Dynamics of smectic membranes as studied by X-ray and neutron scattering

This thesis describes x-ray photon correlation spectroscopy (XPCS) and neutron spin echo (NSE) measurements of fluctuations in smectic A (SmA) liquid crystal membranes. XPCS is a classic dynamic light scattering technique extended into the x-ray domain, which opens the possibility to investigate dynamics at molecular length scales. Using a spatially coherent xray source available these days at third-generation synchrotrons, the time dependence of the scattered intensity is observed. In XPCS experiments spatially coherent x rays are used while working with a single coherence volume. The scattered intensity changes parallel to the positions of the scatterers in the coherence volume. Constructing the intensity-intensity time correlation function one can extract the relaxation times of the fluctuations in the system. We have extended this technique into the sub-microsecond range, approaching the limit of 2.8 ns determined by the bunch structure of the current in the storage ring of the synchrotron. This allows a comparision with NSE spectroscopy which covers times up to about 100 ns. Smectic liquid crystal membranes are highly uniformly ordered, strongly fluctuating and scattering systems that constitute excellent models of low-dimensional ordering. SmA liquid crystal membranes consist of stacks of liquid layers of elongated molecules (no positional ordering within the layers). Consequently, one-dimensional ordering exists along the normal to the layers. Thermal fluctuations in such a system increase with the sample size and eventually destroy the ordering (Landau-Peierls instability). Nevertheless, SmA membranes can be prepared with thicknesses ranging from two to several thousand layers, because the divergence of the fluctuations is weak (logarithmic). We have studied three SmA liquid crystals: 4-octyl- 4’-cyanobiphenyl (8CB), N-(4-n-butoxybenzilidene)-4-n-octylaniline (4O.8) and 4-heptyl-2-[4- (2-perfluorhexylethyl)phenyl]-pyrimidin (FPP). 8CB differs from the other two by a strongly polar end group, 4O.8 is characterized by fast equilibration, and FPP membranes are specific because of the presence of fluorine, leading to strong scattering and a low surface tension. The relaxation behaviour in smectic membranes can be divided into three regimes, characterized by oscillatory relaxation, surface dominated and bulk-elasticity dominated exponential relaxation, respectively. The determining quantity is the largest wavelength from the full spectrum of fluctuations that contributes to the specific situation. Oscillatory relaxations are observed at specular positions for long-wavelength fluctuations. A transition from oscillatory to exponential relaxation is determined by a crossover wave vector qc. Only fluctuations with wave vectors qmn > qc show exponential relaxation, which is in first instance surface dominated. The relaxation time in this regime depends on membrane thickness, surface tension and viscosity and does not depend on bulk smectic elastic parameters. Hence, for surfacedominated relaxation themembrane behaves like a liquid film. For largerwave vectors (shorter wavelengths) the relaxation time becomes dependent on the elastic moduli of the membrane while the dependence on membrane thickness and surface tension disappears. In this bulkelasticity dominated regime the fluctuation pattern is defined exclusively by the bulk smectic elasticity of the membrane. The relaxation time now decreases as 1/q2 mn with the wave vector of the fluctuation. Using XPCS at the Bragg position we observed in thin 4O.8 and 8CB membranes oscillatory relaxations, and in thicker membranes simple exponential relaxation. In contrast, in FPP samples at the first Bragg reflection only oscillatory relaxations were seen. Only by going to the second-order peak we found in FPP exponential relaxation. This can be understood as follows. The finite resolution of the x-ray setup and the size of the coherence volume create a ’window’ of wave vectors defining the experimental results. The position of the crossover wave vector qc in this window determines whether oscillatory or exponential relaxation is observed. As qc decreases with the thickness of the membrane as 1/vL, in thin membranes the wave vector ’window’ is dominated by fluctuations exhibiting oscillatory relaxations, while in thicker samples exponential relaxation prevails. For off-specular scattering geometries only single fluctuationswere probedwith awave vector defined by the rocking angle of the sample. For XPCS, which allows only relatively small offset angles because of intensity considerations, only the surface-dominated exponential relaxation regime was accessed. Scattering at far off-specular angles was possible with NSE revealing the 1/q2 mn decay of the relaxation time characteristic for the bulk-elasticity regime. The role of the coherence volume in determining the ’window’ of detected fluctuations was demonstrated in a series of measurements at the different specular scattering angles and for different settings of the detector slits. The results indicate that the relaxation shifts towards larger times for smaller scattering angles, because of the larger coherence volume due to the increased projection of the beamon themembrane surface. Variation of the setting of the detector slits resulted, apart fromchanges in contrast, also in small shifts of the relaxation times. Finally, XPCS measurements at various off-specular positions close to the specular ridge indicate that the form of the correlation function changes abruptly. Close to specular reflection the finite resolution of the x-ray set-up catches the elastic specular component related to the equilibrium structure of the membrane. This signal acts as a ’natural’ reference signal in addition to the fluctuating part, leading to mixing in a heterodyne detection scheme. In the diffuse tails of a rocking curve this reference signal is absent leading to homodyne detection

Formation of pre-election political discourse in the audiovisual media of Georgia

Based on qualitative and quantitative analysis of the audio-visual material of two Georgian media outlets, the empirical base of research on the formation of political discourses and their influence on the socio-political life of the country is presented. Media content during the pre-election period September - October 2020: about 2337 publications of audio-visual character. The analysis of television content is of interest for understanding the information and political environment in a society where media is directly linked to various interest groups. There is a high concentration of the political factor in the media that determines its discourse. Taking into account the increasing political polarization in Georgia, the mass media also take an appropriate stance towards political actors and form discourse that serves the political agenda of a certain interest group. The public receives ideologically processed information and, despite the factor of democratization of the methods of political struggle, finds itself in a situation where political forces use information and disinformation as a method of combating opponents. The presented analysis is one of the first results of the inter-university project “Mass Media in Political Processes of Georgia”, it is crucial for understanding the trends of Georgian mass media related to both internal and external factors. The results of the research show the patterns inherent in different TV channels in the context of media and political environment in Georgia

Evaluation of Medipix2 detector for recording electron diffraction data in low dose conditions

Biophysical Structural Chemistr

Surface and bulk elasticity determined fluctuation regimes in smectic membranes

We report combined x-ray photon correlation spectroscopy (XPCS) and neutron spin echo (NSE) measurements of the layer-displacement fluctuations in smectic liquid-crystal membranes in the range from 10 ns to 10¿¿¿µs. NSE reveals a new regime, determined by bulk elasticity, in which relaxation times decrease with the wave vector of the fluctuations. XPCS probes slower surface-tension-dominated relaxation times, independent of the wave vector. XPCS gives a difference in correlation times at specular and off-specular positions that can be related to different detection scheme

Surface and bulk elasticity determined fluctuation regimes in smectic membranes

We report combined x-ray photon correlation spectroscopy (XPCS) and neutron spin echo (NSE) measurements of the layer-displacement fluctuations in smectic liquid-crystal membranes in the range from 10 ns to 10¿¿¿µs. NSE reveals a new regime, determined by bulk elasticity, in which relaxation times decrease with the wave vector of the fluctuations. XPCS probes slower surface-tension-dominated relaxation times, independent of the wave vector. XPCS gives a difference in correlation times at specular and off-specular positions that can be related to different detection scheme

Формирование предвыборного политического дискурса в аудиовизуальных СМИ Грузии

Based on qualitative and quantitative analysis of the audio-visual material of two Georgian media outlets, the empirical base of research on the formation of political discourses and their influence on the socio-political life of the country is presented. Media content during the pre-election period September - October 2020: about 2337 publications of audio-visual character. The analysis of television content is of interest for understanding the information and political environment in a society where media is directly linked to various interest groups. There is a high concentration of the political factor in the media that determines its discourse. Taking into account the increasing political polarization in Georgia, the mass media also take an appropriate stance towards political actors and form discourse that serves the political agenda of a certain interest group. The public receives ideologically processed information and, despite the factor of democratization of the methods of political struggle, finds itself in a situation where political forces use information and disinformation as a method of combating opponents. The presented analysis is one of the first results of the inter-university project “Mass Media in Political Processes of Georgia”, it is crucial for understanding the trends of Georgian mass media related to both internal and external factors. The results of the research show the patterns inherent in different TV channels in the context of media and political environment in Georgia.На основе качественного и количественного анализа аудиовизуального материала двух грузинских СМИ представлена эмпирическая база исследования о формировании политических дискурсов и их влиянии на общественно-политическую жизнь страны. Изучен медийный контент за предвыборный период 2020 года (сентябрь - октябрь): 2337 публикаций аудиовизуального формата. Анализ телевизионных материалов представляет интерес для понимания информационной и политической среды в обществе, где медиаресурсы непосредственно связаны с различными группами интересов. В СМИ наблюдается высокая концентрация политического фактора, определяющего дискурс того или иного медиа. Принимая во внимание нарастающую политическую поляризацию в Грузии, средства массовой информации занимают соответствующую позицию по отношению к политическим силам и выстраивают дискурс, отвечающий интересам определенных групп акторов. Общественность получает идеологически обработанную информацию и, несмотря на демократизацию методов политической борьбы, находится в ситуации, когда политические силы в угоду партийным интересам используют дозированное информирование и дезинформацию как методы борьбы с оппонентами. Представленный анализ - один из первых результатов межвузовского проекта «Средства массовой информации в политических процессах Грузии», он важен для понимания трендов грузинских СМИ, относящихся как к внутренним, так и к внешним факторам. Результаты исследования раскрывают закономерности, присущие различным телеканалам в контексте медийного пространства и политической среды в Грузии

Characterisation of a counting imaging detector for electron detection in the energy range 10-20 keV

As part of a feasibility study into the use of novel electron detector for X-ray photoelectron emission microscopes (XPEEM) and related methods, we have characterised the imaging performance of a counting Medipix 2 readout chip bump bonded to a Silicon diode array sensor and directly exposed to electrons in the energy range 10-20 keV. Detective Quantum Efficiency (DQE), Modulation Transfer Function (MTF) and Noise Power Spectra (NPS) are presented, demonstrating very good performance for the case of electrons with an energy of 20 keV. Significant reductions in DQE are observed for electrons with energy of 15 keV and less, down to levels of 20% for electrons of 10 keV. © 2012 Published by Elsevier B.V. All rights reserved