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Publications (10 of 27) Show all publications
Sarman, S. & Laaksonen, A. (2023). Diffusion-driven rotation in cholesteric liquid crystals studied using molecular dynamics simulation of a mixture of the Gay–Berne fluid and the Lennard-Jones fluid. Physical Chemistry, Chemical Physics - PCCP, 25(28), 18833-18843
Open this publication in new window or tab >>Diffusion-driven rotation in cholesteric liquid crystals studied using molecular dynamics simulation of a mixture of the Gay–Berne fluid and the Lennard-Jones fluid
2023 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 25, no 28, p. 18833-18843Article in journal (Refereed) Published
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.

National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:su:diva-221216 (URN)10.1039/d3cp01374j (DOI)001023691900001 ()37403565 (PubMedID)2-s2.0-85165248801 (Scopus ID)
Available from: 2023-09-20 Created: 2023-09-20 Last updated: 2023-09-20Bibliographically approved
Sarman, S. & Laaksonen, A. (2021). Microscopic shear flow simulations of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid. Physical Chemistry, Chemical Physics - PCCP, 23(28), 15183-15195
Open this publication in new window or tab >>Microscopic shear flow simulations of a biaxial smectic A liquid crystal based on the soft ellipsoid string-fluid
2021 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 23, no 28, p. 15183-15195Article in journal (Refereed) Published
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.

National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:su:diva-196161 (URN)10.1039/d1cp00957e (DOI)000669958400001 ()34227627 (PubMedID)
Available from: 2021-09-07 Created: 2021-09-07 Last updated: 2022-02-25Bibliographically approved
Wang, Y.-L., Li, B., Sarman, S., Mocci, F., Lu, Z.-Y., Yuan, J., . . . Fayer, M. D. (2020). Microstructural and Dynamical Heterogeneities in Ionic Liquids. Chemical Reviews, 120(13), 5798-5877
Open this publication in new window or tab >>Microstructural and Dynamical Heterogeneities in Ionic Liquids
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2020 (English)In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 120, no 13, p. 5798-5877Article, review/survey (Refereed) Published
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.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-184504 (URN)10.1021/acs.chemrev.9b00693 (DOI)000550644700002 ()32292036 (PubMedID)
Available from: 2020-09-15 Created: 2020-09-15 Last updated: 2022-03-23Bibliographically approved
Sarman, S., Wang, Y.-L. & Laaksonen, A. (2019). Shear flow simulations of smectic liquid crystals based on the Gay-Berne fluid and the soft sphere string-fluid. Physical Chemistry, Chemical Physics - PCCP, 21(1), 292-305
Open this publication in new window or tab >>Shear flow simulations of smectic liquid crystals based on the Gay-Berne fluid and the soft sphere string-fluid
2019 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 1, p. 292-305Article in journal (Refereed) Published
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.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-165684 (URN)10.1039/c8cp05077e (DOI)000454836700029 ()30520918 (PubMedID)
Available from: 2019-02-05 Created: 2019-02-05 Last updated: 2022-02-26Bibliographically approved
Sarman, S., Wang, Y.-L., Rohlmann, P., Glavatskih, S. & Laaksonen, A. (2018). Rheology of phosphonium ionic liquids: a molecular dynamics and experimental study. Physical Chemistry, Chemical Physics - PCCP, 20(15), 10193-10203
Open this publication in new window or tab >>Rheology of phosphonium ionic liquids: a molecular dynamics and experimental study
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2018 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 15, p. 10193-10203Article in journal (Refereed) Published
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.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-156671 (URN)10.1039/c7cp08349a (DOI)000430537600052 ()29594283 (PubMedID)
Available from: 2018-06-04 Created: 2018-06-04 Last updated: 2022-02-26Bibliographically approved
Wang, Y.-L., Golets, M., Li, B., Sarman, S. & Laaksonen, A. (2017). Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes. ACS Applied Materials and Interfaces, 9(5), 4976-4987
Open this publication in new window or tab >>Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes
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2017 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 9, no 5, p. 4976-4987Article in journal (Refereed) Published
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.

Keywords
trihexyltetradecylphosphonium-bis(mandelato)borate ionic liquid, gold electrodes, atomistic simulations, interfacial structures, molecular arrangements
National Category
Nano Technology Materials Engineering
Identifiers
urn:nbn:se:su:diva-141426 (URN)10.1021/acsami.6b14429 (DOI)000393848900080 ()28099800 (PubMedID)
Available from: 2017-04-05 Created: 2017-04-05 Last updated: 2022-02-28Bibliographically approved
Wang, Y.-L., Li, B., Sarman, S. & Laaksonen, A. (2017). Microstructures and dynamics of tetraalkylphosphonium chloride ionic liquids. Journal of Chemical Physics, 147(22), Article ID 224502.
Open this publication in new window or tab >>Microstructures and dynamics of tetraalkylphosphonium chloride ionic liquids
2017 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 147, no 22, article id 224502Article in journal (Refereed) Published
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.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-150964 (URN)10.1063/1.4995003 (DOI)000418350100024 ()
Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2022-02-28Bibliographically approved
Wang, Y.-L., Shimpi, M. R., Sarman, S., Antzutkin, O. N., Glavatskih, S., Kloo, L. & Laaksonen, A. (2016). Atomistic Insight into Tetraalkylphosphonium Bis(oxalato)borate Ionic Liquid/Water Mixtures. 2. Volumetric and Dynamic Properties. Journal of Physical Chemistry B, 120(30), 7446-7455
Open this publication in new window or tab >>Atomistic Insight into Tetraalkylphosphonium Bis(oxalato)borate Ionic Liquid/Water Mixtures. 2. Volumetric and Dynamic Properties
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2016 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 120, no 30, p. 7446-7455Article in journal (Refereed) Published
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.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-134254 (URN)10.1021/acs.jpcb.6b02921 (DOI)000381235800015 ()27387981 (PubMedID)
Available from: 2016-10-04 Created: 2016-10-03 Last updated: 2022-02-28Bibliographically approved
Nygård, K., Sarman, S., Hyltegren, K., Chodankar, S., Perret, E., Buitenhuis, J., . . . Kjellander, R. (2016). Density Fluctuations of Hard-Sphere Fluids in Narrow Confinement. Physical Review X, 6(1), Article ID 011014.
Open this publication in new window or tab >>Density Fluctuations of Hard-Sphere Fluids in Narrow Confinement
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2016 (English)In: Physical Review X, E-ISSN 2160-3308, Vol. 6, no 1, article id 011014Article in journal (Refereed) Published
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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-168099 (URN)10.1103/PhysRevX.6.011014 (DOI)000370248600002 ()
Available from: 2019-04-17 Created: 2019-04-17 Last updated: 2024-01-17Bibliographically approved
Sarman, S., Wang, Y.-L. & Laaksonen, A. (2016). Self-diffusion in the non-Newtonian regime of shearing liquid crystal model systems based on the Gay-Berne potential. Journal of Chemical Physics, 144(5), Article ID 054901.
Open this publication in new window or tab >>Self-diffusion in the non-Newtonian regime of shearing liquid crystal model systems based on the Gay-Berne potential
2016 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 5, article id 054901Article in journal (Refereed) Published
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.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:su:diva-128193 (URN)10.1063/1.4940731 (DOI)000369893900032 ()26851931 (PubMedID)
Available from: 2016-03-23 Created: 2016-03-21 Last updated: 2022-02-23Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-1702-5645

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