Adriaan De Vries

Bio: Adriaan De Vries is an academic researcher from Liquid Crystal Institute. The author has contributed to research in topics: Liquid crystal & Diffraction. The author has an hindex of 15, co-authored 22 publications receiving 1157 citations.

X-ray Photographic Studies of Liquid Crystals I. A Cybotactic Nematic Phase

Abstract: The X-ray diffraction patterns of the nematic phase of bis-(4′-n-octyloxybenzal)-2-chloro-1, 4-phenylenediamine show major maxima at two diffraction angles. The ones at the larger angle are due to the interaction of neighboring, parallel molecules and the average distance between the long axes of the molecules can be calculated using the formula 2d sin θ = 1.117γ. The maxima at the smaller diffraction angle are related to the length of the molecule; the observed diffraction pattern is explained by assuming that a part of the molecules are organized in groups (cybotactic groups) in such a way that the ends of the molecules lie in fairly well defined planes making an angle of about 45° with the direction of the long axis of the molecules. At lower temperatures the large majority of the molecules are arranged in these groups, and the groups appear to be fairly rigid. At higher temperatures the groups become less prominent, but they do not completely disappear.

Experimental Evidence Concerning Two Different Kinds of Smectic C to Smectic A Transitions

Abstract: Experimental evidence is presented indicating that there are two distinctly different kinds of smectic C to smectic A transitions. The first type is second order with zero tilt in both phases, the second type is first order with a non-zero tilt angle in both phases. The model for the second type of transition is an “uncoupling” of the tilt directions of adjacent layers in the smectic -A phase.

Evidence for the Existence of More Than One Type of Nematic Phase

Abstract: It is suggested that there may be three types of nematic phases: the skewed cybotactic phase, in which the molecules are arranged in groups in such a way that the centers of the molecules in each group lie in a plane making an angle α (significantly different from 90°) with the mean direction of the molecules in that group; the normal cybotactic phase, similar to the skewed one but with α close to 90°; and the classical nematic phase, in which no regular arrangement between neighboring molecules exists Three means of distinguishing between these phase types are discussed: the X-ray diffraction pattern, the type of the preceding smectic phase, and the microscopic textures It is shown that the application of the last two methods to literature data yields support for the concept of different types of nematic phases

Why the Molecules are Tilted in all Smectic A Phases, and How the Layer Thickness Can be Used to Measure Orientational Disorder*

Abstract: Even though the average molecular direction in smectic A phases is perpendicular to the smectic planes, most molecules in these phases have rather large tilt angles because the orientational order parameter S in all smectic A phases is significantly less than unity. This molecular tilt inherent to all A phases accounts for the observed differences between layer thickness d and molecular length l of four quite different compounds, without any need for assuming additional tilt, interdigitation, or kinking of chains. From the differences between d and l , S values from 0.78 to 0.84 are obtained, which yield average molecular tilt angles from 20° to 17°. It is predicted that all smectic A phases with monomolecular layers will follow the same pattern. The concept of “inherent tilt” should apply to all orthogonal smectic phases, thermotropic as well as lytropic.

X-Ray Photographic Studies of Liquid Crystals: II. Apparent Molecular Length and Thickness in Three Phases of Ethyl-p-ethoxybenzal-p-aminobenzoate

Abstract: A general theory is developed for the relation between the molecular length and the position of a diffraction maximum found at diffraction angles of the order of 4°, for nematic and isotropic phases containing elongated molecules. Using this theory and a previously developed one for the relation between the intermolecular distance and the position of the major diffraction maximum at larger diffraction angles, molecular parameters are determined for three phases of ethyl-p-sthoxybenzal-p-aminobenzoate. These parameters are: the thickness of the smectic layers (19.941 A), the length of the molecules in the isotropic phase (21.4 A), and the intermolecular distances in the smectic (4.894 A), nematic (4.932–4.950 A), and isotropic phases (4.950–5.182 A), all as a function of temperature. From these data, densities and volume expansion coefficients are calculated which agree very well with literature data on similar compounds. There appears to be no room for rotation of the molecules around their long ...

Physics of liquid crystals

Abstract: This review discusses the physical properties of nematic, cholesteric, and smectic liquid crystals. Molecular theories of the liquid crystal phases are discussed and the molecular field theories of the phase transitions between the various liquid crystal phases are presented. The elastic theory and hydrodynamics of liquid crystals is developed. A wide variety of phenomena in liquid crystals, including elastic distortions, disclinations, flow properties, fluctuations, light scattering, wave propagation, nuclear magnetic resonance, effects of magnetic and electric fields, electrohydrodynamics, and optical properties, is discussed.

Bent-core liquid crystals: polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems

Abstract: An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.

Ionic Liquid Crystals: Versatile Materials.

Abstract: This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.