# Tomas CarlssonChalmers University of Technology

Tomas Carlsson

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65

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Introduction

## Publications

Publications (65)

The shear flow of nematic Liquid crystals with a positive value of the Leslie viscosity α3 is studied theoretically. Under the assumption that the director will remain in the plane of shear, the equations governing the flow are solved numerically and the director- and velocity-profile are calculated. The validity of this “in-plane” assumption is di...

The flexoelectro-optic effect in short-pitch cholesterics [1] is analysed in terms of applied electric field strengths and material parameters such as the two flexoelectric coefficients es and eb and the three elastic constants. Starting from the free energy density of the uniformly lying-helix (ULH) configuration, including the flexoelectric polar...

Using x-ray and optical methods we have probed the structural organization of an antiferroelectric twist grain boundary phase (TGBC(a)) lying between the regular antiferroelectric smectic-C (SmC(a)* and the smectic-Q (SmQ) or isotropic phase. We find that the twist axis is everywhere perpendicular to the local smectic layer normal and that the heli...

In the ferroelectric helicoidal smectic C* phase the chiral molecules are tilted with their long axes from the normal to smectic layers. The direction of the tilt precesses as one goes from one smectic layer to another resulting in a helicoidal structure. The order parameter of the phase transition from the smectic A to the smectic C* phase is the...

A general deformation of a smectic C liquid crystal is composed of five different distortions, each of which can be made independently. Here we show that to each of these distortions we can assign a simple vector operator. Use of these five basis operators enables us to write down the elastic free energy density as a quadratic form consisting of ni...

This paper presents a theoretical study of the behaviour of smectic A liquid crystals subject to shear flow. The liquid crystal is assumed to consist of uniform, planar smectic layers, on which no external moments and no external boundary conditions have been imposed. It is shown that of the five viscosity coefficients needed for a full description...

The local stability of shear flow of nematic liquid crystals with a positive Leslie viscosity α3 is investigated. Using abstract techniques we show, that there exists a physically realizable in-plane flow for low enough shearing rates. Lower bounds for the onset of instabilities as well as approximate expressions for their thresholds are derived an...

Employing the elastic-hydrodynamic theory of the ferroelectric smectic C* phase, it is easily seen that in most dynamic processes in the system, a coupling between director rotations and macroscopic mass flow exists. Taking this possibility of backflow into account, the equations governing the Goldstone mode dielectric susceptibility are rederived,...

Recently, Carlsson and Osipov reported on observations of the rotation of smectic layers under certain experimental conditions, and a dynamic theory describing such rotations was presented [Carlsson, T., and Osipov, M. A., 1999, Phys. Rev. E , 60, 5619]. In the present work, the rotational motion of the smectic layers of a SmA* liquid crystal, over...

Recently, observations of the rotation of smectic layers under certain experimental conditions have been reported in the literature. In this work, the mechanism of such rotations in chiral smectic-A* liquid crystals under the action of asymmetric, periodic electric fields is studied theoretically. The general conditions for layer rotations have bee...

Today studies of flow phenomena in nematic systems normally employ the so-called Leslie-Ericksen theory. This theory was formulated in its present form in the 1960s, and it represents the culmination of theoretical developments that essentially started at the beginning of this century. This article gives an account of the evolution of the theory.

The equations governing the macroscopic flow behaviour of smectic C and smectic C* liquid crystals are written down and it is demonstrated that the viscoelastic behaviour of these systems demands 20 viscous coefficients and 9 elastic constants for a complete treatment. With the smectic C stress tensor as a starting point, different aspects of the f...

It was recently shown [I. W. Stewart, T. Carlsson, and F. M. Leslie, Phys. Rev. E 49, 2130 (1994)] that chaotic instabilities can occur theoretically in planar samples of smectic-C liquid crystals. These instabilities were found by considering a special perturbation to the dynamic equation when a static electric field was augmented by a weak, slowl...

Continuum theory for nematic liquid crystals predicts two distinct types of flow alignment, one that observed in thermotropic rod-like nematics, and the second not so far observed but thought to be relevant to discotic nematics. This paper examines this latter possibility by considering recent flow alignment predictions for biaxial nematics, partic...

Equations governing the flow behavior of Sm-C and Sm-C* liquid crystals are derived. It is shown that the general theory for the description of the macroscopic flow properties of the system demands 20 viscosity coefficients. Using thermodynamic arguments, one can show that these coefficients must fulfill certain restrictions in terms of inequalitie...

Flow-coupled switching equations are derived for surface-stabilized
ferroelectric liquid crystal devices, starting from Leslie, Stewart and
Nakagawa's continuum theory for smectic C liquid crystals. Comparisons
with nematic like dynamic equations are made.

Professor A. E. Green, FRS, is one of the outstanding British research workers in the field on theoretical and applied mechanics. In addition to two well-known books, he is the author of more than 175 publications. This book contains a set of sixteen up-to-date research papers written by a number of colleagues and friends and cover a range of topic...

The principal components of the dielectric tensor in the isotropic, smectic A∗ and smectic C∗ phases have been measured, with emphasis on the smectic C∗ phase, for a single ferroelectric liquid cystalline compound. The dispersion behaviour of the dielectric biaxiality in the chiral smectic C∗ phase has been determined and it has been established th...

The elasto hydrodynamic equations for coupled reorientation and flow in surface-stabilized ferroelectric liquid-crystal cells have been solved numerically and the results are compared to the case in which the flow effects are neglected. Our simulations show that flow generally makes the switching faster in the case of the dipoles being in an unstab...

An investigation is made of chaos in planar samples of nonchiral smectic-C liquid crystals. When a static electric field is applied at an oblique angle to the layers an explicit solitonlike solution is derived from the smectic continuum equations: this solution describes a moving domain wall. When a suitable oscillating electric field is superimpos...

The soft mode dielectric response of three ferroelectric chiral smectic C* substances is studied in the vicinity of the smectic C*-smectic A* phase transition. For some substances the soft mode dielectric strength exhibits a pronounced anomaly in the C* phase just below the phase transition. We show how this anomaly can be systematically correlated...

A theoretical investigation is made of chaos in planar aligned samples of non-chiral smectic C liquid crystals. Under the application of a suitable oscillating electric field it is possible to determine criteria for the onset of chaos by means of a Melnikov analysis. For any given smectic tilt angle θ the magnitudes and frequencies of the oscillati...

The elastic-hydrodynamic theory for the ferroelectric smectic C* phase is reviewed and the governing equations for the dynamical behaviour of a surface stabilized ferroelectric liquid crystalline cell are written down, taking the possibility of macroscopic mass flow in the system into account. The influence of backflow effects on the dynamical cell...

Backflow effects are familiar from twisted nematic devices, occurring when the electric field director reorientation initiates transient flow of the liquid crystal. We have been exploring similar effects in Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) cells, carrying out computer simulations of coupled director reorientation--flow transi...

The dielectric response of the ferroelectric chiral smectic C phase in a bias electric field is studied theoretically within the Landau model using the constant amplitude approximation. It is argued that the response consists of two modes, one of which is related to the unwinding of the helix (the unwinding mode), whereas the other mode is associat...

This paper presents a formulation of continuum theory for biaxial nematic liquid crystals based upon balance laws for linear
and angular momentum, and derives directly expressions for stress and couple stress. The theory so obtained is more general
than the earlier theory by Saupe, having more terms in the viscous stress, but by determining conditi...

The static dielectric permittivity of the smectic-B* phase is studied as a function of temperature. The field-induced tilt angle, which is due to the electroclinic response, is measured as a function of temperature. The investigated phase precedes the chiral smectic-A* phase upon heating. In the A* phase, the soft-mode dielectric contribution incre...

An expression for the elastic energy of the ferroelectric, chiral smectic C* liquid crystal phase is presented. It is shown that due to the chirality of the system, compared with the elastic energy of the smectic C phase, the elastic energy of the smectic C* phase contains three additional terms, one of which is a surface term. Of the two remaining...

The stress tensor for the biaxial nematic phase is given in such a form that it is easily seen that the dynamic theory of this phase can be formulated as a natural generalization of the Leslie-Ericksen theory of uniaxial nematics. With the biaxial nematic stress tensor as a starting point we introduce the concept of viscous torques and discuss the...

The equations governing the flow behavior of the biaxial nematic phase are derived and presented in such a form that it is easily seen that the fluid dynamic theory of biaxial nematics can be formulated as a natural generalization of the Leslie-Ericksen theory of uniaxial nematics. It is shown that in order to describe the flow behaviour of biaxial...

A theoretical study of the elastic properties of a smectic C liquid-crystalline system consisting of cylindrical smectic layers is presented. We show that the ground state configuration of the c-director for such a system depends on the signs of two combinations of the relevant elastic constants. With these configurations as a starting point, we de...

This article describes the relationships between the alternative formulations of the bulk non-chiral smectic C liquid crystal energies currently available in the literature; the relative equivalence of these energies is discussed and demonstrated.

The behaviour of a biaxial nematic liquid crystal in the presence of electric and magnetic fields is discussed. In terms of the values of the magnetic susceptibilities and the dielectric permittivities, each biaxial nematic compound can be classified to belong to one of thirty-six different states. These states can be grouped together into three di...

The unwinding of the helicoidal structure of ferroelectric liquid crystals by a static electric field is studied theoretically in the constant amplitude approximation. The field dependence of the pitch, of the induced polarization and of the static dielectric susceptibility is evaluated.Close to Tc nonlinear effects in the susceptibility become imp...

A dielectric method for determining the rotational viscosity in thick samples of ferroelectric liquid crystals is described and the results for the DOBA-1-MPC and for the DOBAMBC are presented. In both cases the temperature dependence of the rotational viscosity is of Arrhenius type.

An alternative method, based on dielectric measurements on thick samples, for determining the Goldstone-mode rotational viscosity and the twist elastic constant of ferroelectric, chiral smectic-C* liquid crystals is presented. By the use of this method the temperature dependence of these quantities for p-decyloxibenzilidine-p'-amino-1-methylpropyl-...

Using the generalized Landau model [Phys. Rev. A 36, 1484 (1987)], the temperature and frequency dependence of the complex dielectric constant of the ferroelectric smectic-C* (Sm-C*) phase and the corresponding smectic-A (Sm-A) phase is calculated. It is demonstrated how the dielectric response of the Sm-C* phase generally consists of four modes-tw...

The influence of a bias electric field on the temperature and frequency dependence of the soft mode part of the complex dielectric constant of the chiral smectic A* phase has been studied experimentally. It was found that only close to TS*CS*A does the bias field have any influence on the experimentally determined quantities. Here the relaxation fr...

The concept of rotational viscosities of smectic C and ferroelectric chiral smectic C* liquid crystals is reviewed. We give the expressions for the elastic torque, the rotational torque and the entropy production in spherical polar coordinates for smectic C and ferroelectric chiral smectic C* liquid crystals for pure rotational motion. Comparing th...

The frequency and temperature dependence of the complex dielectric constant was measured near the smectic-C*-smectic-A liquid-crystal phase (Sm-C*-Sm-A) transition for two ferroelectric liquid crystals with a very large spontaneous polarization. The dielectric strengths as well as the corresponding relaxation frequencies of the dielectric modes wer...

The temperature and frequency dependence of the soft mode contribution to the dielectric constant of a ferroelectric liquid crystalline system have been studied experimentally. By applying a bias electric field in the C* phase, strong enough to unwind the helix, we have been able to study the soft mode comparatively deep into the C* phase. From the...

A thermodynamic model of the ferroelectric SmC* phase, based on an extended Landau expansion of the free-energy density, is presented. We discuss how the signs of the parameters entering the model are depending on whether the pitch of the helix is right-handed or left-handed and whether the compound is of the (+) or (-) type in regard to the sign o...

Using the generalized Landau model (Phys. Rev. A, 36, 1484 (1987)), the temperature and wave vector dependence of the eigenfrequencies of the Goldstone mode and the soft mode of ferroelectric SmC* liquid crystals is calculated. The results of the calculations are compared with existing experimental data and it is showed that the qualitative agreeme...

The properties of the extended thermodynamic model (Phys. Rev. A 36, 1484 (1987)) of ferroelectric liquid crystals, which can describe the anomalous thermodynamic behaviour of these systems in the chiral smectic C* phase, are reviewed. In the model the bilinear as well as biquadratic coupling between the tilt of molecules and the spontaneous polari...

The complex dielectric constant as a function of temperature and frequency has been measured in DOBAMBC close to the Sm A-Sm C* phase transition. The relaxation frequency and the dielectric strength of the Goldstone mode and soft mode were determined and the results are compared with the dielectric strength calculated using an extended Landau type...

A Comment on the Letter by S. Dumrongrattane and C. C. Huang, Phys. Rev. Lett. 56, 464 (1986).

The main advances in the field of bulk ferroelectric liquid crystals since 1984 are reviewed. Recent experiments have shown that the molecular tilt and the in-plane spontaneous polarization are not strictly proportional to each other as assumed before. Microscopic experiments such as NMR and NQR have further allowed for a determination of the basic...

A generalized, dimensionless Landau model of the ferroelectric, SmC* phase is presented. The equations governing the tilt, polarization and pitch of ferroelectric, SmC* liquid crystals are given. It is shown that the solutions of these equations agree with available experimental data. It is also shown that the introduction of a biquadratic coupling...

With a generalized Landau expansion of the free-energy density as a basis, we have earlier presented a coherent model, with which we can calculate the temperature dependence of the tilt, polarization, pitch, dielectric susceptibility and heat capacity of the ferroelectric, SmC phase in agreement with experiment (work submitted- to Mol. Cryst. Liq....

Using a generalized Landau type of free-energy density, we develop a theoretical model which, in a unified way, describes the general features of the tilt, polarization, pitch, dielectric susceptibility, and heat capacity of ferroelectric liquid crystals. Rewriting the equations into dimensionless form, we show that six parameters are needed in ord...

Measurements of the spontaneous polarization of DOBAMBC in helicoidal smectic C* phase is presented. It is shown that the polarization shows an anomalous temperature dependence characterized by one square root behaviour close to Tc and by another one far from Tc. This behaviour is explained theoretically to be a consequence of a nonchiral biquadrat...

The complex dielectric constant has been measured in a room-temperature ferroelectric liquid-crystal mixture. We have resolved the experimental data into the Goldstone mode and the soft mode and for each mode determined the dielectric strength and the corresponding relaxation frequency. The results are compared with the dielectric strength calculat...

The generalized Landau model is reviewed, which describes thermodynamic behavior of ferrolectric chiral smectic C* liquid crystals. Polarization and tilt temperature dependences are studied. For small biquadratic tilt-polarization coupling as compared to the bilinear one, the dependences deviate only slightly from the classic square root behavior,...

The unit-sphere description of nematic liquid-crystal configurations first introduced by Thurston [J. Appl. Phys. 52, 3040 (1981)] and dealing with elastic properties of nematic liquid crystals only is extended to include also the study of nematic flows. It is shown how the general features of the flow properties of nematic liquid crystals can be s...

The shear flow behaviour of the nematic liquid crystal 4-n-octyl-4'-cyano-biphenyl (8CB) was studied in a torsional shear flow apparatus. Experiments were performed near the smectic A-nematic phase transition, and they concentrated on the observation of the tumbling instability associated with a positive Leslie viscosity α3. From theoretical consid...

The phase diagrams of proper and improper incommensurate solid ferroelectrics in external electric fields and of ferroelectric liquid crystals in external magnetic fields are reviewed. It is shown that the Landau model for solid improper ferroelectrics with n=4 is isomorphous to the model for ferroelectric liquid crystals in an external magnetic fi...

High-resolution heat-capacity measurement has been carried out on S-4-O-(2-methyl)butyl-resorcylidene-4′-octylaniline near its smectic-A-chiral smectic-C transition. Our heat-capacity results combined with the existing tilt-angle data enable us to calculate the three leading coefficients in the Landau free energy expression for the smectic-A-chiral...

A measurement of the specific heat of DOBAMBC is presented, and it is shown that a Landau expansion of the free-energy density with a sixth-order term is adequate to describe the measurement. The coefficients in the Landau expansion are estimated from this measurement.
The dependence of the tilt angle on applied electric and magnetic fields is disc...

The nature of the flow alignment of nematic liquid crystals is discussed within the hydrodynamic theory of Leslie, Ericksen and Parodi. It is shown that there is a limited range of angles for which flow alignment can occur. With this as a starting point the flow alignment of disc like nematics is discussed. It is proved that only one out of the two...

THE SIGNS OF THE LESLIE VISCOSITIES ″ALPHA″ //2 AND ″ALPHA″//3 FOR NEMATICS CONSISTING OF DISK LIKE MOLECULES ARE DISCUSSED. IT IS PREDICTED ON THEORETICAL GROUNDS THAT FOR DISK LIKE MOLECULES ″ALPHA″ //3 IS ALWAYS POSITIVE WHILE ″ALPHA″ //2 SHOULD BE ABLE TO EXHIBIT BOTH POSITIVE AND NEGATIVE VALUES. THIS IS THE FIRST TIME THAT THE POSSIBILITY OF...

The members 5CB, 6CB, 7CB and 8CB of the cyano-biphenyl series of nematic liquid crystals have been studied in a linear shear flow apparatus to determine the sign of the viscous torque on the director, which is initially parallel to the shear direction. The flow behaviour, particularly the existence of flow alignment, depends critically on the sign...

Shear flow of a nematic liquid crystal with α3>0 is studied. It is shown that applying an electric field across the sample, a stable boundary layer type of flow can be obtained provided the field exceeds a critical value which is calculated. Expressions for the flow alignment angle, the boundary layer and the relaxation time are given as functions...

The temperature dependence of the Leslie viscosities α2 and α3 has been measured in the nematic liquid crystal 8CB (octyl-cyano-biphenyl), using a trosional shear flow apparatus. In most nematics studied so far α3 is found to be everywhere negative, but if the nematic has an adjacent semectic. A phase at lower temperatures, it can be expected that...

Two nematic liquid crystals are studied in a torsional shear flow apparatus. The nematics are MBBA (neg.diel.anisotropy) and 5CB (pos.diel.anisotropy). The director align in the shear at a certain angle θn to the normal to the flow direction, except at two boundary layers near the surfaces. The influence of an electric field on the flow alignment a...