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  • 1.
    Metere, Alfredo
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Oppelstrup, Tomas
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Dzugutov, Mikhail
    Formation of the smectic-B crystal from a simple monatomic liquid2013In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 88, no 6, article id 062502Article in journal (Refereed)
    Abstract [en]

    We report a molecular dynamics simulation demonstrating that the smectic-B crystalline phase (Cry-B), commonly observed in mesogenic systems of anisotropic molecules, can be formed by a system of identical particles interacting via a spherically symmetric potential. The Cry-B phase forms as a result of a first-order transition from an isotropic liquid phase upon isochoric cooling at appropriate number density. Its structure, determined by the design of the pair potential, corresponds to the Cry-B structure formed by elongated particles with the aspect ratio 1.8. The diffraction pattern and the real-space structure inspection demonstrate dominance of the ABC-type of axial layer stacking. This result opens a general possibility of producing smectic phases using isotropic interparticle interaction both in simulations and in colloidal systems.

  • 2.
    Metere, Alfredo
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Oppelstrup, Tomas
    Dzugutov, Mikhail
    Formation of a Columnar Liquid Crystal in a Simple One-Component System of Particles2015In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 11, no 23, p. 4606-4613Article in journal (Refereed)
    Abstract [en]

    We report a molecular dynamics simulation demonstrating that a columnar liquid crystal, commonly formed by disc-shaped molecules, can be formed by identical particles interacting via a spherically symmetric potential. Upon isochoric cooling from a low-density isotropic liquid state the simulated system performed a weak first order phase transition which produced a liquid crystal phase composed of parallel particle columns arranged in a hexagonal pattern in the plane perpendicular to the column axis. The particles within columns formed a liquid structure and demonstrated a significant intracolumn diffusion. Further cooling resulted in another first-order transition whereby the column structure became periodically ordered in three dimensions transforming the liquid-crystal phase into a crystal. This result is the first observation of a liquid crystal formation in a simple one-component system of particles. Its conceptual significance is in that it demonstrated that liquid crystals that have so far only been produced in systems of anisometric molecules, can also be formed by mesoscopic soft-matter and colloidal systems of spherical particles with appropriately tuned interatomic potential.

  • 3. Nygard, K.
    et al.
    Kjellander, R.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Chodankar, S.
    Perret, E.
    Buitenhuis, J.
    van der Veen, J. F.
    Anisotropic Pair Correlations and Structure Factors of Confined Hard-Sphere Fluids: An Experimental and Theoretical Study2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 3, article id 037802Article in journal (Refereed)
    Abstract [en]

    We address the fundamental question: how are pair correlations and structure factors of hard-sphere fluids affected by confinement between hard planar walls at close distance? For this purpose, we combine x-ray scattering from colloid-filled nanofluidic channel arrays and first-principles inhomogeneous liquid-state theory within the anisotropic Percus-Yevick approximation. The experimental and theoretical data are in remarkable agreement at the pair-correlation level, providing the first quantitative experimental verification of the theoretically predicted confinement-induced anisotropy of the pair-correlation functions for the fluid. The description of confined fluids at this level provides, in the general case, important insights into the mechanisms of particle-particle interactions in dense fluids under confinement.

  • 4. Nygard, Kim
    et al.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kjellander, Roland
    Packing frustration in dense confined fluids2014In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 141, no 9, p. 094501-Article in journal (Refereed)
    Abstract [en]

    Packing frustration for confined fluids, i.e., the incompatibility between the preferred packing of the fluid particles and the packing constraints imposed by the confining surfaces, is studied for a dense hard-sphere fluid confined between planar hard surfaces at short separations. The detailed mechanism for the frustration is investigated via an analysis of the anisotropic pair distributions of the confined fluid, as obtained from integral equation theory for inhomogeneous fluids at pair correlation level within the anisotropic Percus-Yevick approximation. By examining the mean forces that arise from interparticle collisions around the periphery of each particle in the slit, we calculate the principal components of the mean force for the density profile - each component being the sum of collisional forces on a particle's hemisphere facing either surface. The variations of these components with the slit width give rise to rather intricate changes in the layer structure between the surfaces, but, as shown in this paper, the basis of these variations can be easily understood qualitatively and often also semi-quantitatively. It is found that the ordering of the fluid is in essence governed locally by the packing constraints at each single solid-fluid interface. A simple superposition of forces due to the presence of each surface gives surprisingly good estimates of the density profiles, but there remain nontrivial confinement effects that cannot be explained by superposition, most notably the magnitude of the excess adsorption of particles in the slit relative to bulk.

  • 5. Nygård, Kim
    et al.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Hyltegren, Kristin
    Chodankar, Shirish
    Perret, Edith
    Buitenhuis, Johan
    van der Veen, J. Friso
    Kjellander, Roland
    Density Fluctuations of Hard-Sphere Fluids in Narrow Confinement2016In: Physical Review X, E-ISSN 2160-3308, Vol. 6, no 1, article id 011014Article in journal (Refereed)
    Abstract [en]

    Spatial confinement induces microscopic ordering of fluids, which in turn alters many of their dynamic and thermodynamic properties. However, the isothermal compressibility has hitherto been largely overlooked in the literature, despite its obvious connection to the underlying microscopic structure and density fluctuations in confined geometries. Here, we address this issue by probing density profiles and structure factors of hard-sphere fluids in various narrow slits, using x-ray scattering from colloid-filled nanofluidic containers and integral-equation-based statistical mechanics at the level of pair distributions for inhomogeneous fluids. Most importantly, we demonstrate that density fluctuations and isothermal compressibilities in confined fluids can be obtained experimentally from the long-wavelength limit of the structure factor, providing a formally exact and experimentally accessible connection between microscopic structure and macroscopic, thermodynamic properties. Our approach will thus, for example, allow direct experimental verification of theoretically predicted enhanced density fluctuations in liquids near solvophobic interfaces.

  • 6. Nygård, Kim
    et al.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kjellander, Roland
    Local order variations in confined hard-sphere fluids2013In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 139, no 16, p. 164701-Article in journal (Refereed)
    Abstract [en]

    Pair distributions of fluids confined between two surfaces at close distance are of fundamental importance for a variety of physical, chemical, and biological phenomena, such as interactions between macromolecules in solution, surface forces, and diffusion in narrow pores. However, in contrast to bulk fluids, properties of inhomogeneous fluids are seldom studied at the pair-distribution level. Motivated by recent experimental advances in determining anisotropic structure factors of confined fluids, we analyze theoretically the underlying anisotropic pair distributions of the archetypical hard-sphere fluid confined between two parallel hard surfaces using first-principles statistical mechanics of inhomogeneous fluids. For this purpose, we introduce an experimentally accessible ensemble-averaged local density correlation function and study its behavior as a function of confining slit width. Upon increasing the distance between the confining surfaces, we observe an alternating sequence of strongly anisotropic versus more isotropic local order. The latter is due to packing frustration of the spherical particles. This observation highlights the importance of studying inhomogeneous fluids at the pair-distribution level.

  • 7.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Petru Poni Institute of Macromolecular Chemistry, Romania; Nanjing Tech University, P. R. China; Luleå University of Technology, Sweden; University of Cagliari, Italy.
    Diffusion-driven rotation in cholesteric liquid crystals studied using molecular dynamics simulation of a mixture of the Gay–Berne fluid and the Lennard-Jones fluid2023In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 25, no 28, p. 18833-18843Article in journal (Refereed)
    Abstract [en]

    Diffusion-driven rotation in cholesteric liquid crystals has been studied using molecular dynamics simulation. Then a chemical potential gradient parallel to the cholesteric axis induces a torque that rotates the director at a constant rate around this axis, besides driving a mass current. An equimolar mixture of Gay–Berne ellipsoids and Lennard-Jones spheres was used as the molecular model. In order to keep the system homogeneous, the color conductivity algorithm was used to apply a color field instead of a chemical potential gradient to drive a mass current. Then the particles are given a color charge that interacts with a color field in the same way as with an electric field, but these charges do not interact with each other. This algorithm is often used to calculate the mutual diffusion coefficient. In the above liquid crystal model, it was found that the color field induces a torque that rotates the director at a constant rate around the cholesteric axis in addition to driving a mass current. The phenomenon was quantified by calculating the cross-coupling coefficient between the color field and the director angular velocity. The results were cross-checked by using a director rotation algorithm to exert a torque to rotate the director at a constant rate. Besides rotation of the director, this resulted in a mass current parallel to the cholesteric axis. The cross-coupling coefficient between the torque and the mass current was equal to the cross-coupling coefficient between the color field and the director rotation rate within a statistical uncertainty of 10 percent, thus fulfilling the Onsager reciprocity relations. As a further cross-check, these cross-coupling coupling coefficients, the color conductivity, and the twist viscosity were calculated by evaluating the corresponding Green–Kubo relations. Finally, it was noted that the orientation of the cholesteric axis parallel to the color field is the one that minimizes the irreversible energy dissipation rate. This is in accordance with a theorem stating that this quantity is minimal in the linear regime of a nonequilibrium steady state.

  • 8.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Director alignment relative to the temperature gradient in nematic liquid crystals studied by molecular dynamics simulation2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 28, p. 14741-14749Article in journal (Refereed)
    Abstract [en]

    The director alignment relative to the temperature gradient in nematic liquid crystal model systems consisting of soft oblate or prolate ellipsoids of revolution has been studied by molecular dynamics simulation. The temperature gradient is maintained by thermostating different parts of the system at different temperatures by using a Gaussian thermostat. It is found that the director of the prolate ellipsoids aligns perpendicularly to the temperature gradient whereas the director of the oblate ellipsoids aligns parallel to this gradient. When the director is oriented in between the parallel and perpendicular orientations a torque is exerted forcing the director to the parallel or perpendicular orientation. Because of symmetry restrictions there is no linear dependence of the torque being a pseudovector on the temperature gradient being a polar vector in an axially symmetric system such as a nematic liquid crystal. The lowest possible order of this dependence is quadratic. Thus the torque is very weak when the temperature gradient is small, which may explain why this orientation phenomenon is hard to observe experimentally. In both cases the director attains the orientation that minimises the irreversible entropy production.

  • 9.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Evaluation of the viscosities of a liquid crystal model system by shear flow simulation2009In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 479, no 1-3, p. 47-51Article in journal (Refereed)
    Abstract [en]

    The three Miesowicz viscosities of a liquid crystal model system consisting of the Gay-Berne fluid have been obtained by shear flow simulations. The viscosities along an isochore have been followed starting in the nematic phase at high temperatures across the nematic-smectic A phase transition down to low temperatures in the smectic A phase. The relative magnitudes of the viscosities as a function of the structure of the liquid crystal are discussed. The viscosities obtained by the shear flow simulations agree very well with those obtained by Green-Kubo relations in a previous work

  • 10.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
    Flow alignment phenomena in liquid crystals studied by molecular dynamics simulation2009In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 14, p. -144904Article in journal (Refereed)
    Abstract [en]

    The flow alignment of a nematic liquid crystal has been studied as a function of temperature, beginning at high temperature in the nematic phase and down to the nematic-smectic A phase transition. The alignment angle is obtained by estimating the twist viscosities by nonequilibrium molecular dynamics (NEMD) methods. These estimates are cross-checked by evaluating the corresponding equilibrium fluctuation relations. As a further comparison, shear flow simulations are carried out by application of the SLLOD equations of motion (so named because of their close relationship to the Doll's equation of motion, which can be derived from the Doll's tensor Hamiltonian), whereby the alignment angle is obtained directly. All these methods give consistent results for the alignment angle. At low temperatures near the nematic-smectic A transition the system becomes flow unstable. In this region the alignment angle has been calculated as a function of time.

  • 11.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Nanjing Tech University, China; Petru Poni Institute of Macromolecular Chemistry, Romania; Luleå University of Technology, Sweden.
    Microscopic shear flow simulations of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid2021In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 23, no 28, p. 15183-15195Article in journal (Refereed)
    Abstract [en]

    We have studied the behaviour of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid in shear flow by molecular dynamics simulation using the SLLOD equation of motion. This is facilitated by the fact that the biaxial symmetry allows linear relations between the pressure and the velocity gradient. This means that linear irreversible thermodynamics can be applied independently of the simulations to obtain the torques determining the orientations of the system and that the predictions of this theory can be cross-checked by the simulations. It turns out that there is a torque turning the smectic layers to the orientation parallel to the vorticity plane if the simulation is started in another orientation. In the orientation parallel to the vorticity plane where the director formed by the long axes of the molecules, nw, is perpendicular to the vorticity plane there is another torque keeping the director formed by the normals of the broadsides of the molecules, nu, parallel to this plane at a constant alignment angle, ψ relative to the streamlines independently of the strain rate. Moreover, this alignment angle seems to be the one where the irreversible energy dissipation rate, , is minimal. This is in agreement with a recently proven theorem according to which is minimal in the linear regime of a nonequilibrium steady state. Finally, we studied the orientation of nu when the smectic layers are parallel to the shear plane. In a simulation this orientation is stabilised by the periodic boundary conditions. Then we found that there was a nonlinear torque turning nu to the orientation perpendicular to the streamlines thus minimising the value of even though this value is larger than the value of in the orientation parallel to the vorticity plane. This means that is minimized given the external boundary conditions.

  • 12.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Molecular dynamics simulation of planar elongational flow in a nematic liquid crystal based on the Gay-Berne potential2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 5, p. 3332-3342Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics simulations of planar elongational flow in a nematic liquid crystal model system based on the Gay-Berne fluid were undertaken by applying the SLLOD equations of motion with an elongational velocity field or strain rate. In order to facilitate the simulation, Kraynik-Reinelt periodic boundary conditions allowing arbitrarily long simulations were used. A Lagrangian constraint algorithm was utilized to fix the director at different angles relative to the elongation direction, so that the various pressure tensor elements could be calculated as a function of this angle. This made it possible to obtain accurate values of the shear viscosities which were found to agree with results previously obtained by shear flow simulations. The torque needed to fix the director at various angles relative to the elongation direction was evaluated in order to determine the stable orientation of the director, where this torque is equal to zero. This orientation was found to be parallel to the elongation direction. It was also noted that the irreversible entropy production was minimal when the director attained this orientation. Since the simulated system was rather large and fairly long simulation runs were undertaken it was also possible to study the cross coupling between the strain rate and the order tensor. It turned out to be very weak at low strain rates but at higher strain rates it could lead to break down of the liquid crystalline order.

  • 13.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Molecular Dynamics Simulation of Viscous Flow and Heat Conduction in Liquid Crystals2011In: Journal of Computational and Theoretical Nanoscience, ISSN 1546-1955, Vol. 8, no 7, p. 1081-1100Article, review/survey (Refereed)
    Abstract [en]

    We have reviewed the work on molecular dynamics simulation of transport properties of liquid crystals during the last 25 years. To begin with, we present the model systems that have been used in molecular dynamics simulation of liquid crystals, such as the hard sphere fluid, the Gay-Berne fluid and atomistic models. Then we explain the theory necessary for obtaining numerical estimates of the viscosities. They fall into three classes: evaluation of equilibrium fluctuation relations or Green-Kubo relations, nonequilibrium methods such as shear flow simulations and approximate methods based on the rotational diffusion coefficient. Then we review various simulation results for the Miesowicz viscosities which are of interest for lubrication and the twist viscosity, which is the technologically most important viscosity because it determines the switching time in liquid crystal displays. A special section is devoted to flow alignment and flow stability of nematic liquid crystals, which can be studied by obtaining the actual values of the viscosity coefficients or by direct shear flow simulations. The ever increasing speed of electronic computers has made it possible to study flow properties directly in very large systems. We finally review some work on the heat conductivity and the Lehmann effect, where a temperature gradient parallel to the cholesteric axis in a cholesteric liquid crystal causes the director to rotate. This rotation can also be induced by electric fields or flow of matter. This phenomenon and some of the flow properties are of technological importance for construction of molecular motors.

  • 14.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    The temperature dependence of the heat conductivity of a liquid crystal studied by molecular dynamics simulation2010In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 485, no 03-jan, p. 77-82Article in journal (Refereed)
    Abstract [en]

    The temperature dependence of the heat conductivity has been obtained for a liquid crystal model based on the Gay-Berne fluid, from the isotropic phase at high temperatures through the nematic phase to the smectic A phase at low temperatures. The ratio of the parallel and the perpendicular components of the heat conductivity is about 2.5:1 in the nematic phase, which is similar to that of real systems. Both Green-Kubo methods and nonequilibrium molecular dynamics methods have been applied and the results agree within in a relative error of a couple of percent, but the latter method is much more efficient.

  • 15.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Thermomechanical coupling, heat conduction and director rotation in cholesteric liquid crystals studied by molecular dynamics simulation2013In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 10, p. 3442-3453Article in journal (Refereed)
    Abstract [en]

    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 pseudovectors, respectively. This makes it possible for a temperature gradient parallel to the cholesteric axis to induce a torque that rotates the director, a phenomenon known as the Lehmann effect or thermomechanical coupling. The converse is also possible: a torque applied parallel to the cholesteric axis rotates the director and drives a heat flow. In order to study this phenomenon, nonequilibrium molecular dynamics simulation algorithms and Green-Kubo relations evaluated by equilibrium molecular dynamics simulation have been used to calculate the Leslie coefficient, i.e. the cross coupling coefficient between the temperature gradient and the director angular velocity, for a model system composed of soft prolate ellipsoids of revolution interacting via the Gay-Berne potential augmented by a chiral interaction potential causing the formation of a cholesteric phase. It is found that the Leslie coefficient is two orders of magnitudes smaller than other transport coefficients such as the heat conductivity and the twist viscosity, so that very long simulations are required to evaluate it. The Leslie coefficient decreases with the pitch but it has not been possible to determine the exact functional dependence of this coefficient on the pitch. Since very long simulations have been performed to evaluate the Leslie coefficient, very accurate values have been obtained for the twist viscosity and the heat conductivity as a by-product and it is found that they are very similar to the values of the corresponding quantities in the achiral nematic phase that arises when the pitch goes to infinity.

  • 16.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK), Physical Chemistry.
    Twist viscosities and flow alignment of biaxial nematic liquid crystal phases of a soft ellipsoid string fluid studied by molecular dynamics simulation2012In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 14, no 34, p. 11999-12013Article in journal (Refereed)
    Abstract [en]

    We have calculated the twist viscosity and the alignment angle between the director and the stream lines in shear flow of a liquid crystal model system, which forms biaxial nematic liquid crystals, as functions of the density, from the Green-Kubo relations by equilibrium molecular dynamics simulation and by a nonequilibrium molecular dynamics algorithm, where a torque conjugate to the director angular velocity is applied to rotate the director. The model system consists of a soft ellipsoid-string fluid where the ellipsoids interact according a repulsive version of the Gay-Berne potential. Four different length-to-width-to-breadth ratios have been studied. On compression, this system forms discotic or calamitic uniaxial nematic phases depending on the dimensions of the molecules, and on further compression a biaxial nematic phase is formed. In the uniaxial nematic phase there is one twist viscosity and one alignment angle. In the biaxial nematic phase there are three twist viscosities and three alignment angles corresponding to the rotation around the various directors and the different alignments of the directors relative to the stream lines, respectively. It is found that the smallest twist viscosity arises by rotation around the director formed by the long axes, the second smallest one arises by rotation around the director formed by the normals of the broadsides, and the largest one by rotation around the remaining director. The first twist viscosity is rather independent of the density whereas the last two ones increase strongly with density. One finds that there is one stable director alignment relative to the streamlines, namely where the director formed by the long axes is almost parallel to the stream lines and where the director formed by the normals of the broadsides is almost parallel to the shear plane. The relative magnitudes of the components of the twist viscosities span a fairly wide interval so this model should be useful for parameterisation experimental data.

  • 17.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wang, Yong-Lei
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Non-Newtonian rheological properties of shearing nematic liquid crystal model systems based on the Gay-Berne potential2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 25, p. 16615-16623Article in journal (Refereed)
    Abstract [en]

    The viscosities and normal stress differences of various liquid crystal model systems based on the Gay-Berne potential have been obtained as functions of the shear rate in the non-Newtonian regime. Various molecular shapes such as regular convex calamitic and discotic ellipsoids and non-convex shapes such as bent core molecules and soft ellipsoid strings have been examined. The isotropic phases were found to be shear thinning with the shear rate dependence of the viscosity following a power law in the same way as alkanes and other non-spherical molecules. The nematic phases turned out to be shear thinning but the logarithm of the viscosity proved to be an approximately linear function of the square root of the shear rate. The normal stress differences were found to display a more or less parabolic dependence on the shear rate in the isotropic phase whereas this dependence was linear at low to intermediate shear rates in the nematic phase.

  • 18.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wang, Yong-Lei
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Self-diffusion in the non-Newtonian regime of shearing liquid crystal model systems based on the Gay-Berne potential2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 5, article id 054901Article in journal (Refereed)
    Abstract [en]

    The self-diffusion coefficients of nematic phases of various model systems consisting of regular convex calamitic and discotic ellipsoids and non-convex bodies such as bent-core molecules and soft ellipsoid strings have been obtained as functions of the shear rate in a shear flow. Then the self-diffusion coefficient is a second rank tensor with three different diagonal components and two off-diagonal components. These coefficients were found to be determined by a combination of two mechanisms, which previously have been found to govern the self-diffusion of shearing isotropic liquids, namely, (i) shear alignment enhancing the diffusion in the direction parallel to the streamlines and hindering the diffusion in the perpendicular directions and (ii) the distortion of the shell structure in the liquid whereby a molecule more readily can escape from a surrounding shell of nearest neighbors, so that the mobility increases in every direction. Thus, the diffusion parallel to the streamlines always increases with the shear rate since these mechanisms cooperate in this direction. In the perpendicular directions, these mechanisms counteract each other so that the behaviour becomes less regular. In the case of the nematic phases of the calamitic and discotic ellipsoids and of the bent core molecules, mechanism (ii) prevails so that the diffusion coefficients increase. However, the diffusion coefficients of the soft ellipsoid strings decrease in the direction of the velocity gradient because the broadsides of these molecules are oriented perpendicularly to this direction due the shear alignment (i). The cross coupling coefficient relating a gradient of tracer particles in the direction of the velocity gradient and their flow in the direction of the streamlines is negative and rather large, whereas the other coupling coefficient relating a gradient in the direction of the streamlines and a flow in the direction of the velocity gradient is very small.

  • 19.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wang, Yong-Lei
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Uppsala University, Sweden; Petru Poni Institute of Macromolecular Chemistry, Romania.
    Shear flow simulations of smectic liquid crystals based on the Gay-Berne fluid and the soft sphere string-fluid2019In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 1, p. 292-305Article in journal (Refereed)
    Abstract [en]

    We have studied the shear flow of the smectic A phase of three coarse grained liquid crystal model systems, namely two versions of the Gay-Berne fluid and the soft sphere string-fluid. At low shear rates, the orientation where the smectic layers are parallel to the shear plane and the orientation parallel to the vorticity plane are both stable in all the systems. In one of the Gay-Berne fluids, there is a transition from the orientation parallel to the shear plane to the orientation parallel to the vorticity plane. At higher shear rates, a nonequilibrium nematic phase is obtained in all the systems in the same way as in linear alkanes under shear. If the initial configuration is an equilibrium smectic A phase or a nematic phase with the molecules parallel to the streamlines, the orientation parallel to the shear plane is attained at low shear rates in the Gay-Berne fluids. In order to analyze the stability of the different orientations, the torque acting on the liquid crystal is calculated. It consists of an elastic torque caused by deformations due to the shape of the simulation cell and the periodic boundary conditions and a shear-induced torque. The elastic torque stabilizes both the orientation parallel to the shear plane and the orientation parallel to the vorticity plane because the liquid crystal is deformed if it is turned away from these orientations. The shear-induced torque, on the other hand, always turns the liquid crystal to the orientation parallel to the vorticity plane where the viscosity and the irreversible energy dissipation rate are minimal. Since the latter torque is proportional to the square of the shear rate, rather high shear rates are required for it to overwhelm the elastic torque. However, the elastic torque decreases with the system size so that it is likely that the shear-induced torque will dominate in large systems and that the orientation parallel to the vorticity plane will be attained at low or even zero shear rate.

  • 20.
    Sarman, Sten
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Wang, Yong-Lei
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Stanford University, USA.
    Rohlmann, Patrick
    Glavatskih, Sergei
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Uppsala University, Sweden.
    Rheology of phosphonium ionic liquids: a molecular dynamics and experimental study2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 15, p. 10193-10203Article in journal (Refereed)
    Abstract [en]

    We have studied the rheological behavior of the ionic liquid trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P-66614][BMB], and compared it with that of another ionic liquid, namely trihexyl(tetradecyl)phosphonium chloride, [P-66614][Cl]. The non-halogenated [P-66614][BMB] has been selected as it is known to provide enhanced lubrication performance and is, consequently, of technological importance. The ionic liquid [P-66614][Cl], despite its relatively simple anion, exhibits viscosities very similar to those of [P-66614][BMB], making it an excellent reference fluid for the modeling study. The viscosities of the ionic liquids have been obtained by equilibrium atomistic simulations using the Green-Kubo relation, and by performing nonequilibrium shear flow simulations. The influence of the simulation system size and a reduction of the atomic charges on the viscosities of the ionic liquids are systematically studied. The atomic charges are reduced to mimic the temperature dependent charge transfer and polarization effects. It has been found that scaling the point charges with factors between 0.60 and 0.80 from full ion charges can provide reliable viscosities of [P-66614][BMB], consistent with the experimentally measured viscosities within the studied temperature interval from 373 to 463 K. The viscosities of [P-66614][Cl] have been obtained with scaling factors between 0.80 and 1.0 reflecting the lower polarizability and charge transfer effects of the chloride anion.

  • 21.
    Wang, Yong-Lei
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Golets, Mikhail
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Li, Bin
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes2017In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 5, p. 4976-4987Article in journal (Refereed)
    Abstract [en]

    Atomistic molecular dynamics simulations have been performed to study microscopic the interfacial ionic structures, molecular arrangements, and orientational preferences of trihexyltetradecylphosphonium-bis(mandelato)borate ([P-6,P-6,P-6,P-14][BM]) ionic liquid confined between neutral and charged gold electrodes. It was found that both [P-6,P-6,P-6,P-14] cations and [BMB] anions are coabsorbed onto neutral electrodes at different temperatures. The hexyl and tetradecyl chains in [P-6,P-6,P-6,P-14] cations lie preferentially flat on neutral electrodes. The oxalato and phenyl rings in [BMB] anions are characterized by alternative parallel perpendicular orientations in the mixed innermost ionic layer adjacent to neutral electrodes. An increase in temperature has a marginal effect on the interfacial ionic structures and molecular orientations of [P-6,P-6,P-6,P-14] [BMB] ionic species in a confined environment. Electrifying gold electrodes leads to peculiar changes in the interfacial ionic structures and molecular orientational arrangements of [p(6,6,414)] cations and [BMB] anions in negatively and positively charged gold electrodes, respectively. As surface charge density increases (but lower than 20 mu C/cm(2)), the layer thickness of the mixed innermost interfacial layer gradually increases due to a consecutive accumulation of [P6,6,614] cations and [BMB] anions at negatively and positively charged electrodes, respectively, before the formation of distinct cationic and anionic innermost layers. Meanwhile, the molecular orientations of two oxalato rings in the same [BMB] anions change gradually from a parallel perpendicular feature to being partially characterized by a tilted arrangement at an angle of 45 from the electrodes and finally to a dominant parallel coordination pattern along positively charged electrodes. Distinctive interfacial distribution patterns are also observed accordingly for phenyl rings that are directly connected to neighboring oxalato rings in [BMB] anions.

  • 22.
    Wang, Yong-Lei
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Stanford University, USA.
    Li, Bin
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Microstructures and dynamics of tetraalkylphosphonium chloride ionic liquids2017In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 22, article id 224502Article in journal (Refereed)
    Abstract [en]

    Atomistic simulations have been performed to investigate the effect of aliphatic chain length in tetraalkylphosphonium cations on liquid morphologies, microscopic ionic structures, and dynamical quantities of tetraalkylphosphonium chloride ionic liquids. The liquid morphologies are characterized by sponge-like interpenetrating polar and apolar networks in ionic liquids consisting of tetraalkylphosphonium cations with short aliphatic chains. The lengthening aliphatic chains in tetraalkylphosphonium cations lead to polar domains consisting of chloride anions and central polar groups in cations being partially or totally segregated in ionic liquid matrices due to a progressive expansion of apolar domains in between. Prominent polarity alternation peaks and adjacency correlation peaks are observed at low and high q range in total X-ray scattering structural functions, respectively, and their peak positions gradually shift to lower q values with lengthening aliphatic chains in tetraalkylphosphonium cations. The charge alternation peaks registered in the intermediate q range exhibit complicated tendencies due to a cancellation of peaks and anti-peaks in partial structural functions for ionic subcomponents. The particular microstructures and liquid morphologies in tetraalkylphosphonium chloride ionic liquids intrinsically contribute to distinct dynamics characterized by mean square displacements, van Hove correlation functions, and non-Gaussian parameters for ionic species in the heterogeneous ionic environment. Most tetraalkylphosphonium cations have higher translational mobilities than their partner anions due to strong coordination of chloride anions with central polar groups in tetraalkylphosphonium cations through strong Coulombic and hydrogen bonding interactions. The increase of aliphatic chain length in tetraalkylphosphonium cations leads to a concomitant shift of van Hove correlation functions and non-Gaussian parameters to larger radial distances and longer time scales, respectively, indicating the enhanced translational dynamical heterogeneities of tetraalkylphosphonium cations and the corresponding chloride anions.

  • 23.
    Wang, Yong-Lei
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Li, Bin
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Mocci, Francesca
    Lu, Zhong-Yuan
    Yuan, Jiayin
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Nanjing Tech University, P. R. China; Petru Poni Institute of Macromolecular Chemistry, Romania; Luleå University of Technology, Sweden.
    Fayer, Michael D.
    Microstructural and Dynamical Heterogeneities in Ionic Liquids2020In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 120, no 13, p. 5798-5877Article, review/survey (Refereed)
    Abstract [en]

    Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of Its lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

  • 24.
    Wang, Yong-Lei
    et al.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Gavatskth, Sergei
    Antzutkin, Oleg N.
    Rutland, Mark W.
    Laaksonen, Aatto
    Atomistic Insight into Tetraalkylphosphonium-Bis(oxalato)borate Ionic Liquid/Water Mixtures. I. Local Microscopic Structure2015In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 16, p. 5251-5264Article in journal (Refereed)
    Abstract [en]

    Atomistic simulations have been performed to investigate the microscopic structural organization of aqueous solutions of trihexyltetradecylphosphonium bis(oxalato)borate ([P-6,P-6,P-6,P-14] [BOB]) ionic liquid (IL). The evolution of the microscopic liquid structure and the local ionic organization of IL/water mixtures as a function of the water concentration is visualized and systematically analyzed via radial and spatial distribution functions, coordination numbers, hydrogen bond network, and water clustering analysis. The microscopic liquid structure in neat IL is characterized by a connected apolar network composed of the alkyl chains of [P-6,P-6,P-6,P-14] cations and isolated polar domains consisting of the central segments of [P-6,P-6,P-6,P-14] cations and [BOB] anions, and the corresponding local ionic environment is described by direct contact ion pairs. In IL/water mixtures with lower water mole fractions, the added water molecules are dispersed and embedded in cavities between neighboring ionic species and the local ionic structure is characterized by solvent-shared ion pairs through cation-water-anion triple complexes. With a gradual increase in the water concentration in IL/water mixtures, the added water molecules tend to aggregate and form small clusters, intermediate chain-like structures, large aggregates, and eventually a water network in water concentrated simulation systems. A further progressive dilution of IL/water mixtures leads to the formation of self-organized micelle-like aggregates characterized by a hydrophobic core and hydrophilic shell consisting of the central polar segments in [P-6,P-6,P-6,P-14] cations and [BOB] anions in a highly branched water network. The striking structural evolution of the [P-6,P-6,P-6,P-14] [BOB] IL/water mixtures is rationalized by the competition between favorable hydrogen bonded interactions and strong electrostatic interactions between the polar segments in ionic species and the dispersion interactions between the hydrophobic alkyl chains in [P-6,P-6,P-6,P-14] cations.

  • 25. Wang, Yong-Lei
    et al.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Kloo, Lars
    Antzutkin, Oleg N.
    Glavatskih, Sergei
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Solvation structures of water in trihexyltetradecylphosphonium-orthoborate ionic liquids2016In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 145, no 6, article id 064507Article in journal (Refereed)
    Abstract [en]

    Atomistic molecular dynamics simulations have been performed to investigate effective interactions of isolated water molecules dispersed in trihexyltetradecylphosphonium-orthoborate ionic liquids (ILs). The intrinsic free energy changes in solvating one water molecule from gas phase into bulk IL matrices were estimated as a function of temperature, and thereafter, the calculations of potential of mean force between two dispersed water molecules within different IL matrices were performed using umbrella sampling simulations. The systematic analyses of local ionic microstructures, orientational preferences, probability and spatial distributions of dispersed water molecules around neighboring ionic species indicate their preferential coordinations to central polar segments in orthoborate anions. The effective interactions between two dispersed water molecules are partially or totally screened as their separation distance increases due to interference of ionic species in between. These computational results connect microscopic anionic structures with macroscopically and experimentally observed difficulty in completely removing water from synthesized IL samples and suggest that the introduction of hydrophobic groups to central polar segments and the formation of conjugated ionic structures in orthoborate anions can effectively reduce residual water content in the corresponding IL samples.

  • 26. Wang, Yong-Lei
    et al.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Li, Bin
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 34, p. 22125-22135Article in journal (Refereed)
    Abstract [en]

    A multiscale modeling protocol was sketched for the trihexyltetradecylphosphonium chloride ([P-6,P-6,P-6,P-14]Cl) ionic liquid (IL). The optimized molecular geometries of an isolated [P-6,P-6,P-6,P-14] cation and a tightly bound [P-6,P-6,P-6,P-14]Cl ion pair structure were obtained from quantum chemistry ab initio calculations. A cost-effective united-atom model was proposed for the [P-6,P-6,P-6,P-14] cation based on the corresponding atomistic model. Atomistic and coarse-grained molecular dynamics simulations were performed over a wide temperature range to validate the proposed united-atom [P-6,P-6,P-6,P-14] model against the available experimental data. Through a systemic analysis of volumetric quantities, microscopic structures, and transport properties of the bulk [P-6,P-6,P-6,P-14]Cl IL under varied thermodynamic conditions, it was identified that the proposed united-atom [P-6,P-6,P-6,P-14] cationic model could essentially capture the local intermolecular structures and the nonlocal experimental thermodynamics, including liquid density, volume expansivity and isothermal compressibility, and transport properties, such as zero-shear viscosity, of the bulk [P-6,P-6,P-6,P-14]Cl IL within a wide temperature range.

  • 27. Wang, Yong-Lei
    et al.
    Shimpi, Manishkumar R.
    Sarman, Sten
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Antzutkin, Oleg N.
    Glavatskih, Sergei
    Kloo, Lars
    Laaksonen, Aatto
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Atomistic Insight into Tetraalkylphosphonium Bis(oxalato)borate Ionic Liquid/Water Mixtures. 2. Volumetric and Dynamic Properties2016In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 120, no 30, p. 7446-7455Article in journal (Refereed)
    Abstract [en]

    Atomistic molecular dynamics simulations have been performed to investigate volumetric quantities and dynamic properties of binary trihexyltetradecylphosphonium bis(oxalato)-borate ([P-6,P-6,P-6,P-14][BOB]) ionic liquid (IL)/water mixtures with different water concentrations. The predicted liquid densities for typical [P-6,P-6,P-6,P-14][BOB] IL/water mixtures are consistent with available experimental data with a relative discrepancy of less than 3%. The liquid densities and excess molar volumes of all studied [P-6,P-6,P-6,P-14][BOB] IL/water mixtures are characterized by concave and convex features, respectively, within full water concentration range. The dynamic properties of [P-6,P-6,P-6,P-14] cations, [BOB] anions, and water molecules are particularly analyzed through calculation of velocity autocorrelation functions, diffusion coefficients, and reorientational autocorrelation functions and correlation times. The translational and reorientational mobilities of three species become faster upon increasing water concentration in [P-6,P-6,P-6,P-14][BOB] IL/water mixtures and present complex dynamical characteristics arising from three distinct microscopic diffusion features within the full water concentration range. The obtained striking volumetric quantities and particular dynamic properties are well correlated to microscopic liquid structural organization and distinct local ionic environment of all studied [P-6,P-6,P-6,P-14][BOB] IL/water mixtures.

1 - 27 of 27
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