Volodymyr Borshch

Nematic twist-bend phase with nanoscale modulation of molecular orientation.

A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).

Direct observation of liquid crystals using cryo‐TEM: Specimen preparation and low‐dose imaging

Liquid crystals (LCs) represent a challenging group of materials for direct transmission electron microscopy (TEM) studies due to the complications in specimen preparation and the severe radiation damage. In this paper, we summarize a series of specimen preparation methods, including thin film and cryo‐sectioning approaches, as a comprehensive toolset enabling high‐resolution direct cryo‐TEM observation of a broad range of LCs. We also present comparative analysis using cryo‐TEM and replica freeze‐fracture TEM on both thermotropic and lyotropic LCs. In addition to the revisits of previous practices, some new concepts are introduced, e.g., suspended thermotropic LC thin films, combined high‐pressure freezing and cryo‐sectioning of lyotropic LCs, and the complementary applications of direct TEM and indirect replica TEM techniques. The significance of subnanometer resolution cryo‐TEM observation is demonstrated in a few important issues in LC studies, including providing direct evidences for the existence of nanoscale smectic domains in nematic bent‐core thermotropic LCs, comprehensive understanding of the twist‐bend nematic phase, and probing the packing of columnar aggregates in lyotropic chromonic LCs. Direct TEM observation opens ways to a variety of TEM techniques, suggesting that TEM (replica, cryo, and in situ techniques), in general, may be a promising part of the solution to the lack of effective structural probe at the molecular scale in LC studies. Microsc. Res. Tech. 77:754–772, 2014.

Viscoelasticity, dielectric anisotropy, and birefringence in the nematic phase of three four-ring bent-core liquid crystals with an L-shaped molecular frame

Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid four-ring core in the shape of the letter ‘L’. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K1 and bend K3 elastic constants, splay viscosity ηsplay and flow viscosities η|| and η⊥. The bend–splay anisotropy δK31 = K3 − K1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of V-shape. The dielectric anisotropy Δe and birefringence are positive in the entire nematic range. The splay viscosity ηsplay and the flow viscosities η|| and η⊥ are smaller than the viscosities measured for the symmetric V-shaped bent-core materials at similar temperatures. The ratio Γ = ηsplay/η||,⊥ is in the range 5–4 that is typical for rod-like LCs. The reported L-shaped bent-core nematic LCs combine the useful features of bent-core LCs (such as a negative δK31, suitable for formulation of broad-range blue phases) with the relatively low viscosities, a property typical for rod-like LCs and beneficial for electro-optic switching applications.

Electro-optic switching of dielectrically negative nematic through nanosecond electric modification of order parameter

We present experimental studies of nanosecond electric modification of the order parameter (NEMOP) in a variety of nematic materials with negative dielectric anisotropy. The study demonstrates that NEMOP enables a large amplitude of fast (nanoseconds) electro-optic response with the field-induced birefringence on the order of 0.01 and a figure of merit (FoM) on the order of 104 μm2/s; the latter is orders of magnitude higher than the FoM of the Frederiks effect traditionally used in electro-optic nematic devices. The amplitude of the NEMOP response is generally stronger in nematics with larger dielectric anisotropy and with higher natural (field-free) birefringence.

Properties of the self-deforming Ntb phase in mesogenic dimers

An overview of the results obtained from the most recent experiments performed for revealing the structure of the twistbend nematic Ntb phase will be presented at the conference. This new phase provides typical X-ray diffraction pattern for the nematic phase and is found at temperatures below the conventional nematic phase in odd-chain hydrocarbon linked mesogenic dimers. The materials in the Ntb phase form self-deformed striped pattern parallel to the rubbing direction in planarly aligned rubbed cells with a well-defined period. The period is found to depend on the cell spacing. The selfdeformation stripes appear without any external electromagnetic field or thickness gradient across the cell. Although the materials are composed of non-chiral molecules, the low temperature nematic phase exhibits fast linear optical response of the order of a few microseconds. This response is reminiscent of the phase exhibiting chirality. Moreover, at higher fields some of the materials form striped domains with opposite direction of the optical response. These stripes appear normal to the rubbing direction and their periodicity depends on voltage and frequency. The Freedericksz transition in this phase also shows unusual properties and this is proven to be of the first order. The techniques to characterize this phase include polarized microscopy observation and optical contrast spectroscopy. Possible causes of the phenomena will be discussed.

Nanosecond electro-optics of a nematic liquid crystal with negative dielectric anisotropy

We study a nanosecond electro-optic response of a nematic liquid crystal in a geometry where an applied electric field E modifies the tensor order parameter but does not change the orientation of the optic axis (director N ̂). We use a nematic with negative dielectric anisotropy with the electric field applied perpendicularly to N ̂. The field changes the dielectric tensor at optical frequencies (optic tensor) due to the following mechanisms: (a) nanosecond creation of the biaxial orientational order, (b) uniaxial modification of the orientational order that occurs over time scales of tens of nanoseconds, and (c) the quenching of director fluctuations with a wide range of characteristic times up to milliseconds. We develop a model to describe the dynamics of all three mechanisms. We design the experimental conditions to selectively suppress the contributions from the quenching of director fluctuations (c) and from the biaxial order effect (a) and thus, separate the contributions of the three mechanisms in the electro-optic response. As a result, the experimental data can be well fitted with the model parameters. The analysis provides a rather detailed physical picture of how the liquid crystal responds to a strong electric field on a time scale of nanoseconds. The paper provides a useful guidance in the current search for the biaxial nematic phase. Namely, the temperature dependence of the biaxial susceptibility allows one to estimate the temperature of the potential uniaxial-to-biaxial phase transition. An analysis of the quenching of director fluctuations indicates that on a time scale of nanoseconds, the classic model with constant viscoelastic material parameters might reach its limit of validity. The effect of nanosecond electric modification of the order parameter can be used in applications in which one needs to achieve ultrafast (nanosecond) changes in optical characteristics, such as birefringence.

Kerr effect at high electric field in the isotropic phase of mesogenic materials.

The well-known Kerr effect in isotropic fluids consists in the appearance of uniaxial orientational order and birefringence that grows as the square of the applied electric field. We predict and observe that at a high electric field, the Kerr effect displays features caused by the nonlinear dependence of dielectric permittivity on the field-induced orientational order parameter. Namely, the field-induced birefringence grows faster than the square of the electric field and the dynamics of birefringence growth slows down as the field increases. As a function of temperature, the field-induced birefringence is inversely proportional to the departure from an asymptotic critical temperature, but this temperature is no longer a constant (corresponding to the lower limit of the supercooled isotropic phase) and increases proportionally to the square of the electric field.

Electric Field Induced Biaxial Order and Differential Quenching of Uniaxial Fluctuations in a Nematic with Negative Dielectric Anisotropy

We explore the response of a uniaxial nematic liquid crystal with a negative dielectric anisotropy to the electric field applied normally to the director. The geometry causes two effects: field-induced biaxiality order (FIBO) and differential quenching of uniaxial fluctuations (DQUF). We separate the two effects by tracing the field-induced changes in optical response at the timescales of nanoseconds. The study paves the way to establish the possibility of field-assisted formation of the biaxial nematic phase and to explore the likelihood of the appearance of biaxial order in the field-free conditions.

Nanosecond electric modification of order parameter in nematic and isotropic phases of materials with negative and positive dielectric anisotropy

We demonstrate nanosecond electro-optic switching in the nematic and isotropic phases of two nematic materials, one with a negative dielectric anisotropy (HNG715600-100) and another with a positive dielectric anisotropy (8CB). In both cases, the effect is caused by the nanosecond electric modification of the order parameter (NEMOP). In NEMOP effect, the electric field is applied in such a way that the director alignment is not distorted. The NEMOP effects in the nematic phases are compared to the Kerr effects of the isotropic phases of the same two materials. Although the amplitudes of NEMOP and Kerr effects are comparable, we observe differences in temperature dependencies. We also observe that the field-induced Kerr birefringence in the isotropic phases does not follow the expected quadratic dependence on the applied field. Namely, it grows slower in the dielectrically negative material and faster in the dielectrically positive material.

Liquid crystal-directed polymer nanostructure for vertically aligned nematic cells

We demonstrate a fast switching surface polymer-assisted vertically aligned (SPA-VA) liquid crystal (LC) cell. The deposition of the polymer nano spikes as a part of the alignment layer is achieved by polymerizing a small amount of a reactive monomer in vertically aligned liquid crystal in the absent of an applied voltage. The phase-separated polymer localized at the both substrate surfaces and formed nano-sized spikes. These polymer nano-spikes act as internal surface alignment layers and enhance the speed of field-induced reorientation of liquid crystal molecules.