@article{geier_wandrei_skutnik_schoen_2019, title={Molecular theory of a ferromagnetic nematic liquid crystal}, volume={100}, ISSN={["2470-0053"]}, DOI={10.1103/PhysRevE.100.022702}, abstractNote={We employ a version of classical density functional theory to study the phase behavior of a simple model liquid crystal in an external field. The uniaxially symmetric molecules have a spherically symmetric core with superimposed orientation-dependent attractions. The interaction between the cores consists of a hard-sphere repulsion plus an isotropic square-well attraction. The anisotropic part of the interaction potential allows for the formation of a uniaxially symmetric nematic phase. The orientation of the molecules couples to an external polar field. The external field is capable of rotating the nematic director n[over ̂] in the x-z plane. The field is also capable of changing the topology of the phase diagram in that it suppresses the phase coexistence between an isotropic liquid and a nematic phase observed in the absence of the field. We study the transition from an unpolar to a polar nematic phase in terms of the orientation-distribution function (odf), nematic and polar order parameters, and components of n[over ̂]. If represented suitably the odf allows us to study orientational changes during the switching process between nonpolar and polar nematic phases. We also give a simple argument that explains why nematic order is lost whereas polar order persists up to the gas-liquid critical point along the coexistence curve. We also discuss the relevance of our theory for future experimental studies.}, number={2}, journal={PHYSICAL REVIEW E}, author={Geier, Immanuel S. and Wandrei, Stefanie M. and Skutnik, Robert A. and Schoen, Martin}, year={2019}, month={Aug} }
 @article{skutnik_lehmann_pueschel-schlotthauer_jackson_schoen_2019, title={The formation of biaxial nematic phases in binary mixtures of thermotropic liquid-crystals composed of uniaxial molecules}, volume={117}, ISSN={["1362-3028"]}, DOI={10.1080/00268976.2019.1581292}, abstractNote={ABSTRACT Monte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species. GRAPHICAL ABSTRACT}, number={20}, journal={MOLECULAR PHYSICS}, author={Skutnik, Robert A. and Lehmann, Louis and Pueschel-Schlotthauer, Sergej and Jackson, George and Schoen, Martin}, year={2019}, month={Oct}, pages={2830–2845} }
 @article{perrin_schoen_coudert_boutin_2018, title={Structure and dynamics of solvated polymers near a silica surface: On the different roles played by solvent}, volume={122}, DOI={10.1021/acs.jpcb.7b11753}, abstractNote={Whereas it is experimentally known that the inclusion of nanoparticles in hydrogels can lead to a mechanical reinforcement, a detailed molecular understanding of the adhesion mechanism is still lacking. Here we use coarse-grained molecular dynamics simulations to investigate the nature of the interface between silica surfaces and solvated polymers. We show how differences in the nature of the polymer and the polymer-solvent interactions can lead to drastically different behavior of the polymer-surface adhesion. Comparing explicit and implicit solvent models, we conclude that this effect cannot be fully described in an implicit solvent. We highlight the crucial role of polymer solvation for the adsorption of the polymer chain on the silica surface, the significant dynamics of polymer chains on the surface, and details of the modifications in the structure solvated polymer close to the interface.}, number={16}, journal={Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces & Biophysical}, author={Perrin, E. and Schoen, M. and Coudert, F. X. and Boutin, A.}, year={2018}, pages={4573–4582} }
 @article{stieger_agha_schoen_mazza_sengupta_2017, title={Hydrodynamic cavitation in Stokes flow of anisotropic fluids}, volume={8}, ISSN={["2041-1723"]}, DOI={10.1038/ncomms15550}, abstractNote={Abstract}, journal={NATURE COMMUNICATIONS}, author={Stieger, Tillmann and Agha, Hakam and Schoen, Martin and Mazza, Marco G. and Sengupta, Anupam}, year={2017}, month={May}, pages={1–11} }
 @article{pueschel-schlotthauer_turrion_hall_mazza_schoen_2017, title={The Impact of Colloidal Surface-Anchoring on the Smectic A Phase}, volume={33}, ISSN={["0743-7463"]}, DOI={10.1021/acs.langmuir.6b03941}, abstractNote={Liquid-crystalline phases are known for their unique properties, i.e., the combination of fluidity and long-range orientational and/or positional order. The presence of a colloidal particle gives rise to perturbations of this order locally. These perturbations are the origin of intercolloidal forces driving the colloidal self-assembly in a directed manner. Hence, the understanding of these perturbations is the first step in understanding and controlling the self-assembly process. Here, we perform Monte Carlo simulations to investigate the perturbations of orientational and positional order in a smectic A phase caused by a spherical colloid. We model the host phase via an interaction potential that reproduces characteristic features of phase behavior, structure, dynamics, and elasticity [S. Püschel-Schlotthauer et al. J. Chem. Phys. 2016, 145, 164903]. For strong homeotropic anchoring conditions, we find a Saturn ring defect and an onion structure in the smectic A phase; the latter has never been reported for colloids so far. For strong planar anchoring conditions, we find Boojum defects that become elongated at low temperature, similar to what is observed in experiments. However, for weak planar anchoring conditions, a double surface ring defect is exhibited in the smectic A phase.}, number={9}, journal={LANGMUIR}, author={Pueschel-Schlotthauer, Sergej and Turrion, Victor Meiwes and Hall, Carol K. and Mazza, Marco G. and Schoen, Martin}, year={2017}, month={Mar}, pages={2222–2234} }
 @article{pueschel-schlotthauer_turrion_stieger_grotjahn_hall_mazza_schoen_2016, title={A novel model for smectic liquid crystals: Elastic anisotropy and response to a steady-state flow}, volume={145}, ISSN={["1089-7690"]}, DOI={10.1063/1.4965711}, abstractNote={By means of a combination of equilibrium Monte Carlo and molecular dynamics simulations and nonequilibrium molecular dynamics we investigate the ordered, uniaxial phases (i.e., nematic and smectic A) of a model liquid crystal. We characterize equilibrium behavior through their diffusive behavior and elastic properties. As one approaches the equilibrium isotropic-nematic phase transition, diffusion becomes anisotropic in that self-diffusion D⊥ in the direction orthogonal to a molecule’s long axis is more hindered than self-diffusion D∥ in the direction parallel to that axis. Close to nematic-smectic A phase transition the opposite is true, D∥ < D⊥. The Frank elastic constants K1, K2, and K3 for the respective splay, twist, and bend deformations of the director field n̂ are no longer equal and exhibit a temperature dependence observed experimentally for cyanobiphenyls. Under nonequilibrium conditions, a pressure gradient applied to the smectic A phase generates Poiseuille-like or plug flow depending on whether the convective velocity is parallel or orthogonal to the plane of smectic layers. We find that in Poiseuille-like flow the viscosity of the smectic A phase is higher than in plug flow. This can be rationalized via the velocity-field component in the direction of the flow. In a sufficiently strong flow these smectic layers are not destroyed but significantly bent.}, number={16}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Pueschel-Schlotthauer, Sergej and Turrion, Victor Meiwes and Stieger, Tillmann and Grotjahn, Robin and Hall, Carol K. and Mazza, Marco G. and Schoen, Martin}, year={2016}, month={Oct} }
 @article{cordoba_stieger_mazza_schoen_pablo_2016, title={Anisotropy and probe-medium interactions in the microrheology of nematic fluids}, volume={60}, ISSN={["0148-6055"]}, DOI={10.1122/1.4935849}, abstractNote={A theoretical formalism is presented to analyze and interpret microrheology experiments in anisotropic fluids with nematic order. The predictions of that approach are examined in the context of a simple coarse-grained molecular model which is simulated using nonequilibrium molecular dynamics calculations. The proposed formalism is used to study the effect of confinement, the type of anchoring at the probe-particle surface, and the strength of the nematic field on the rheological response functions obtained from probe-particle active microrheology. As expected, a stronger nematic field leads to increased anisotropy in the rheological response of the material. It is also found that the defect structures that arise around the probe particle, which are determined by the type of anchoring and the particle size, have a significant effect on the rheological response observed in microrheology simulations. Independent estimates of the bulk dynamic modulus of the model nematic fluid considered here are obtained from small-amplitude oscillatory shear simulations with Lees–Edwards boundary conditions. The results of simulations indicate that the dynamic modulus extracted from particle-probe microrheology is different from that obtained in the absence of the particle, but that the differences decrease as the size of the defect also decreases. Importantly, the results of the nematic microrheology theory proposed here are in much closer agreement with simulations than those from earlier formalisms conceived for isotropic fluids. As such, it is anticipated that the theoretical framework advanced in this study could provide a useful tool for interpretation of microrheology experiments in systems such as liquid crystals and confined macromolecular solutions or gels.}, number={1}, journal={JOURNAL OF RHEOLOGY}, author={Cordoba, Andres and Stieger, Tillmann and Mazza, Marco G. and Schoen, Martin and Pablo, Juan J.}, year={2016}, pages={75–95} }
 @article{pueschel-schlotthauer_stieger_melle_mazza_schoen_2016, title={Coarse-grained treatment of the self-assembly of colloids suspended in a nematic host phase}, volume={12}, ISSN={["1744-6848"]}, DOI={10.1039/c5sm01860a}, abstractNote={Plots of the three-dimensional defect topologies of a pair of colloids (grey spheres) immersed in a nematic host fluid for various anglesθbetween the center-of-mass distance vector r12and the far-field nematic director n̂0.}, number={2}, journal={SOFT MATTER}, author={Pueschel-Schlotthauer, Sergej and Stieger, Tillmann and Melle, Michael and Mazza, Marco G. and Schoen, Martin}, year={2016}, pages={469–480} }
 @article{stieger_pueschel-schlotthauer_schoen_mazza_2016, title={Flow-induced deformation of closed disclination lines near a spherical colloid immersed in a nematic host phase}, volume={114}, ISSN={["1362-3028"]}, DOI={10.1080/00268976.2015.1096973}, abstractNote={ABSTRACT We present nonequilibrium molecular dynamics simulations of a spherical colloidal particle with a chemically homogeneous surface immersed in a nematic liquid-crystal host phase. This setup is then placed between planar and atomically structured substrate surfaces that serve to fix the nematic far-field director . The substrates are separated by a sufficiently large distance such that they do not interfere directly with the environment of the colloid. Because of a mismatch between and the local homeotropic anchoring of molecules of the liquid crystal (i.e., mesogens) at the surface of the colloid circular defect (Saturn) rings ℓ arise if the host is in thermodynamic equilibrium (i.e., in the absence of flow). The size of these rings depends on the range of the mesogen–colloid interactions which we model via an attractive Yukawa potential. As Poiseuille flow is initiated, ℓ is deformed. The degree of deformation is analysed quantitatively in terms of characteristic geometric parameters fitted to suitable projections of ℓ. Our results suggest that smaller ℓ are shifted downstream while approximately maintaining their circular shape, whereas larger ones exhibit an elastic deformation in addition. We provide a simple geometric argument to predict the downstream shift of smaller, circular ℓs in excellent agreement with the simulation data over the range of steady-state flows considered. GRAPHICAL ABSTRACT}, number={2}, journal={MOLECULAR PHYSICS}, author={Stieger, Tillmann and Pueschel-Schlotthauer, Sergej and Schoen, Martin and Mazza, Marco G.}, year={2016}, month={Jan}, pages={259–275} }
 @article{cattes_gubbins_schoen_2016, title={Mean-field density functional theory of a nanoconfined classical, three-dimensional Heisenberg fluid. I. The role of molecular anchoring}, volume={144}, ISSN={["1089-7690"]}, DOI={10.1063/1.4949330}, abstractNote={In this work, we employ classical density functional theory (DFT) to investigate for the first time equilibrium properties of a Heisenberg fluid confined to nanoscopic slit pores of variable width. Within DFT pair correlations are treated at modified mean-field level. We consider three types of walls: hard ones, where the fluid-wall potential becomes infinite upon molecular contact but vanishes otherwise, and hard walls with superimposed short-range attraction with and without explicit orientation dependence. To model the distance dependence of the attractions, we employ a Yukawa potential. The orientation dependence is realized through anchoring of molecules at the substrates, i.e., an energetic discrimination of specific molecular orientations. If the walls are hard or attractive without specific anchoring, the results are “quasi-bulk”-like in that they can be linked to a confinement-induced reduction of the bulk mean field. In these cases, the precise nature of the walls is completely irrelevant at coexistence. Only for specific anchoring nontrivial features arise, because then the fluid-wall interaction potential affects the orientation distribution function in a nontrivial way and thus appears explicitly in the Euler-Lagrange equations to be solved for minima of the grand potential of coexisting phases.}, number={19}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Cattes, Stefanie M. and Gubbins, Keith E. and Schoen, Martin}, year={2016}, month={May} }
 @article{cattes_klapp_schoen_2015, title={Condensation, demixing, and orientational ordering of magnetic colloidal suspensions}, volume={91}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.91.052127}, abstractNote={In this work we study the phase behavior of magnetic particles suspended in a simple nonmagnetic solvent. Magnetic particles are modelled as spherical particles carrying a three-dimensional, classical Heisenberg spin, whereas solvent molecules are treated as spherically symmetric Lennard-Jones particles. The binary mixture of magnetic particles and solvent is studied within the framework of classical density functional theory (DFT). Within DFT pair correlations are treated at the modified mean-field level at which they are approximated by orientation dependent Mayer f functions. In the absence of an external magnetic field four generic types of phase diagrams are observed depending on the concentration of magnetic particles. In this case we observe liquid-liquid phase coexistence between an orientationally ordered (polarized) and a disordered phase characterized by slightly different concentrations of magnetic particles. Liquid-liquid phase coexistence is suppressed by an external field and vanishes completely if the strength of the field is sufficiently large.}, number={5}, journal={PHYSICAL REVIEW E}, author={Cattes, Stefanie M. and Klapp, Sabine H. L. and Schoen, Martin}, year={2015}, month={May} }
 @article{schlotthauer_skutnik_stieger_schoen_2015, title={Defect topologies in chiral liquid crystals confined to mesoscopic channels}, volume={142}, ISSN={["1089-7690"]}, DOI={10.1063/1.4920979}, abstractNote={We present Monte Carlo simulations in the grand canonical and canonical ensembles of a chiral liquid crystal confined to mesochannels of variable sizes and geometries. The mesochannels are taken to be quasi-infinite in one dimension but finite in the two other directions. Under thermodynamic conditions chosen and for a selected value of the chirality coupling constant, the bulk liquid crystal exhibits structural characteristics of a blue phase II. This is established through the tetrahedral symmetry of disclination lines and the characteristic simple-cubic arrangement of double-twist helices formed by the liquid-crystal molecules along all three axes of a Cartesian coordinate system. If the blue phase II is then exposed to confinement, the interplay between its helical structure, various anchoring conditions at the walls of the mesochannels, and the shape of the mesochannels gives rise to a broad variety of novel, qualitative disclination-line structures that are reported here for the first time.}, number={19}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Schlotthauer, Sergej and Skutnik, Robert A. and Stieger, Tillmann and Schoen, Martin}, year={2015}, month={May} }
 @article{giura_schoen_2014, title={Density-functional theory and Monte Carlo simulations of the phase behavior of a simple model liquid crystal}, volume={90}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.90.022507}, abstractNote={We consider the phase behavior of a simple model of a liquid crystal by means of modified mean-field density-functional theory (MMF DFT) and Monte Carlo simulations in the grand canonical ensemble (GCEMC). The pairwise additive interactions between liquid-crystal molecules are modeled via a Lennard-Jones potential in which the attractive contribution depends on the orientation of the molecules. We derive the form of this orientation dependence through an expansion in terms of rotational invariants. Our MMF DFT predicts two topologically different phase diagrams. At weak to intermediate coupling of the orientation dependent attraction, there is a discontinuous isotropic-nematic liquid-liquid phase transition in addition to the gas-isotropic liquid one. In the limit of strong coupling, the gas-isotropic liquid critical point is suppressed in favor of a fluid- (gas- or isotropic-) nematic phase transition which is always discontinuous. By considering three representative isotherms in parallel GCEMC simulations, we confirm the general topology of the phase diagram predicted by MMF DFT at intermediate coupling strength. From the combined MMF DFT-GCEMC approach, we conclude that the isotropic-nematic phase transition is very weakly first order, thus confirming earlier computer simulation results for the same model [see M. Greschek and M. Schoen, Phys. Rev. E 83, 011704 (2011)].}, number={2}, journal={PHYSICAL REVIEW E}, author={Giura, Stefano and Schoen, Martin}, year={2014}, month={Aug} }
 @article{melle_schlotthauer_hall_diaz-herrera_schoen_2014, title={Disclination lines at homogeneous and heterogeneous colloids immersed in a chiral liquid crystal}, volume={10}, ISSN={["1744-6848"]}, DOI={10.1039/c4sm00959b}, abstractNote={In the present work we perform Monte Carlo simulations in the isothermal-isobaric ensemble to study defect topologies formed in a cholesteric liquid crystal due to the presence of a spherical colloidal particle.}, number={30}, journal={SOFT MATTER}, author={Melle, Michael and Schlotthauer, Sergej and Hall, Carol K. and Diaz-Herrera, Enrique and Schoen, Martin}, year={2014}, pages={5489–5502} }
 @article{stieger_schoen_mazza_2014, title={Effects of flow on topological defects in a nematic liquid crystal near a colloid}, volume={140}, ISSN={["1089-7690"]}, DOI={10.1063/1.4862953}, abstractNote={We perform molecular dynamics simulations of a nematic liquid crystal flowing around a colloidal particle. We study the flow-induced modifications of the topological defects in the liquid crystal due to the presence of the colloid. We show that flow distorts Boojum defects into an asymmetrically larger downstream lobe, and that Saturn ring defects are convected downstream along the flow direction, which is in agreement with experimental observations. Additionally, for a Janus colloid with both parallel and perpendicular patches, exhibiting a Boojum defect and a Saturn ring defect, we find that the Boojum defect facing the upstream direction is destroyed and the Saturn ring is convected downstream.}, number={5}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Stieger, Tillmann and Schoen, Martin and Mazza, Marco G.}, year={2014}, month={Feb} }
 @article{schoen_giura_klapp_2014, title={Phase behavior of an amphiphilic fluid}, volume={89}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.89.012310}, abstractNote={We invoke mean-field density functional theory (DFT) to investigate the phase behavior of an amphiphilic fluid composed of a hard-sphere core plus a superimposed anisometric Lennard-Jones perturbation. The orientation dependence of the interactions consists of a contribution analogous to the interaction potential between a pair of "spins" in the classical, three-dimensional Heisenberg fluid and another one reminiscent of the interaction between (electric or magnetic) point dipoles. At fixed orientation both contributions are short-range in nature decaying as r-6 (r being the separation between the centers of mass of a pair of amphiphiles). Based upon two mean-field-like approximations for the pair correlation function that differ in the degree of sophistication we derive expressions for the phase boundaries between various isotropic and polar phases that we solve numerically by the Newton-Raphson method. For sufficiently strong coupling between the Heisenberg "spins" both mean-field approximations generate three topologically different and generic types of phase diagrams that are observed in agreement with earlier work [see, for example, Tavares et al., Phys. Rev. E 52, 1915 (1995)]. Whereas the dipolar contribution alone is incapable of stabilizing polar phases on account of its short-range nature it is nevertheless important for details of the phase diagram such as location of the gas-isotropic liquid critical point, triple, and tricritical points. By tuning the dipolar coupling constant suitably one may, in fact, switch between topologically different phase diagrams. Employing also Monte Carlo simulations in the isothermal-isobaric ensemble the general topology of the DFT phase diagrams is confirmed.}, number={1}, journal={PHYSICAL REVIEW E}, author={Schoen, Martin and Giura, Stefano and Klapp, Sabine H. L.}, year={2014}, month={Jan} }
 @article{ilnytskyi_trokhymchuk_schoen_2014, title={Topological defects around a spherical nanoparticle in nematic liquid crystal: Coarse-grained molecular dynamics simulations}, volume={141}, ISSN={["1089-7690"]}, DOI={10.1063/1.4894438}, abstractNote={We consider the applicability of coarse-grained molecular dynamics for the simulation of defects in a nematic liquid crystal around a colloidal particle. Two types of colloids are considered, a soft colloid resembling a liquid crystal dendrimer or a similar macromolecule. In addition, a decorated colloid is used which could represent a gold nanoparticle with mesogen-modified surface. For both models we consider homeotropic and tangential anchoring. Precise control of the easy axis on the colloid's surface enables us to focus on specific planar arrangements in the case of a decorated colloid. The nematic phase is modelled explicitly via soft spherocylinders interacting through a potential, suggested by Lintuvuori and Wilson [J. Chem. Phys. 128, 044906 (2008)]. Properties of the nematic phase are studied by computing the Frank elastic constants. In addition, estimates for the nematic-isotropic transition and the coherence length allow us to establish a relation between energy and length scales with respect to experimental systems. Both models exhibit similar defect topologies, namely, that of a Saturn ring and a boojum-type of defect for homeotropic and tangential surface anchoring, respectively. In the decorated colloid model we tune the anchoring strength through the density of the mesogenic shell on the surface. We also found the biaxial boojum defect for the special case of longitudinal planar anchoring. The study demonstrates the potential of coarse-grained simulation methods for studying defects in liquid crystals.}, number={11}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Ilnytskyi, Jaroslav M. and Trokhymchuk, Andrij and Schoen, Martin}, year={2014}, month={Sep} }
 @article{giura_markus_klapp_schoen_2013, title={Isotropic-polar phase transitions in an amphiphilic fluid: Density functional theory versus computer simulations}, volume={87}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.87.012313}, abstractNote={We investigate the critical line separating isotropic from polar phases in an amphiphilic bulk fluid by means of density functional theory (DFT) and Monte Carlo (MC) simulations in the isothermal-isobaric ensemble. The intermolecular interactions are described by a Lennard-Jones potential in which the attractive contribution is modified by an orientation-dependent function. The latter consists of two terms: The first one has the orientation dependence of a classical three-dimensional Heisenberg interaction, whereas, the second one has the orientation dependence of a classical dipole-dipole interaction. However, both contributions are short range. Employing DFT together with a modified mean-field (MMF) approximation for the orientation-dependent pair correlation function, we derive an analytical expression for the critical line separating isotropic from polar liquidlike phases. In parallel MC simulations, we locate the line of critical points through an analysis of Binder's second-order cumulant of the polar-order parameter. Comparison with DFT shows that the dipolelike contribution is irrelevant for the isotropic-polar phase transition. As far as the Heisenberg contribution is concerned, the MC data are in semiquantitative agreement with the DFT predictions for sufficiently strong coupling between molecular orientations. For weaker coupling, the variation in the ratio of critical density and temperature ρ(c)/T(c) with the Heisenberg coupling constant ε(H) is underestimated by the MMF treatment. The MC results suggest that this is because ρ(c) increases with decreasing ε(H) such that the assumption on which the MMF approach rests becomes less applicable in the weaker-coupling limit.}, number={1}, journal={PHYSICAL REVIEW E}, author={Giura, Stefano and Markus, Bence G. and Klapp, Sabine H. L. and Schoen, Martin}, year={2013}, month={Jan} }
 @article{melle_theile_hall_schoen_2013, title={Nanoconfinement-Induced Structures in Chiral Liquid Crystals}, volume={14}, ISSN={["1422-0067"]}, DOI={10.3390/ijms140917584}, abstractNote={We employ Monte Carlo simulations in a specialized isothermal-isobaric and in the grand canonical ensemble to study structure formation in chiral liquid crystals as a function of molecular chirality. Our model potential consists of a simple Lennard-Jones potential, where the attractive contribution has been modified to represent the orientation dependence of the interaction between a pair of chiral liquid-crystal molecules. The liquid crystal is confined between a pair of planar and atomically smooth substrates onto which molecules are anchored in a hybrid fashion. Hybrid anchoring allows for the formation of helical structures in the direction perpendicular to the substrate plane without exposing the helix to spurious strains. At low chirality, we observe a cholesteric phase, which is transformed into a blue phase at higher chirality. More specifically, by studying the unit cell and the spatial arrangement of disclination lines, this blue phase can be established as blue phase II. If the distance between the confining substrates and molecular chirality are chosen properly, we see a third structure, which may be thought of as a hybrid, exhibiting mixed features of a cholesteric and a blue phase.}, number={9}, journal={INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES}, author={Melle, Michael and Theile, Madlona and Hall, Carol K. and Schoen, Martin}, year={2013}, month={Sep}, pages={17584–17607} }
 @article{kaehlitz_schoen_stark_2012, title={Clustering and mobility of hard rods in a quasicrystalline substrate potential}, volume={137}, ISSN={["0021-9606"]}, DOI={10.1063/1.4769839}, abstractNote={Recently, we have studied the self-assembly of hard needles in a quasicrystalline substrate potential with decagonal symmetry [P. Kählitz and H. Stark, J. Chem. Phys. 136, 174705 (2012)10.1063/1.4711086]. We have identified new structure formation using Monte Carlo simulations. However, hard needles have a zero width. To investigate how the excluded volume of rod-shaped particles influences their phase ordering, we extend here our studies to spherocylinders. We determine phase diagrams and plot them in the relevant variables, strength of substrate potential versus area fraction. At increasing area fraction η short rods form clusters that ultimately destroy directional ordering along the decagonal symmetry directions while surface-induced positional order exists for all η. In contrast, long rods show directional order in the whole density range. However, at high area fractions they assemble into compact clusters which destroy positional ordering. Finally, we also study the rod mobility using the kinetic Monte Carlo method and discuss an unexpected mobility enhancement with increasing density. All these features crucially depend on the non-zero excluded volume of the spherocylinders.}, number={22}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Kaehlitz, Philipp and Schoen, Martin and Stark, Holger}, year={2012}, month={Dec} }
 @article{melle_schlotthauer_mazza_klapp_schoen_2012, title={Defect topologies in a nematic liquid crystal near a patchy colloid}, volume={136}, ISSN={["1089-7690"]}, DOI={10.1063/1.4717619}, abstractNote={Using isothermal-isobaric Monte Carlo simulations we investigate defect topologies due to a spherical colloidal particle immersed in a nematic liquid crystal. Defects arise because of the competition between the preferential orientation at the colloid's surface and the far-field director \documentclass[12pt]{minimal}\begin{document}$\widehat{\bm {n}}_{0}$\end{document}n̂0. Considering a chemically homogeneous colloid as a special case we observe the well-known surface and saturn ring defect topologies for weak and strong perpendicular anchoring, respectively; for homogeneous, strong parallel anchoring we find a boojum defect topology that has been seen experimentally [see P. Poulin and D. A. Weitz, Phys. Rev. E 57, 626 (1998)] but not in computer simulations. We also consider a heterogeneous, patchy colloid where the liquid-crystal molecules anchor either preferentially planar or perpendicular at the surface of the colloid. For a patchy colloid we observe a boojum ring defect topology in agreement with recent experimental studies [see M. Conradi, M. Ravnik, M. Bele, M. Zorko, S. Žumer, and I. Muševič, Soft Matter 5, 3905 (2009)]. We also observe two other novel defect topologies that have not been reported thus far neither experimentally nor theoretically.}, number={19}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Melle, Michael and Schlotthauer, Sergej and Mazza, Marco G. and Klapp, Sabine H. L. and Schoen, Martin}, year={2012}, month={May} }
 @article{stieger_mazza_schoen_2012, title={Diffusivity Maximum in a Reentrant Nematic Phase}, volume={13}, ISSN={["1422-0067"]}, DOI={10.3390/ijms13067854}, abstractNote={We report molecular dynamics simulations of confined liquid crystals using the Gay–Berne–Kihara model. Upon isobaric cooling, the standard sequence of isotropic–nematic–smectic A phase transitions is found. Upon further cooling a reentrant nematic phase occurs. We investigate the temperature dependence of the self-diffusion coefficient of the fluid in the nematic, smectic and reentrant nematic phases. We find a maximum in diffusivity upon isobaric cooling. Diffusion increases dramatically in the reentrant phase due to the high orientational molecular order. As the temperature is lowered, the diffusion coefficient follows an Arrhenius behavior. The activation energy of the reentrant phase is found in reasonable agreement with the reported experimental data. We discuss how repulsive interactions may be the underlying mechanism that could explain the occurrence of reentrant nematic behavior for polar and non-polar molecules.}, number={6}, journal={INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES}, author={Stieger, Tillmann and Mazza, Marco G. and Schoen, Martin}, year={2012}, month={Jun}, pages={7854–7871} }
 @article{melle_giura_schlotthauer_schoen_2012, title={Finite-size scaling analysis of isotropic-polar phase transitions in an amphiphilic fluid}, volume={24}, ISSN={["0953-8984"]}, DOI={10.1088/0953-8984/24/3/035103}, abstractNote={We present Monte Carlo simulations of the isotropic–polar (IP) phase transition in an amphiphilic fluid carried out in the isothermal–isobaric ensemble. Our model consists of Lennard-Jones spheres where the attractive part of the potential is modified by an orientation-dependent function. This function gives rise to an angle dependence of the intermolecular attractions corresponding to that characteristic of point dipoles. Our data show a substantial system-size dependence of the dipolar order parameter. We analyze the system-size dependence in terms of the order-parameter distribution and a cumulant involving its first and second moments. The order parameter, its distribution, and susceptibility observe the scaling behavior characteristic of the 3D Ising universality class. Because of this scaling behavior and because all cumulants have a common intersection irrespective of system size we conclude that the IP phase transition is continuous. Considering pressures 1.3 ≤ P ≤ 3.0 we demonstrate that a line of continuous phase transitions exists which is analogous to the Curie line in systems exhibiting a ferroelectric transition. Our results are qualitatively consistent with Landau’s theory of continuous phase transitions.}, number={3}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Melle, Michael and Giura, Stefano and Schlotthauer, Sergej and Schoen, Martin}, year={2012}, month={Jan} }
 @article{greschek_gubbins_schoen_2012, title={Imprinting substrate structures onto a nematic liquid crystal}, volume={137}, ISSN={["1089-7690"]}, DOI={10.1063/1.4757391}, abstractNote={By means of Monte Carlo simulations in the grand canonical ensemble we study the morphology of the nematic phase of a simple model liquid crystal interacting with an alternating sequence of chemically different stripes. The stripes anchor molecules such that their orientation is either parallel or perpendicular with the substrate plane. The different molecular orientations are realized through anchoring functions that cause an energetic penalty for molecules oriented in an undesired fashion. We consider combinations of monostable and degenerate anchoring fields. The nature of the nematic phase is characterized through both the local nematic order parameter and the associated local director field. We observe states of uniaxial or biaxial symmetry depending on the ratio of stripe widths and the range of fluid-substrate attraction. In some cases the specific substrate pattern causes regions of biaxial symmetry to coexist with a bulk-like regime sufficiently far away from the substrates in which the local director field indicates a (homogeneous) bent state of the nematic liquid crystal.}, number={14}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Greschek, Manuel and Gubbins, Keith E. and Schoen, Martin}, year={2012}, month={Oct} }
 @article{schoen_guenther_2011, title={Capillary condensation in deformable mesopores: wetting versus nanomechanics}, volume={109}, ISSN={["1362-3028"]}, DOI={10.1080/00268976.2010.513346}, abstractNote={We employ grand canonical ensemble Monte Carlo simulations to investigate the strain experienced by a nanoscopic slit pore when this pore fills with fluid material. Both solid substrates of our model system consist of a single layer of solid atoms bound to their equilibrium lattice sites by a harmonic potential such that these atoms are thermally coupled to molecules of a fluid phase confined between them. Parameters are tuned such that they represent an experimental situation in which pentane is adsorbed by mesoporous silica. Our focus is on strain isotherms, that is the net deformation of the solid as fluid material is imbibed by the pore. At low pressures prior to pore filling, strain isotherms are dominated by wetting characteristics of the fluid–solid interface whereas nanomechanical properties of the pore may be deduced quantitatively from high-pressure portions of the strain isotherm after the pore is completely filled with fluid. To that end we introduce a thermodynamic analysis of the high-pressure portion of the sorption isotherm that permits us to determine the elasticity of the confining solid material in terms of a so-called pore-load modulus which is also experimentally accessible.}, number={1}, journal={MOLECULAR PHYSICS}, author={Schoen, Martin and Guenther, Gerrit}, year={2011}, pages={83–95} }
 @article{greschek_schoen_2011, title={Finite-size scaling analysis of isotropic-nematic phase transitions in an anisometric Lennard-Jones fluid}, volume={83}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.83.011704}, abstractNote={By means of Monte Carlo simulations in the isothermal-isobaric ensemble, we perform a finite-size scaling analysis of the isotropic-nematic (IN) phase transition. Our model consists of egg-shaped anisometric Lennard-Jones molecules. We employ the cumulant intersection method to locate the pressure P* at which the IN phase transition occurs at a given temperature T. In particular, we focus on second-order cumulants of the largest and middle eigenvalues of the alignment tensor. At fixed T, cumulants for various system sizes intersect at a unique pressure P*. Various known scaling relations for these cumulants are verified numerically. At P*, the isobaric heat capacity passes through a maximum value c(P)(m), which depends on the number of molecules N. This dependency can accurately be described by a power law such that lim(N→∞)c(P)(m)(N)→∞. For sufficiently large N, the pressure at which c(P)(m) is located shifts only very slightly in agreement with the apparent insensitivity of the cumulant intersection to N. In addition, we analyze our data in terms of Landau's theory of phase transitions. Our results are consistent with a weakly discontinuous entropy-driven phase transition.}, number={1}, journal={PHYSICAL REVIEW E}, author={Greschek, Manuel and Schoen, Martin}, year={2011}, month={Jan} }
 @article{greschek_schoen_2011, title={Orientational prewetting of planar solid substrates by a model liquid crystal}, volume={135}, ISSN={["0021-9606"]}, DOI={10.1063/1.3660377}, abstractNote={We present grand canonical ensemble Monte Carlo simulations of prewetting transitions in a model liquid crystal at structureless solid substrates. Molecules of the liquid crystal interact via anisometric Lennard-Jones potentials and can be anchored planar or homeotropically at the substrates. Fluid-substrate attraction is modeled by a Yukawa potential of variable range. By monitoring the grand-potential density and the nematic order parameter as functions of the chemical potential μ, several discontinuous prewetting, wetting, and isotropic-nematic phase transitions are observed. These transitions depend on both the range of the fluid-substrate attraction and the specific anchoring at the substrate. Our results show that at substrates characterized by degenerate anchoring prewetting occurs at lower μ compared with cases in which the anchoring is monostable. This indicates that prewetting transitions are driven by orientational entropy because degenerate anchoring allows for more orientationally distinct configurations of molecules compared with monostable anchoring. In addition, by analyzing local density and various local order parameters, a detailed picture of the structure of various phases emerges from our simulations.}, number={20}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Greschek, Manuel and Schoen, Martin}, year={2011}, month={Nov} }
 @article{mazza_greschek_valiullin_schoen_2011, title={Role of stringlike, supramolecular assemblies in reentrant supernematic liquid crystals}, volume={83}, ISSN={["1550-2376"]}, DOI={10.1103/physreve.83.051704}, abstractNote={Using a combination of isothermal-isobaric Monte Carlo and microcanonical molecular dynamics we investigate the relation between structure and self-diffusion in various phases of a model liquid crystal using the Gay-Berne-Kihara potential. These molecules are confined to a mesoscopic slit pore with atomically smooth substrate surfaces. As reported recently [seeM. G. Mazza et al., Phys. Rev. Lett. 105, 227802 (2010)], a reentrant nematic (RN) phase may form at sufficiently high pressures and densities. This phase is characterized by a high degree of nematic order and a substantially enhanced self-diffusivity in the direction of the director n that exceeds that of the lower-density nematic and an intermittent smectic-A phase by about an order of magnitude. Here we demonstrate that the unique transport behavior in the RN phase may be linked to a confinement-induced packing effect that causes the formation of supramolecular, stringlike conformations. The strings consist of several molecules traveling in the direction of n as individual "trains" consisting of chains of molecular "cars."}, number={5}, journal={PHYSICAL REVIEW E}, author={Mazza, Marco G. and Greschek, Manuel and Valiullin, Rustem and Schoen, Martin}, year={2011}, month={May} }
 @misc{mazza_schoen_2011, title={Structure and Dynamics of Reentrant Nematics: Any Open Questions after Almost 40 Years?}, volume={12}, ISSN={["1422-0067"]}, DOI={10.3390/ijms12085352}, abstractNote={Liquid crystals have attracted enormous interest because of the variety of their phases and richness of their application. The interplay of general physical symmetries and specific molecular features generates a myriad of different phenomena. A surprising behavior of liquid crystals is the reentrancy of phases as temperature, pressure, or concentration are varied. Here, we review the main experimental facts and the different theoretical scenarios that have guided the understanding of bulk reentrant nematics. Recently, some computer simulations of a system confined to nanoscopic scales have found new dynamical features of the reentrant nematic phase. We discuss this prediction in relation with the available experimental evidence on reentrant nematics and with the dynamics of liquids in strongly confined environments.}, number={8}, journal={INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES}, author={Mazza, Marco G. and Schoen, Martin}, year={2011}, month={Aug}, pages={5352–5372} }
 @article{mazza_greschek_valiullin_kaerger_schoen_2010, title={Entropy-Driven Enhanced Self-Diffusion in Confined Reentrant Supernematics}, volume={105}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.105.227802}, abstractNote={We present a molecular dynamics study of reentrant nematic phases using the Gay-Berne-Kihara model of a liquid crystal in nanoconfinement. At densities above those characteristic of smectic A phases, reentrant nematic phases form that are characterized by a large value of the nematic order parameter S≃1. Along the nematic director these "supernematic" phases exhibit a remarkably high self-diffusivity, which exceeds that for ordinary, lower-density nematic phases by an order of magnitude. Enhancement of self-diffusivity is attributed to a decrease of rotational configurational entropy in confinement. Recent developments in the pulsed field gradient NMR technique are shown to provide favorable conditions for an experimental confirmation of our simulations.}, number={22}, journal={PHYSICAL REVIEW LETTERS}, author={Mazza, Marco G. and Greschek, Manuel and Valiullin, Rustem and Kaerger, Joerg and Schoen, Martin}, year={2010}, month={Nov} }
 @article{greschek_schoen_2010, title={Frustration of nanoconfined liquid crystals due to hybrid substrate anchoring}, volume={6}, ISSN={["1744-6848"]}, DOI={10.1039/c0sm00197j}, abstractNote={We present Monte Carlo simulations of liquid-crystalline material confined to a nanoscopic slit-pore. The simulations are carried out under isothermal conditions in a specialized isostress ensemble in which N fluid molecules are exposed to a compressional stress τ‖ acting on the fluid in directions parallel with the substrate planes. Fluid-fluid and fluid-substrate interactions are modelled as in our previous work (M. Greschek et al., Soft Matter, 2010, 6, 1898). We study several anchoring mechanisms at the solid substrate by introducing an anchoring function 0 ≤ g(u) ≤ 1 that depends on the orientation u of a fluid molecule relative to the substrate plane; g(u) “switches” the fluid-substrate attraction on or off. Here we focus on various heterogeneous (i.e., hybrid) anchoring scenarios imposing different anchoring functions at the opposite substrates. As in our previous study we compute the isostress heat capacity which allows us to identify states at which the confined fluid undergoes a structural transformation. The isotropic-nematic transformation turns out to be nearly independent of the specific anchoring scenario. This is because the director in the nematic phase assumes a direction that is a compromise between the directions enforced by the competing anchoring scenarios at either substrate. On the contrary, at lower compressional stresses molecules prealign near the slit-pore's midplane in specific directions that depend on details of the anchoring scenario.}, number={19}, journal={SOFT MATTER}, author={Greschek, Manuel and Schoen, Martin}, year={2010}, pages={4931–4941} }
 @article{greschek_melle_schoen_2010, title={Isotropic-nematic phase transitions in confined mesogenic fluids. The role of substrate anchoring}, volume={6}, ISSN={["1744-683X"]}, DOI={10.1039/b924417d}, abstractNote={The anchoring of rodlike liquid-crystalline molecules at solid surfaces plays an important role in the design of novel nanotechnological devices such as, for example, biosensors. In this work we investigate the impact of various anchoring scenarios on the isotropic–nematic phase transition of weakly anisometric, prolate mesogens confined to a mesoscopic slit-pore. We employ isostress ensemble Monte Carlo simulations in which the isotropic–nematic transition is driven by applying an external stress in the direction parallel to the substrate plane. The fluid–fluid interaction is described by a Lennard-Jones potential with an orientation-dependent attractive term. Our simulations show that different anchoring scenarios may shift the isotropic–nematic phase transition considerably. We locate this transition through response functions such as an isostress heat capacity, isothermal compressibility, and the Maier–Saupe order parameter. Our results suggest that for the present model system the isotropic–nematic transition is likely to be continuous. For the planar anchoring scenario we observe the formation of a new layer of fluid molecules during a structural transformation preceding the isotropic–nematic transition.}, number={9}, journal={SOFT MATTER}, author={Greschek, Manuel and Melle, Michael and Schoen, Martin}, year={2010}, pages={1898–1909} }
 @article{schoen_guenther_2010, title={Phase transitions in nanoconfined fluids: Synergistic coupling between soft and hard matter}, volume={6}, ISSN={["1744-6848"]}, DOI={10.1039/c0sm00440e}, abstractNote={We report recent theoretical developments in the study of gas-liquid transitions of fluids confined to deformable mesoporous materials. Due to a synergistic coupling the phase behavior of the confined soft matter phase is significantly affected by the deformation of the confining material which in turn is deformed as a result of phase changes occurring in the confined phase. If the confined fluid is gas-like its wetting characteristics affect the strain isotherm such that the pore may expand or contract as more gas is adsorbed prior to capillary condensation. Directly at capillary condensation the pore abruptly shrinks on account of fluid-substrate attraction. If the density of the confined liquid-like phase is then enhanced further the pore expands again. This expansion allows one to determine nanomechanical properties of the confining solid directly from the deformation isotherm. In the future it might be possible to fabricate sensors that allow one to measure mechanically changes in thermodynamic properties of confined soft matter phases.}, number={23}, journal={SOFT MATTER}, author={Schoen, Martin and Guenther, Gerrit}, year={2010}, pages={5832–5838} }
 @article{schoen_paris_guenther_mueter_prass_fratzl_2010, title={Pore-lattice deformations in ordered mesoporous matrices: experimental studies and theoretical analysis}, volume={12}, ISSN={["1463-9084"]}, DOI={10.1039/c000782j}, abstractNote={The sorption of fluids in mesoporous silica is an important physical phenomenon with a wide range of applications. Traditionally, mesoporous materials have been considered as inert scaffolds for the sorption and condensation reaction of the fluid. Here we present in situ small angle X-ray diffraction experiments providing evidence for a sorption strain induced in the solid that manifests itself as a change in the lattice parameter of the ordered mesopore array as the pores gradually adsorb fluid material. The experimental data are analyzed by means of Monte Carlo simulations carried out in a grand canonical ensemble describing a fluid confined by deformable substrates. We show that-in agreement with experimental data-sorption of a nonpolar fluid causes the pores to expand initially, to shrink abruptly when capillary condensation sets in, and to expand again as more liquid-like fluid is adsorbed subsequently. We show that the pore pressure can be extracted from a thermodynamic analysis of sorption isotherms in the liquid-like regime and that this information can be used for an estimation of the Young's modulus of the porous silica material. In addition, our Monte Carlo simulations indicate that the phase behavior of confined fluids is considerably changed by the deformability of the confining solid. This is reflected by a change of the location of phase boundaries at sufficiently subcritical temperatures.}, number={37}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Schoen, Martin and Paris, Oskar and Guenther, Gerrit and Mueter, Dirk and Prass, Johannes and Fratzl, Peter}, year={2010}, pages={11267–11279} }
 @article{guenther_schoen_2009, title={Sorption strain as a packing phenomenon}, volume={11}, ISSN={["1463-9084"]}, DOI={10.1039/b903514a}, abstractNote={We employ Monte Carlo simulations in a semi-grand canonical ensemble to analyze the relation between sorption strains and the thermodynamic state of a confined fluid composed of "simple" fluid molecules that possess only translational degrees of freedom. Fluid molecules are confined to a slit-pore whose walls are composed of individual atoms distributed across the plane of each substrate according to the (100) structure of the face-centered cubic lattice. The substrates can be deformed to a certain extent on account of their own thermal energy and due to the interaction with the fluid molecules. We determine the phase diagram in both the bulk and in confinement for both rigid and deformable solid substrates. By using finite-size scaling concepts the location of the critical point is determined accurately. Our results indicate for the first time that the previously observed variation of sorption strains with the amount of adsorbed fluid material [G. Günther et al., Phys. Rev. Lett., 2008, 101, 086104] is caused by packing effects (i.e. stratification of the confined fluid) but is largely independent of the precise nature of the thermodynamic state considered.}, number={40}, journal={PHYSICAL CHEMISTRY CHEMICAL PHYSICS}, author={Guenther, Gerrit and Schoen, Martin}, year={2009}, pages={9082–9092} }