in Mechanoelectrical Transduction of Ion-Containing Polymers. ACS Sustain. Chem. & Eng. 2021, 0, doi:10.1021/acssuschemeng.0c08252.

Microlens Arrays The LC-based microlens array has attractive applications in optoelectronics, integrated optics, optical fiber switches, information processing, optical communications, and imaging processing etc. Most of the Liquid crystal microlens arrays required very delicate fabrication process. We study cholesteric liquid crystal microlens arrays with different configurations which shows unique geometrical and optical properties. Our focus is to develop simple, cost-effective fabrication process for microlenses using different cholesteric liquid crystal mixtures and investigate the optical capabilities of the microlenses for various applications. [1] K. Perera, A. Nemati, E. Mann, T. Hegmann and A. Jákli, ”Converging microlens array using nematic liquid crystals doped with chiral nanoparticles”, ACS Applied Materials and Interfaces, 13, 4574-4582 (2021); DOI: https://dx.doi.org/10.1021/acsami.0c21044 -Cover Picture [2] Kelum Perera, H Nilanthi Padmini, Elizabeth Mann and Antal Jákli, “Polymer Stabilized Paraboloid Liquid Crystal Microlenses with Integrated Pancharatnam–Berry Phase”, Advanced Optical Materials, 2101510 (1-8) (2021) [3] Kelum Perera, Nilanthi Haputhantrige, Md Sakhawat Hossain Himel, Md Mostafa, Alex Adaka, Oleg D. Lavrentovich and Antal Jákli, “Electrically tunable chiral liquid crystal lens arrays” SPIE 12658-30 (August 1.2023)

Recent Cover Images

Textiles and Wearable Sensors Based on a simple process we proposed recently[1] to clad conventional monofilament fibers with liquid crystal/polymer composites, we are developing fibers that retain the responsive properties of the LCs but in a highly flexible/drapable format. The monofilament core makes these fibers much more rugged with a magnified response to external stimuli when compared to previously reported LC-core fibers produced by electrospinning or airbrushing. The sensitivity of the response of individual fibers can be tuned over a broad range by varying the composition of the LCs. Complex fabrics can be easily woven from fibers that respond to different external stimuli, such as temperature variation, chemical concentrations, and pressure. The fabrics can be fashioned into garments that can sense and report the state of health of the wearer or the status of their environment. [1] Guan Y, Agra-Kooijman DM, Fu S, Jákli A, West JL (2019) Responsive Liquid-Crystal-Clad Fibers for Advanced Textiles and Wearable Sensors. Advanced Materials, :1902168-1–5. https://doi.org/10.1002/adma.201902168 [2] Deña Mae Agra-Kooijman, Md Mostafa, Mourad Krifa, Linda Ohrn-McDaniel, John L. West, Antal Jákli, Liquid Crystal Coated Yarns for Thermo-Responsive Textile Structures, Fibers, 11, 3 (2023); https://doi.org/10.3390/fib1101000

Ferroelectric Nematic Liquid Crystals The recently discovered ferroelectric nematic (N_F) liquid crystals (LCs) with over 0.04 C/m2 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. We studied electrical, optical, and electro-optical studies of a ferroelectric nematic LC material doped with commercially available chiral dopants. While the N_F phase of the undoped LC is only monotropic, the chiral N_F phase is enantiotropic, indicating a chirality induced stabilization of the polar nematic order. Compared to undoped N_F 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 N_F phase, can be reversibly tuned by 0.02-0.1V/µm in-plane electric fields, which are much smaller than typically required in full-color cholesteric LC displays and do not require complicated driving scheme.[1] The fast switchable reflection color at low fields has potential applications for LC displays without backlight, smart windows, shutters and e-papers. We studied the physical properties of a new compound, 4-nitrophenyl 4-[(2,4- dimethoxylbenzoyl)oxy]-2-fluorobenzoate (RT11001) synthesized by RobertTwieg. This material exhibits multiple, highly polar, ferroelectric nematic phases that have not been previously reported. We employ a wide range of physical characterization methods including differential scanning calorimetry (DSC), mass density measurement, optical birefringence, polarizing optical microscopy (POM), dielectric spectroscopy, electric current analysis, electro-optical switching, small-angle and wide-angle x-ray scattering measurements to show that RT11001 has multiple, distinct ferroelectric phases. We argue that the highest temperature phase is a polar nematic fluid with non-polar smectic clusters. Directly below appears to be a transition to another polar nematic phase containing polar positionally ordered clusters. Lastly, there are indications of an additional, polar biaxial liquid crystal phase at lower temperatures. [2] We presented 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 1mV/m 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. [3] In collaboration with researchers at the Wigner Research Centre for Physics, we studied RM734 sessile drops. The director profile was found to be radial at the LC-air interface in the N phase. When the top surface transitions to the N_F phase while the bottom is still at the N phase, the director is spiraling. When the entire drop is in the N_F phase, the director and the ferroelectric polarization have a circular distribution. Additionally, it was demonstrated that a thermal gradient- induces a circular motion of particles placed on the drop. A simple model shows that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become electrically charged in the fluid.[4] In frame of this collaboration we also studied 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. [5] Freestanding slender ferroelectric nematic fluid filaments forming at zero electric field or stabilized under axial electric fields between suspending wires are described. The electric stabilization is similar to those observed in dielectric fluids such as water, but the voltages required are three orders of magnitude smaller in ferroelectric nematic materials than in dielectric fluids. The stabilization can be also achieved by low frequency AC voltages that lead to transversal coupled damped vibrations. The analysis of the results can provide the ferroelectric tension and the viscosity of the material, while the measurements of the transient electric current can provide the measure of the ferroelectric polarization and the switching time. [6] The observed effects are not only unique and novel, thus have fundamental scientific merits, they may also prove to have practical applications such as their use in nano-fluidic in micron-size logic devices, switches, and relays. [1] Chenrun Feng, Rony Saha, Eva Korblova, David Walba, Samuel N. Sprunt, and Antal Jákli, “Electrically Tunable Reflection Color of Chiral Ferroelectric Nematic Liquid Crystals”, Adv. Optical Mater. 2021, 2101230 [2] Rony Saha, Pawan Nepal, Chenrun Feng, Md Sakhawat Hossein, James T. Gleeson, Samuel Sprunt, Robert J. Twieg, Antal Jákli, “Multiple ferroelectric nematic phases of a highly polar liquid crystal compound”, arXiv:2104.06520v and Liq. Cryst., 49, (13), 1784-1796 [3] Kelum Perera, Rony Saha, Pawan Nepal, Rohan Dharmarthna, Md Sakhawat Hossain, Md Mostafa, Alex Adaka, Ronan Waroquet, Robert J. Twieg and Antal Jákli, Ferroelectric Nematic Droplets in their Isotropic Melt, ArXiv 2209.09363 (2022); Soft Matter, 19, 347 – 354 (2023), DOI: 10.1039/D2SM01395A [4] Marcell Tibor Máthé, Ágnes Buka, Antal Jákli, Péter Salamon, “Ferroelectric nematic liquid crystal thermo-motor”, ArXiv: 2201.07556 (2022); Physical Review E, 105, L052701 (2022) [5] Marcell Tibor Máthé, Bendegúz Farkas, László Péter, Ágnes Buka, Antal Jákli, Péter Salamon, “Electric field-induced interfacial instability in a ferroelectric nematic liquid crystal”, arXiv:2210.14329v1, Scientific Reports, 13:6981 (2023) [6] Marcell Tibor Máthé, Kelum Perera, Ágnes Buka, Péter Salamon, Antal Jákli, “Fluid ferroelectric filaments”, ArXiv: 2307.16588,