We have calculated the viscosities of a nematic liquid crystal phase of the Gay–Berne fluid [J. G... more We have calculated the viscosities of a nematic liquid crystal phase of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by using nonequilibrium molecular dynamics simulation methods. The calculations are facilitated by applying a Gaussian constraint method that makes it possible to fix the orientation of the director. The viscosity is a fourth rank tensor. In an isotropic fluid it has got three independent components whereas it has got seven components in an axially symmetric liquid crystal. Our estimates of the shear viscosities and the twist viscosities agree with the equilibrium fluctuation results of a previous study [S. Sarman and D. J. Evans, J. Chem. Phys. 99, 9021 (1993)]. We have also found that the streaming angular velocity is different from zero even though the angular velocity of the director is constrained to be zero thus demonstrating that these two angular velocities are different quantities. Finally we have calculated the irreversible entropy production due to the symmetric traceless strain rate as a function of the alignment angle. We have found it to be minimal near the preferred alignment angle. This is in agreement with the principle of minimum entropy production of linear irreversible thermodynamics.
We propose a new expression for the irreversible entropy production of a nematic liquid crystal s... more We propose a new expression for the irreversible entropy production of a nematic liquid crystal subject to a velocity gradient. This is done by adding a contribution due to the streaming angular velocity, ω, which is distinct from the contribution from the angular velocity of the director, Ω. This removes the inconsistency between the isotropic fluid entropy production and the liquid crystal entropy production. The new entropy production means that the traditional viscosity coefficients must be replaced by a new set of coefficients. This can be done in a few different ways depending on how one defines the thermodynamic forces and fluxes. We derive equilibrium fluctuation relations for the viscosities by applying linear response theory. One finds that it is very important to select the proper equilibrium ensemble in order to obtain simple expressions, i.e., linear combinations of time correlation function integrals (TCFI’s), for the viscosities. It turns out that the thermodynamic forces must be given external parameters whereas the fluxes must be fluctuating phase functions. This means that one sometimes must use equilibrium ensembles where Ω and ω are constrained to be zero. Most TCFI’s are the same in those ensembles as in ordinary equilibrium ensembles such as the canonical or isokinetic ensemble. There are relations between those TCFI’s that are different. It is particularly convenient to constrain Ω to be zero because this makes a director based coordinate system an inertial frame. It also prevents the director reorientation from affecting the tails of the time correlation functions. In order to test some of the fluctuation relations numerically, we have evaluated them for a nematic liquid crystal phase of an oblate version of the Gay–Berne fluid. We have compared the ordinary isokinetic ensemble to an ensemble where Ω has been constrained to be zero by performing equilibrium molecular dynamics (EMD) simulations. The results were either the same or satisfied relations between the TCFI’s in the two ensembles. We cross check these results by applying the SLLOD nonequilibrium molecular dynamics (NEMD) algorithm (so named because of its close relationship to the Dolls tensor algorithm) for planar Couette flow. The NEMD estimates and the EMD fluctuation results are consistent. Constraining Ω to be zero also makes it possible to fix the director at different angles relative to the stream lines. In particular, one can calculate the entropy production as a function of the alignment angle. It seems to be minimal very close to the preferred alignment angle.
The lack of a centre of inversion in a cholesteric liquid crystal allows linear cross-couplings b... more The lack of a centre of inversion in a cholesteric liquid crystal allows linear cross-couplings between thermodynamic forces and fluxes that are polar vectors and pseudo vectors respectively. This makes it possible for a temperature gradient parallel to the cholesteric axis to induce a torque which rotates the director. This phenomenon is known as the Lehmann effect. The converse is
We derive Green–Kubo relations for the viscosities of a biaxial nematic liquid crystal. In this s... more We derive Green–Kubo relations for the viscosities of a biaxial nematic liquid crystal. In this system there are seven shear viscosities, three twist viscosities, and three cross coupling coefficients between the antisymmetric strain rate and the symmetric traceless pressure tensor. According to the Onsager reciprocity relations these couplings are equal to the cross couplings between the symmetric traceless strain rate and the antisymmetric pressure. Our method is based on a comparison of the microscopic linear response generated by the SLLOD equations of motion for planar Couette flow (so named because of their close connection to the Doll’s tensor Hamiltonian) and the macroscopic linear phenomenological relations between the pressure tensor and the strain rate. In order to obtain simple Green–Kubo relations we employ an equilibrium ensemble where the angular velocities of the directors are identically zero. This is achieved by adding constraint torques to the equations for the molecular angular accelerations. One finds that all the viscosity coefficients can be expressed as linear combinations of time correlation function integrals (TCFIs). This is much simpler compared to the expressions in the conventional canonical ensemble, where the viscosities are complicated rational functions of the TCFIs. The reason for this is, that in the constrained angular velocity ensemble, the thermodynamic forces are given external parameters whereas the thermodynamic fluxes are ensemble averages of phase functions. This is not the case in the canonical ensemble. The simplest way of obtaining numerical estimates of viscosity coefficients of a particular molecular model system is to evaluate these fluctuation relations by equilibrium molecular dynamics simulations.
A cholesteric liquid crystal lacks a center of inversion and it is consequently different from it... more A cholesteric liquid crystal lacks a center of inversion and it is consequently different from its mirror image. The low symmetry allows linear cross couplings between thermodynamic forces and fluxes that are polar vectors and pseudovectors, respectively. This makes it possible for a temperature gradient, which is a polar vector to induce a director angular velocity, which is a pseudovector. The reverse is also possible; the torque conjugate to the director angular velocity can drive a heat current. This is the basis for the Lehman effect where a temperature gradient parallel to the cholesteric axis causes the local director to rotate. We use linear response theory to derive Green–Kubo relations and nonequilibrium molecular dynamics simulation algorithms for the transport coefficient that couples the temperature gradient and the director angular velocity. The theory is completely general and can consequently be used to find relations for any linear cross coupling coefficient between a polar vector and a pseudovector.
We have calculated the phase diagram of a fluid, the molecules of which are composed of a string ... more We have calculated the phase diagram of a fluid, the molecules of which are composed of a string of partially overlapping oblate soft ellipsoid interaction sites, by molecular dynamics simulation. The molecules are elongated in one direction and flattened in the perpendicular direction so that they become biaxial. We find that when the fluid consisting of 8-site or 9-site molecules is compressed, a discotic uniaxial nematic phase is formed. Further compression causes a transition to a biaxial phase. The same phase behaviour is found by compressing a fluid composed of molecules with 10 or 11 sites, but in this case the uniaxial phase is calamitic. The 9-site and 10-site molecules consequently bracket the Landau bicritical point where there is a continuous transition between the isotropic phase and the biaxial nematic phase. This behaviour is consistent with the Landau–de Gennes theory and mean field theories. However, the molecular dimensions for which the Landau point appears differ from the theoretical predictions of the mean field theories.
We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the ... more We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by performing molecular dynamics simulations. The equations of motion have been augmented by a director constraint torque that fixes the orientation of the directors. This makes it possible to fix them at different angles relative to the stream lines in shear flow simulations. In equilibrium simulations the constraints generate a new ensemble. One finds that the Green–Kubo relations for the viscosities become linear combinations of time correlation function integrals in this ensemble whereas they are complicated rational functions in the conventional canonical ensemble. We have evaluated these Green–Kubo relations for all the shear viscosities and all the twist viscosities. We have also calculated the alignment angles, which are functions of the viscosity coefficients. We find that there are three real alignment angles but a linear stability analysis shows that only one of them corresponds to a stable director orientation. The Green–Kubo results have been cross checked by nonequilibrium shear flow simulations. The results from the different methods agree very well. Finally, we have evaluated the Miesowicz viscosities [D. Baalss, Z. Naturforsch. Teil A 45, 7 (1990)]. They vary by more than 2 orders of magnitude. The viscosity is consequently highly orientation dependent.
We devise a constraint algorithm that makes the angular velocity of the director of a liquid crys... more We devise a constraint algorithm that makes the angular velocity of the director of a liquid crystal a constant of motion. When the angular velocity is set equal to zero, a director based coordinate system becomes an inertial frame. This is a great advantage because most thermodynamic properties and time correlation functions of a liquid crystal are best expressed relative to a director based coordinate system. One also prevents the director reorientation from interfering with the tails of the time correlation functions. When the angular velocity is forced to be zero the constraints do not do any work on the system. This makes it possible to prove that ensemble averages of phase functions and time correlation functions are unaffected by the director constraint torques. The constraint algorithm also facilitates generalization of nonequilibrium molecular dynamics algorithms to liquid crystal phases. In order to test the algorithm numerically we have simulated a biaxial nematic phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)]. The director constraint algorithm works very well. We have calculated the velocity autocorrelation functions and the self diffusion coefficients. In a biaxial nematic liquid crystal there are three independent components of the self-diffusion tensor. They have been found to be finite and different thus proving that we really simulate a liquid rather than a solid and that the symmetry is biaxial. Simulation of biaxial liquid crystals requires fairly large systems. We have therefore developed an algorithm that we run on a parallel computer instead of an ordinary work station.
We have calculated the viscosities of a variant of the Gay–Berne fluid as a function of the tempe... more We have calculated the viscosities of a variant of the Gay–Berne fluid as a function of the temperature by performing molecular dynamics simulations. We have evaluated the Green–Kubo relations for the various viscosity coefficients. The results have been cross-checked by performing shear flow simulations. At high temperatures there is a nematic phase that is transformed to a smectic A phase as the temperature is decreased. The nematic phase is found to be flow stable. Close to the nematic–smectic transition point the liquid crystal model system becomes flow unstable. This is in agreement with the theoretical predictions by Jähnig and Brochard [F. Jähnig and F. Brochard, J. Phys. 35, 301 (1974)]. In a planar Couette flow one can define the three Miesowicz viscosities or effective viscosities η1, η2, and η3. The coefficient η1 is the viscosity when the director is parallel to the streamlines, η2 is the viscosity when the director is perpendicular to the shear plane, and η3 is the viscosity when the director is perpendicular to the vorticity plane. In the smectic phase η1 is undefined because the strain rate field is incommensurate with the smectic layer structure when the director is parallel to the streamlines. The viscosity η3 is found to be fairly independent of the temperature. The coefficient η2 increases with the temperature. This is unusual because the viscosity of most isotropic liquids decreases with the temperature. This anomaly is due to the smectic layer structure that is present at low temperatures. This lowers the friction because the layers can slide past each other fairly easily.
The structure of a Lennard-Jones fluid in a narrow planar slit is studied for different kinds of ... more The structure of a Lennard-Jones fluid in a narrow planar slit is studied for different kinds of wall–fluid interaction potentials by applying anisotropic integral equation theories. Density profiles and in some cases the force between the walls as a function of the slit width are calculated. It is found that when the state of the fluid in the slit is close to the liquid–vapor coexistence line, there must exist a distinct attractive minimum in the wall potential in order to induce an oscillatory density profile in the slit. Then, the force as a function of the slit width is also oscillatory. The repulsive part of the wall potential has little influence on the oscillatory behavior. When the wall potential consists of a very soft repulsive part close to the wall followed by a diffuse attractive minimum further out, the layer structure disappears completely. However, when the pressure of the fluid is raised above the vapor pressure, the oscillatory structure slowly reappears. These results are utilized to discuss certain experimental surface force measurements where solid, molecularly smooth walls have been modified by adsorption of amphiphiles. It is suggested that the main reason why the oscillatory forces do not appear in such systems is that the attractive part of the wall potential becomes diffuse and not that the repulsive wall becomes soft.
International Journal of Thermophysics, Nov 1, 1994
During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied ... more During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied to study transport phenomena in fluids that are isotropic at equilibrium. A natural extension is therefore to study liquid crystals, which are anisotropic al equilibrium. The lower symmetry of these systems means that the linear transport coefficients are considerably more complicated than in an isotropic system. Part of the reason for this is that there are crosscouplings between tensors of different rank and parity. Such couplings arc symmetry-forbidden in isotropic phases. In this paper. we review some of fundamental theoretical results we have derived concerning the rheology of liquid crystals. report NEMD simulations of thermal conductivity and shear viscosity of liquid crystals, and present NEMD simulations of shear cessation phenomena. All of the NEMD results are presented for a model liquid crystal fluid which is a modification of the Gay-Borne fluid. The results obtained are in qualitative agreement with experimental measurements on liquid crystal systems.
A representation of alkanes and alkane/gas mixtures is proposed in terms of simple fluids interac... more A representation of alkanes and alkane/gas mixtures is proposed in terms of simple fluids interacting through pairwise square well potentials parameterised by the range of attractive forces. Using the model, vapour–liquid equilibria and interfacial tension (IFT) are studied both for pure alkane fluids (C4, C5, C6, C8, C10 and C14) and their high pressure binary mixtures formed with methane, nitrogen
Journal of the Chemical Society, Faraday Transactions, 1991
The anisotropic pair-correlation functions for a hard-sphere fluid between hard walls have been c... more The anisotropic pair-correlation functions for a hard-sphere fluid between hard walls have been calculated using the anisotropic Percus–Yevick (PY) approximation. The non-uniform fluid in the slit is in equilibrium with a hard-sphere bulk fluid of known density. Density profiles and pair distribution functions are displayed graphically for various slit widths and are used to give a detailed description of the mechanism for the oscillatory interaction between the walls and other structural properties of the fluid in the slit. A major goal in the discussion is a conceptual understanding of the physical factors that determine these properties.
We have calculated the viscosities of a nematic liquid crystal phase of the Gay–Berne fluid [J. G... more We have calculated the viscosities of a nematic liquid crystal phase of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by using nonequilibrium molecular dynamics simulation methods. The calculations are facilitated by applying a Gaussian constraint method that makes it possible to fix the orientation of the director. The viscosity is a fourth rank tensor. In an isotropic fluid it has got three independent components whereas it has got seven components in an axially symmetric liquid crystal. Our estimates of the shear viscosities and the twist viscosities agree with the equilibrium fluctuation results of a previous study [S. Sarman and D. J. Evans, J. Chem. Phys. 99, 9021 (1993)]. We have also found that the streaming angular velocity is different from zero even though the angular velocity of the director is constrained to be zero thus demonstrating that these two angular velocities are different quantities. Finally we have calculated the irreversible entropy production due to the symmetric traceless strain rate as a function of the alignment angle. We have found it to be minimal near the preferred alignment angle. This is in agreement with the principle of minimum entropy production of linear irreversible thermodynamics.
We propose a new expression for the irreversible entropy production of a nematic liquid crystal s... more We propose a new expression for the irreversible entropy production of a nematic liquid crystal subject to a velocity gradient. This is done by adding a contribution due to the streaming angular velocity, ω, which is distinct from the contribution from the angular velocity of the director, Ω. This removes the inconsistency between the isotropic fluid entropy production and the liquid crystal entropy production. The new entropy production means that the traditional viscosity coefficients must be replaced by a new set of coefficients. This can be done in a few different ways depending on how one defines the thermodynamic forces and fluxes. We derive equilibrium fluctuation relations for the viscosities by applying linear response theory. One finds that it is very important to select the proper equilibrium ensemble in order to obtain simple expressions, i.e., linear combinations of time correlation function integrals (TCFI’s), for the viscosities. It turns out that the thermodynamic forces must be given external parameters whereas the fluxes must be fluctuating phase functions. This means that one sometimes must use equilibrium ensembles where Ω and ω are constrained to be zero. Most TCFI’s are the same in those ensembles as in ordinary equilibrium ensembles such as the canonical or isokinetic ensemble. There are relations between those TCFI’s that are different. It is particularly convenient to constrain Ω to be zero because this makes a director based coordinate system an inertial frame. It also prevents the director reorientation from affecting the tails of the time correlation functions. In order to test some of the fluctuation relations numerically, we have evaluated them for a nematic liquid crystal phase of an oblate version of the Gay–Berne fluid. We have compared the ordinary isokinetic ensemble to an ensemble where Ω has been constrained to be zero by performing equilibrium molecular dynamics (EMD) simulations. The results were either the same or satisfied relations between the TCFI’s in the two ensembles. We cross check these results by applying the SLLOD nonequilibrium molecular dynamics (NEMD) algorithm (so named because of its close relationship to the Dolls tensor algorithm) for planar Couette flow. The NEMD estimates and the EMD fluctuation results are consistent. Constraining Ω to be zero also makes it possible to fix the director at different angles relative to the stream lines. In particular, one can calculate the entropy production as a function of the alignment angle. It seems to be minimal very close to the preferred alignment angle.
The lack of a centre of inversion in a cholesteric liquid crystal allows linear cross-couplings b... more The lack of a centre of inversion in a cholesteric liquid crystal allows linear cross-couplings between thermodynamic forces and fluxes that are polar vectors and pseudo vectors respectively. This makes it possible for a temperature gradient parallel to the cholesteric axis to induce a torque which rotates the director. This phenomenon is known as the Lehmann effect. The converse is
We derive Green–Kubo relations for the viscosities of a biaxial nematic liquid crystal. In this s... more We derive Green–Kubo relations for the viscosities of a biaxial nematic liquid crystal. In this system there are seven shear viscosities, three twist viscosities, and three cross coupling coefficients between the antisymmetric strain rate and the symmetric traceless pressure tensor. According to the Onsager reciprocity relations these couplings are equal to the cross couplings between the symmetric traceless strain rate and the antisymmetric pressure. Our method is based on a comparison of the microscopic linear response generated by the SLLOD equations of motion for planar Couette flow (so named because of their close connection to the Doll’s tensor Hamiltonian) and the macroscopic linear phenomenological relations between the pressure tensor and the strain rate. In order to obtain simple Green–Kubo relations we employ an equilibrium ensemble where the angular velocities of the directors are identically zero. This is achieved by adding constraint torques to the equations for the molecular angular accelerations. One finds that all the viscosity coefficients can be expressed as linear combinations of time correlation function integrals (TCFIs). This is much simpler compared to the expressions in the conventional canonical ensemble, where the viscosities are complicated rational functions of the TCFIs. The reason for this is, that in the constrained angular velocity ensemble, the thermodynamic forces are given external parameters whereas the thermodynamic fluxes are ensemble averages of phase functions. This is not the case in the canonical ensemble. The simplest way of obtaining numerical estimates of viscosity coefficients of a particular molecular model system is to evaluate these fluctuation relations by equilibrium molecular dynamics simulations.
A cholesteric liquid crystal lacks a center of inversion and it is consequently different from it... more A cholesteric liquid crystal lacks a center of inversion and it is consequently different from its mirror image. The low symmetry allows linear cross couplings between thermodynamic forces and fluxes that are polar vectors and pseudovectors, respectively. This makes it possible for a temperature gradient, which is a polar vector to induce a director angular velocity, which is a pseudovector. The reverse is also possible; the torque conjugate to the director angular velocity can drive a heat current. This is the basis for the Lehman effect where a temperature gradient parallel to the cholesteric axis causes the local director to rotate. We use linear response theory to derive Green–Kubo relations and nonequilibrium molecular dynamics simulation algorithms for the transport coefficient that couples the temperature gradient and the director angular velocity. The theory is completely general and can consequently be used to find relations for any linear cross coupling coefficient between a polar vector and a pseudovector.
We have calculated the phase diagram of a fluid, the molecules of which are composed of a string ... more We have calculated the phase diagram of a fluid, the molecules of which are composed of a string of partially overlapping oblate soft ellipsoid interaction sites, by molecular dynamics simulation. The molecules are elongated in one direction and flattened in the perpendicular direction so that they become biaxial. We find that when the fluid consisting of 8-site or 9-site molecules is compressed, a discotic uniaxial nematic phase is formed. Further compression causes a transition to a biaxial phase. The same phase behaviour is found by compressing a fluid composed of molecules with 10 or 11 sites, but in this case the uniaxial phase is calamitic. The 9-site and 10-site molecules consequently bracket the Landau bicritical point where there is a continuous transition between the isotropic phase and the biaxial nematic phase. This behaviour is consistent with the Landau–de Gennes theory and mean field theories. However, the molecular dimensions for which the Landau point appears differ from the theoretical predictions of the mean field theories.
We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the ... more We have calculated the viscosities of a biaxial nematic liquid crystal phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)] by performing molecular dynamics simulations. The equations of motion have been augmented by a director constraint torque that fixes the orientation of the directors. This makes it possible to fix them at different angles relative to the stream lines in shear flow simulations. In equilibrium simulations the constraints generate a new ensemble. One finds that the Green–Kubo relations for the viscosities become linear combinations of time correlation function integrals in this ensemble whereas they are complicated rational functions in the conventional canonical ensemble. We have evaluated these Green–Kubo relations for all the shear viscosities and all the twist viscosities. We have also calculated the alignment angles, which are functions of the viscosity coefficients. We find that there are three real alignment angles but a linear stability analysis shows that only one of them corresponds to a stable director orientation. The Green–Kubo results have been cross checked by nonequilibrium shear flow simulations. The results from the different methods agree very well. Finally, we have evaluated the Miesowicz viscosities [D. Baalss, Z. Naturforsch. Teil A 45, 7 (1990)]. They vary by more than 2 orders of magnitude. The viscosity is consequently highly orientation dependent.
We devise a constraint algorithm that makes the angular velocity of the director of a liquid crys... more We devise a constraint algorithm that makes the angular velocity of the director of a liquid crystal a constant of motion. When the angular velocity is set equal to zero, a director based coordinate system becomes an inertial frame. This is a great advantage because most thermodynamic properties and time correlation functions of a liquid crystal are best expressed relative to a director based coordinate system. One also prevents the director reorientation from interfering with the tails of the time correlation functions. When the angular velocity is forced to be zero the constraints do not do any work on the system. This makes it possible to prove that ensemble averages of phase functions and time correlation functions are unaffected by the director constraint torques. The constraint algorithm also facilitates generalization of nonequilibrium molecular dynamics algorithms to liquid crystal phases. In order to test the algorithm numerically we have simulated a biaxial nematic phase of a variant of the Gay–Berne fluid [J. G. Gay and B. J. Berne, J. Chem. Phys. 74, 3316 (1981)]. The director constraint algorithm works very well. We have calculated the velocity autocorrelation functions and the self diffusion coefficients. In a biaxial nematic liquid crystal there are three independent components of the self-diffusion tensor. They have been found to be finite and different thus proving that we really simulate a liquid rather than a solid and that the symmetry is biaxial. Simulation of biaxial liquid crystals requires fairly large systems. We have therefore developed an algorithm that we run on a parallel computer instead of an ordinary work station.
We have calculated the viscosities of a variant of the Gay–Berne fluid as a function of the tempe... more We have calculated the viscosities of a variant of the Gay–Berne fluid as a function of the temperature by performing molecular dynamics simulations. We have evaluated the Green–Kubo relations for the various viscosity coefficients. The results have been cross-checked by performing shear flow simulations. At high temperatures there is a nematic phase that is transformed to a smectic A phase as the temperature is decreased. The nematic phase is found to be flow stable. Close to the nematic–smectic transition point the liquid crystal model system becomes flow unstable. This is in agreement with the theoretical predictions by Jähnig and Brochard [F. Jähnig and F. Brochard, J. Phys. 35, 301 (1974)]. In a planar Couette flow one can define the three Miesowicz viscosities or effective viscosities η1, η2, and η3. The coefficient η1 is the viscosity when the director is parallel to the streamlines, η2 is the viscosity when the director is perpendicular to the shear plane, and η3 is the viscosity when the director is perpendicular to the vorticity plane. In the smectic phase η1 is undefined because the strain rate field is incommensurate with the smectic layer structure when the director is parallel to the streamlines. The viscosity η3 is found to be fairly independent of the temperature. The coefficient η2 increases with the temperature. This is unusual because the viscosity of most isotropic liquids decreases with the temperature. This anomaly is due to the smectic layer structure that is present at low temperatures. This lowers the friction because the layers can slide past each other fairly easily.
The structure of a Lennard-Jones fluid in a narrow planar slit is studied for different kinds of ... more The structure of a Lennard-Jones fluid in a narrow planar slit is studied for different kinds of wall–fluid interaction potentials by applying anisotropic integral equation theories. Density profiles and in some cases the force between the walls as a function of the slit width are calculated. It is found that when the state of the fluid in the slit is close to the liquid–vapor coexistence line, there must exist a distinct attractive minimum in the wall potential in order to induce an oscillatory density profile in the slit. Then, the force as a function of the slit width is also oscillatory. The repulsive part of the wall potential has little influence on the oscillatory behavior. When the wall potential consists of a very soft repulsive part close to the wall followed by a diffuse attractive minimum further out, the layer structure disappears completely. However, when the pressure of the fluid is raised above the vapor pressure, the oscillatory structure slowly reappears. These results are utilized to discuss certain experimental surface force measurements where solid, molecularly smooth walls have been modified by adsorption of amphiphiles. It is suggested that the main reason why the oscillatory forces do not appear in such systems is that the attractive part of the wall potential becomes diffuse and not that the repulsive wall becomes soft.
International Journal of Thermophysics, Nov 1, 1994
During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied ... more During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied to study transport phenomena in fluids that are isotropic at equilibrium. A natural extension is therefore to study liquid crystals, which are anisotropic al equilibrium. The lower symmetry of these systems means that the linear transport coefficients are considerably more complicated than in an isotropic system. Part of the reason for this is that there are crosscouplings between tensors of different rank and parity. Such couplings arc symmetry-forbidden in isotropic phases. In this paper. we review some of fundamental theoretical results we have derived concerning the rheology of liquid crystals. report NEMD simulations of thermal conductivity and shear viscosity of liquid crystals, and present NEMD simulations of shear cessation phenomena. All of the NEMD results are presented for a model liquid crystal fluid which is a modification of the Gay-Borne fluid. The results obtained are in qualitative agreement with experimental measurements on liquid crystal systems.
A representation of alkanes and alkane/gas mixtures is proposed in terms of simple fluids interac... more A representation of alkanes and alkane/gas mixtures is proposed in terms of simple fluids interacting through pairwise square well potentials parameterised by the range of attractive forces. Using the model, vapour–liquid equilibria and interfacial tension (IFT) are studied both for pure alkane fluids (C4, C5, C6, C8, C10 and C14) and their high pressure binary mixtures formed with methane, nitrogen
Journal of the Chemical Society, Faraday Transactions, 1991
The anisotropic pair-correlation functions for a hard-sphere fluid between hard walls have been c... more The anisotropic pair-correlation functions for a hard-sphere fluid between hard walls have been calculated using the anisotropic Percus–Yevick (PY) approximation. The non-uniform fluid in the slit is in equilibrium with a hard-sphere bulk fluid of known density. Density profiles and pair distribution functions are displayed graphically for various slit widths and are used to give a detailed description of the mechanism for the oscillatory interaction between the walls and other structural properties of the fluid in the slit. A major goal in the discussion is a conceptual understanding of the physical factors that determine these properties.
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Papers by Sten Sarman