Publications by authors named "Roland Roth"

64 Publications

Dynamical Density Functional Theory for the Drying and Stratification of Binary Colloidal Dispersions.

Langmuir 2021 Feb 20;37(4):1399-1409. Epub 2021 Jan 20.

Department of Mathematical Sciences, Loughborough University, Loughborough LE11 3TU, United Kingdom.

We develop a dynamical density functional theory based model for the drying of colloidal films on planar surfaces. We consider mixtures of two different sizes of hard-sphere colloids. Depending on the solvent evaporation rate and the initial concentrations of the two species, we observe varying degrees of stratification in the final dried films. Our model predicts the various structures described in the literature previously from experiments and computer simulations, in particular the small-on-top stratified films. Our model also includes the influence of adsorption of particles to the interfaces.
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http://dx.doi.org/10.1021/acs.langmuir.0c02825DOI Listing
February 2021

Toward a density-functional theory for the Jagla fluid.

Phys Rev E 2020 Dec;102(6-1):062112

Institut für Theoretische Physik, Universität Tübingen, D-72076 Tübingen, Germany.

The so-called Jagla fluid is well known to exhibit, in addition to the usual gas-liquid critical point, also a liquid-liquid critical point, as well as a density anomaly. This makes it an interesting toy model for water, for which a liquid-liquid critical point is considered to exist but so far eludes experimental verification due to crystallization occurring in the corresponding metastable, deeply supercooled state. With the Jagla fluid being understood quite well in bulk-mostly via simulation studies-the focus of the present study is to describe the spatially inhomogeneous fluid in terms of classical density-functional theory (DFT) with the aim to be able to control its phase behavior on changing the shape or the nature of the confinement of the fluid. This information might contribute to guide potential experimental tests of the liquid-liquid critical point of actual water. We first determine the bulk phase diagram for the Jagla fluid by using thermodynamical perturbation theory. In doing so we explain why the perturbation theories of Barker and Henderson as well as of Weeks, Chandler, and Anderson fail to describe the Jagla fluid. We then continue to construct a perturbative DFT based on our bulk model, which shows significant improvement over the standard mean-field DFT valid at high temperatures. But ultimately the perturbative DFT breaks down at state points close to the binodal line and at low temperatures. This prevents us from achieving the original aim to study a highly confined, inhomogeneous Jagla fluid close to its liquid-liquid binodal.
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http://dx.doi.org/10.1103/PhysRevE.102.062112DOI Listing
December 2020

Statics and dynamics of a finite two-dimensional colloidal system with competing attractive critical Casimir and repulsive magnetic dipole interactions.

Phys Rev E 2020 Oct;102(4-1):042608

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

We continue our theoretical study of a recently proposed two-dimensional colloidal system with attractive critical Casimir and repulsive magnetic dipole forces that can be tuned easily and independently from each other via the temperature and the strength of an external magnetic field, respectively [K. Marolt, M. Zimmermann, and R. Roth, Phys. Rev. E 100, 052602 (2019)2470-004510.1103/PhysRevE.100.052602]. Using this freedom, it is possible to construct a competing interaction potential that causes microphase separation featuring spatially inhomogeneous cluster, stripe, and bubble phases in the bulk, i.e., in an infinite system without an external potential. In the present work, we demonstrate by means of density functional theory that microphase separation can also occur in finite geometries. In a square cell with a side length of 20 or 30 colloid diameters, we observe the emergence of highly structured cluster and ring phases at intermediate bulk densities in addition to almost uniform fluid phases for lower and higher bulk densities. We then employ dynamic density functional theory to determine how the system reacts when the temperature and the magnetic field are altered over time, and we show how to induce a transition from the liquid to the cluster/ring phase and also from the cluster directly to the ring phase. We find that often a slowly varying and nontrivial path in parameter space is required to reach a stable state, whereas abrupt changes are prone to lead to metastable configurations.
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http://dx.doi.org/10.1103/PhysRevE.102.042608DOI Listing
October 2020

Low-intensity blood flow restriction calf muscle training leads to similar functional and structural adaptations than conventional low-load strength training: A randomized controlled trial.

PLoS One 2020 30;15(6):e0235377. Epub 2020 Jun 30.

Institute of Sports Sciences, Goethe-Universität Frankfurt am Main, Frankfurt am Main, Germany.

The purpose of this study was to investigate whether a six-week, twice weekly resistance training (4 sets at 30% 1-RM until failure) with practical blood flow restriction (BFR) using 7cm wide cuffs with a twist lock placed below the patella is superior to training without BFR (NoBFR) concerning muscle mass and strength gains in calf muscles. A two-group (BFR n = 12, mean age 27.33 (7.0) years, training experience 7.3 (7.0) years; NoBFR n = 9, mean age 28.9 (7.4) years, training experience 7.1 (6.6) years) randomized matched pair design based on initial 1-RM was used to assess the effects on structural and functional adaptations in healthy males (Perometer calf volume [CV], gastrocnemius muscle thickness using ultrasound [MT], 7-maximal hopping test for leg stiffness [LS], 1-RM smith machine calf raise [1-RM], and visual analogue scale as a measure of pain intensity [VAS]). The mean number of repetitions completed per training session across the intervention period was higher in the NoBFR group compared to the BFR group (70 (16) vs. 52 (9), p = 0.002). VAS measured during the first session increased similarly in both groups from first to fourth set (p<0.001). No group effects or time×group interactions were found for CV, MT, LS, and 1-RM. However, there were significant time effects for MT (BFR +0.07 cm; NoBFR +0.04; p = 0.008), and 1-RM (BFR +40 kg; NoBFR +34 kg; p<0.001). LS and CV remained unchanged through training. VAS in both groups were similar, and BFR and NoBFR were equally effective for increasing 1-RM and MT in trained males. However, BFR was more time efficient, due to lesser repetition per training session.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0235377PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7326162PMC
September 2020

Enhanced protein adsorption upon bulk phase separation.

Sci Rep 2020 06 25;10(1):10349. Epub 2020 Jun 25.

Institute for Applied Physics, Auf der Morgenstelle 10, University of Tübingen, 72076, Tübingen, Germany.

In all areas related to protein adsorption, from medicine to biotechnology to heterogeneous nucleation, the question about its dominant forces and control arises. In this study, we used ellipsometry and quartz-crystal microbalance with dissipation (QCM-D), as well as density-functional theory (DFT) to obtain insight into the mechanism behind a wetting transition of a protein solution. We established that using multivalent ions in a net negatively charged globular protein solution (BSA) can either cause simple adsorption on a negatively charged interface, or a (diverging) wetting layer when approaching liquid-liquid phase separation (LLPS) by changing protein concentration (c) or temperature (T). We observed that the water to protein ratio in the wetting layer is substantially larger compared to simple adsorption. In the corresponding theoretical model, we treated the proteins as limited-valence (patchy) particles and identified a wetting transition for this complex system. This wetting is driven by a bulk instability introduced by metastable LLPS exposed to an ion-activated attractive substrate.
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http://dx.doi.org/10.1038/s41598-020-66562-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7316800PMC
June 2020

Remnants of the disappearing critical point in chain-forming patchy fluids.

J Chem Phys 2020 Mar;152(11):111101

H. H. Wills Physics Laboratory, University of Bristol, Royal Fort, Bristol BS8 ITL, United Kingdom.

For a standard model of patchy colloidal fluids with patch number M = 2, where chain formation (polymerization) occurs, we show that Wertheim theory predicts critical behavior at vanishing density and temperature. The analysis is based on determining lines in the phase diagram of maximal correlation length and compressibility. Simulation studies identify the latter line and confirm our prediction of Fisher-Widom crossover, i.e., the asymptotic decay of the pair correlation function changes from monotonic to damped oscillatory as the density is increased. For M > 2, it is known that phase separation occurs with a true critical point. Our results support the notion that a "disappearing" critical point occurs in the limit M = 2 and we uncover its remnants.
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http://dx.doi.org/10.1063/1.5141059DOI Listing
March 2020

Many-body correlations from integral geometry.

Phys Rev E 2019 Dec;100(6-1):062126

H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom.

In a recent letter we presented a framework for predicting the concentrations of many-particle local structures inside the bulk liquid as a route to assessing changes in the liquid approaching dynamical arrest. Central to this framework was the morphometric approach, a synthesis of integral geometry and liquid-state theory, which has traditionally been derived from fundamental measure theory. We present the morphometric approach in a new context as a generalization of scaled-particle theory, and we derive several morphometric theories for hard spheres of fundamental and practical interest. Our central result is a new theory that is particularly suited to the treatment of many-body correlation functions in the hard-sphere liquid, which we demonstrate by numerical tests against simulation.
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http://dx.doi.org/10.1103/PhysRevE.100.062126DOI Listing
December 2019

Microphase separation in a two-dimensional colloidal system with competing attractive critical Casimir and repulsive magnetic dipole interactions.

Phys Rev E 2019 Nov;100(5-1):052602

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

We propose and study theoretically a colloidal system in two dimensions with attractive critical Casimir and repulsive magnetic dipole forces, wherein the strength of attraction and repulsion can be easily and independently tuned by adjusting the temperature and an external magnetic field, respectively. We expect this setup to be experimentally accessible and are confident that it can serve to deepen our understanding of the mechanisms behind microphase separation due to competing interactions. We develop a density functional theory for our model and present first results of our calculations in the form of a phase diagram for fixed temperature, but varying magnetic fields and bulk densities. For certain values of these parameters, we are able to confirm the existence of thermodynamically stable inhomogeneous density profiles in the bulk, such as parallel lamellar stripes, as well as clusters and voids on a hexagonal lattice.
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http://dx.doi.org/10.1103/PhysRevE.100.052602DOI Listing
November 2019

On the decay of the pair correlation function and the line of vanishing excess isothermal compressibility in simple fluids.

J Chem Phys 2019 Jul;151(1):014501

H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1 TL, United Kingdom.

We revisit the competition between attractive and repulsive interparticle forces in simple fluids and how this governs and connects the macroscopic phase behavior and structural properties, as manifested in pair correlation functions. We focus on the asymptotic decay of the total correlation function h(r) which is, in turn, controlled by the form of the pair direct correlation function c(r). The decay of rh(r) to zero can be exponential (monotonic) if attraction dominates repulsion and exponentially damped oscillatory otherwise. The Fisher-Widom (FW) line separates the phase diagram into two regions characterized by the two different types of asymptotic decays. We show that there is a new and physically intuitive thermodynamic criterion which approximates well the actual FW line. This new criterion defines a line where the isothermal compressibility takes its ideal gas value χ=χ. We test our hypothesis by considering four commonly used models for simple fluids. In all cases, the new criterion yields a line in the phase diagram that is close to the actual FW line for the thermodynamic state points that are most relevant. We also investigate (Widom) lines of maximal correlation length, emphasizing the importance of distinguishing between the true and Ornstein-Zernike correlation lengths.
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http://dx.doi.org/10.1063/1.5110044DOI Listing
July 2019

An updated algorithm recommendation for the management of knee osteoarthritis from the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO).

Semin Arthritis Rheum 2019 12 30;49(3):337-350. Epub 2019 Apr 30.

Division of Public Health, Epidemiology and Health Economics, University of Liège, CHU Sart Tilman, 4000, Liège, Belgium; WHO Collaborating Centre for Public Health Aspects of Musculoskeletal Health and Aging, Liège, Belgium; Chair for Biomarkers of Chronic Diseases, Biochemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia.

Objectives: The European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases (ESCEO) sought to revisit the 2014 algorithm recommendations for knee osteoarthritis (OA), in light of recent efficacy and safety evidence, in order to develop an updated stepwise algorithm that provides practical guidance for the prescribing physician that is applicable in Europe and internationally.

Methods: Using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) process, a summary of evidence document for each intervention in OA was provided to all members of an ESCEO working group, who were required to evaluate and vote on the strength of recommendation for each intervention. Based on the evidence collected, and on the strength of recommendations afforded by consensus of the working group, the final algorithm was constructed.

Results: An algorithm for management of knee OA comprising a stepwise approach and incorporating consensus on 15 treatment recommendations was prepared by the ESCEO working group. Both "strong" and "weak" recommendations were afforded to different interventions. The algorithm highlights the continued importance of non-pharmacological interventions throughout the management of OA. Benefits and limitations of different pharmacological treatments are explored in this article, with particular emphasis on safety issues highlighted by recent literature analyses.

Conclusions: The updated ESCEO stepwise algorithm, developed by consensus from clinical experts in OA and informed by available evidence for the benefits and harms of various treatments, provides practical, current guidance that will enable clinicians to deliver patient-centric care in OA practice.
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http://dx.doi.org/10.1016/j.semarthrit.2019.04.008DOI Listing
December 2019

Safety of Oral Non-Selective Non-Steroidal Anti-Inflammatory Drugs in Osteoarthritis: What Does the Literature Say?

Drugs Aging 2019 04;36(Suppl 1):15-24

WHO Collaborating Centre for Public Heath Aspects of Musculoskeletal Health and Aging, Liège, Belgium.

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely recommended and prescribed to treat pain in osteoarthritis. While measured to have a moderate effect on pain in osteoarthritis, NSAIDs have been associated with wide-ranging adverse events affecting the gastrointestinal, cardiovascular, and renal systems. Gastrointestinal toxicity is found with all NSAIDs, which may be of particular concern when treating older patients with osteoarthritis, and gastric adverse events may be reduced by taking a concomitant gastroprotective agent, although intestinal adverse events are not ameliorated. Cardiovascular toxicity is associated with all NSAIDs to some extent and the degree of risk appears to be pharmacotherapy specific. An increased risk of acute myocardial infarction and heart failure is observed with all NSAIDs, while an elevated risk of hemorrhagic stroke appears to be restricted to the use of diclofenac and meloxicam. All NSAIDs have the potential to induce acute kidney injury, and patients with osteoarthritis with co-morbid conditions including hypertension, heart failure, and diabetes mellitus are at increased risk. Osteoarthritis is associated with excess mortality, which may be explained by reduced levels of physical activity owing to lower limb pain, presence of comorbid conditions, and the adverse effects of anti-osteoarthritis medications especially NSAIDs. This narrative review of recent literature identifies data on the safety of non-selective NSAIDs to better understand the risk:benefit of using NSAIDs to manage pain in osteoarthritis.
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http://dx.doi.org/10.1007/s40266-019-00660-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6509083PMC
April 2019

Morphometric Approach to Many-Body Correlations in Hard Spheres.

Phys Rev Lett 2019 Feb;122(6):068004

H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, United Kingdom.

We model the thermodynamics of local structures within the hard sphere liquid at arbitrary volume fractions through the morphometric calculation of n-body correlations. We calculate absolute free energies of local geometric motifs in excellent quantitative agreement with molecular dynamics simulations across the liquid and supercooled liquid regimes. We find a bimodality in the density library of states where fivefold symmetric structures appear lower in free energy than fourfold symmetric structures and from a single reaction path predict a dynamical barrier which scales linearly in the compressibility factor. The method provides a new route to assess changes in the free energy landscape at volume fractions dynamically inaccessible to conventional techniques.
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http://dx.doi.org/10.1103/PhysRevLett.122.068004DOI Listing
February 2019

Type 2 diabetes mellitus and osteoarthritis.

Semin Arthritis Rheum 2019 08 11;49(1):9-19. Epub 2019 Jan 11.

Division of Diabetes, Nutrition and Metabolic Disorders and Clinical Pharmacology Unit, Department of Medicine, University of Liège, CHU Liège, Sart Tilman B35, B-4000 Liège, Belgium.

Objectives: Type 2 diabetes mellitus (T2DM) and osteoarthritis (OA) are common diseases that frequently co-exist, along with overweight/obesity. While the mechanical impact of excess body weight on joints may explain lower limb OA, we sought to explore whether T2DM is linked to OA outside of excess weight and whether T2DM may play a role in OA pathophysiology. The consequence of T2DM on OA outcomes is a question of research interest.

Methods: We conducted a critical review of the literature to explore the association between T2DM and OA, whether any association is site-specific for OA, and whether the presence of T2DM impacts on OA outcomes. We also reviewed the literature to assess the safety of anti-OA treatments in patients with T2DM.

Results: T2DM has a pathogenic effect on OA through 2 major pathways involving oxidative stress and low-grade chronic inflammation resulting from chronic hyperglycemia and insulin resistance. T2DM is a risk factor for OA progression and has a negative impact on arthroplasty outcomes. Evidence is mounting for safety concerns with some of the most frequently prescribed anti-OA medications, including paracetamol, non-steroidal anti-inflammatory drugs, and corticosteroid injections, while other anti-OA medications may be safely prescribed in OA patients with T2DM, such as glucosamine and intra-articular hyaluronic acid.

Conclusions: Future research is needed to better understand whether diabetes control and prevention can modulate OA occurrence and progression. The selection of therapy to treat OA symptoms in patients with T2DM may require careful consideration of the evidence based to avoid untoward safety issues.
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http://dx.doi.org/10.1016/j.semarthrit.2019.01.005DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6642878PMC
August 2019

Bulk structural information from density functionals for patchy particles.

J Chem Phys 2018 Dec;149(22):224503

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

We investigate bulk structural properties of tetravalent associating particles within the framework of classical density functional theory, building upon Wertheim's thermodynamic perturbation theory. To this end, we calculate density profiles within an effective test-particle geometry and compare to radial distribution functions obtained from computer simulations. We demonstrate that a modified version of the functional proposed by Yu and Wu [J. Chem. Phys. , 7094 (2002)] based on fundamental measure theory for hard spheres produces accurate results, although the functional does not satisfy the exactly known low-density limit. In addition, at low temperatures where particles start to form an amorphous tetrahedral network, quantitative differences between simulations and theory emerge due to the absence of geometrical information regarding the patch arrangement in the latter. Indeed, here we find that the theory fits better to simulations of the floating-bond model [E. Zaccarelli , J. Chem. Phys. , 174501 (2007)], which exhibits a weaker tetrahedral order due to more flexible bonds between particles. We also demonstrate that another common density functional approach by Segura [Mol. Phys. , 759 (1997)] fails to capture fundamental structural properties.
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http://dx.doi.org/10.1063/1.5064780DOI Listing
December 2018

Reentrant Phase Behavior in Protein Solutions Induced by Multivalent Salts: Strong Effect of Anions Cl Versus NO

J Phys Chem B 2018 12 30;122(50):11978-11985. Epub 2018 Nov 30.

Institut für Angewandte Physik , Universität Tübingen , Auf der Morgenstelle 10 , 72076 Tübingen , Germany.

In this work, the effects of the two anions Cl and NO on the phase behavior of bovine serum albumin (BSA) in solution with trivalent salts are compared systematically. In the presence of trivalent metal salts, negatively charged proteins such as BSA in solution undergo a reentrant condensation (RC) phase behavior, which has been established for several proteins with chlorides of trivalent salts. Here, we show that replacing Cl by NO leads to a marked change in the phase behavior. The effect is investigated for the two different cations Y and La. The salts are thus YCl, Y(NO), LaCl, and La(NO). The experimental phase behavior shows that while the chloride salts induce both liquid-liquid phase separation (LLPS) and RC, the nitrate salts also induce LLPS, but RC becomes partial with La(NO) and disappears with Y(NO). The observed phase behavior is rationalized by effective protein-protein interactions which are characterized using small-angle X-ray scattering. The results based on the reduced second virial coefficients B/ B and 1/ I( q → 0) demonstrate that the NO salts induce a stronger attraction than the Cl salts. Overall, the effective attraction, the width of the condensed regime in the RC phase diagram, and the nature of LLPS follow the order LaCl < YCl < La(NO) < Y(NO). Despite the decisive role of cations in RC phase behavior, isothermal titration calorimetry measurements indicate that replacing anions does not significantly influence the cation binding to proteins. The experimental results observed are discussed based on an "enhanced Hofmeister effect" including electrostatic and hydrophobic interactions between protein-cation complexes.
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http://dx.doi.org/10.1021/acs.jpcb.8b10268DOI Listing
December 2018

Thermodynamics of the hard-disk fluid at a planar hard wall: Generalized scaled-particle theory and Monte Carlo simulation.

J Chem Phys 2018 Aug;149(8):084701

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

A generalized scaled-particle theory for the uniform hard-disk mixture is derived in the spirit of the White Bear II free energy of the hard-sphere fluid [H. Hansen-Goos and R. Roth, J. Phys. C: Condens. Matter , 8413 (2006)]. The theory provides a very simple result for the interfacial free energy of the hard-disk fluid at a planar hard wall (which in = 2 is a line) in terms of the equation of state. To complement and assess the theory, we perform Monte Carlo simulations from which we obtain using Gibbs-Cahn integration. While we find excellent overall agreement between theory and simulation, it also becomes apparent that the set of scaled-particle variables available in = 2 is too limited, prohibiting a quasi-exact result for . Furthermore, this is reflected in the mixture equation of state resulting from our theory, which, similar to a previous attempt by Santos [Mol. Phys. , 1 (1999)], displays a small but systematic deviation from simulations.
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http://dx.doi.org/10.1063/1.5043185DOI Listing
August 2018

Microscopic density functional theory for monolayers of diblock copolymers.

J Chem Phys 2018 Aug;149(6):064902

Department for the Modeling of Physico-Chemical Processes, Maria Curie-Skłodowska University, 20-031 Lublin, Poland.

We propose density functional theory for diblock copolymers in two dimensions. Our theoretical framework is based on Wertheim's first order thermodynamic perturbation theory. Using the proposed approach, we investigate the structure and phase behavior of monolayers of symmetric diblock copolymers. We find that the phase behavior of symmetric diblock copolymer monolayers is similar to that in 3D. This includes the scaling of the equilibrium lamellar width with chain length. We find that the topology of the resulting phase diagrams depends on the chain length and the unlike segment interaction incompatibility and involves either one, two, or three triple points (one of them being the peritectic point). We expect that a similar phase behavior could be obtained for monolayers of colloidal suspensions with carefully tuned interparticle interactions.
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http://dx.doi.org/10.1063/1.5039522DOI Listing
August 2018

Mean-field density functional theory of a nanoconfined classical, three-dimensional Heisenberg fluid. II. The interplay between molecular packing and orientational order.

J Chem Phys 2018 Aug;149(5):054704

Stranski-Laboratorium für Physikalische und Theoretische Chemie, Fakultät für Mathematik und Naturwissenschaften, Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany.

As in Paper I of this series of papers [S. M. Cattes et al., J. Chem. Phys. 144, 194704 (2016)], we study a Heisenberg fluid confined to a nanoscopic slit pore with smooth walls. The pore walls can either energetically discriminate specific orientations of the molecules next to them or are indifferent to molecular orientations. Unlike in Paper I, we employ a version of classical density functional theory that allows us to explicitly account for the stratification of the fluid (i.e., the formation of molecular layers) as a consequence of the symmetry-breaking presence of the pore walls. We treat this stratification within the White Bear version (Mark I) of fundamental measure theory. Thus, in this work, we focus on the interplay between local packing of the molecules and orientational features. In particular, we demonstrate why a critical end point can only exist if the pore walls are not energetically discriminating specific molecular orientations. We analyze in detail the positional and orientational order of the confined fluid and show that reorienting molecules across the pore space can be a two-dimensional process. Last but not least, we propose an algorithm based upon a series expansion of Bessel functions of the first kind with which we can solve certain types of integrals in a very efficient manner.
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http://dx.doi.org/10.1063/1.5040934DOI Listing
August 2018

Nonequilibrium phase transitions of sheared colloidal microphases: Results from dynamical density functional theory.

Phys Rev E 2018 Jun;97(6-1):062602

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

By means of classical density functional theory and its dynamical extension, we consider a colloidal fluid with spherically symmetric competing interactions, which are well known to exhibit a rich bulk phase behavior. This includes complex three-dimensional periodically ordered cluster phases such as lamellae, two-dimensional hexagonally packed cylinders, gyroid structures, or spherical micelles. While the bulk phase behavior has been studied extensively in earlier work, in this paper we focus on such structures confined between planar repulsive walls under shear flow. For sufficiently high shear rates, we observe that microphase separation can become fully suppressed. For lower shear rates, however, we find that, e.g., the gyroid structure undergoes a kinetic phase transition to a hexagonally packed cylindrical phase, which is found experimentally and theoretically in amphiphilic block copolymer systems. As such, besides the known similarities between the latter and colloidal systems regarding the equilibrium phase behavior, our work reveals further intriguing nonequilibrium relations between copolymer melts and colloidal fluids with competing interactions.
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http://dx.doi.org/10.1103/PhysRevE.97.062602DOI Listing
June 2018

Bulk dynamics of Brownian hard disks: Dynamical density functional theory versus experiments on two-dimensional colloidal hard spheres.

J Chem Phys 2018 Mar;148(10):104501

Institute for Theoretical Physics, University of Tübingen, 72076 Tübingen, Germany.

Using dynamical density functional theory (DDFT), we theoretically study Brownian self-diffusion and structural relaxation of hard disks and compare to experimental results on quasi two-dimensional colloidal hard spheres. To this end, we calculate the self-van Hove correlation function and distinct van Hove correlation function by extending a recently proposed DDFT-approach for three-dimensional systems to two dimensions. We find that the theoretical results for both self-part and distinct part of the van Hove function are in very good quantitative agreement with the experiments up to relatively high fluid packing fractions of roughly 0.60. However, at even higher densities, deviations between the experiment and the theoretical approach become clearly visible. Upon increasing packing fraction, in experiments, the short-time self-diffusive behavior is strongly affected by hydrodynamic effects and leads to a significant decrease in the respective mean-squared displacement. By contrast, and in accordance with previous simulation studies, the present DDFT, which neglects hydrodynamic effects, shows no dependence on the particle density for this quantity.
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http://dx.doi.org/10.1063/1.5019447DOI Listing
March 2018

Phase behavior and bulk structural properties of a microphase former with anisotropic competing interactions: A density functional theory study.

Phys Rev E 2017 Oct 20;96(4-1):042607. Epub 2017 Oct 20.

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

Using classical density functional theory, we investigate systems exhibiting interactions where a short-range anisotropic attractive force competes with a long-range spherically symmetric repulsive force. The former is modelled within Wertheim's first-order perturbation theory for patchy particles, and the repulsive part is assumed to be a Yukawa potential which is taken into account via a mean-field approximation. From previous studies of systems with spherically symmetric competing interactions, it is well known that such systems can exhibit stable bulk cluster phases (microphase separation) provided that the repulsion is sufficiently weak compared to the attraction. For the present model system, we find rich phase diagrams including both reentrant clustering and liquid-gas binodals. In particular, the model predicts inhomogeneous bulk phases at extremely low packing fractions, which cannot be observed in systems with isotropic competing interactions.
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http://dx.doi.org/10.1103/PhysRevE.96.042607DOI Listing
October 2017

Multivalent-Ion-Activated Protein Adsorption Reflecting Bulk Reentrant Behavior.

Phys Rev Lett 2017 Dec 1;119(22):228001. Epub 2017 Dec 1.

Institute for Applied Physics, University of Tübingen, 72076 Tübingen, Germany.

Protein adsorption at the solid-liquid interface is an important phenomenon that often can be observed as a first step in biological processes. Despite its inherent importance, still relatively little is known about the underlying microscopic mechanisms. Here, using multivalent ions, we demonstrate the control of the interactions and the corresponding adsorption of net-negatively charged proteins (bovine serum albumin) at a solid-liquid interface. This is demonstrated by ellipsometry and corroborated by neutron reflectivity and quartz-crystal microbalance experiments. We show that the reentrant condensation observed within the rich bulk phase behavior of the system featuring a nonmonotonic dependence of the second virial coefficient on salt concentration c_{s} is reflected in an intriguing way in the protein adsorption d(c_{s}) at the interface. Our findings are successfully described and understood by a model of ion-activated patchy interactions within the framework of the classical density functional theory. In addition to the general challenge of connecting bulk and interface behavior, our work has implications for, inter alia, nucleation at interfaces.
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http://dx.doi.org/10.1103/PhysRevLett.119.228001DOI Listing
December 2017

Solvent-Mediated Interactions Close to the Fisher-Widom Line.

Authors:
Roland Roth

J Phys Chem B 2018 04 4;122(13):3556-3561. Epub 2018 Jan 4.

Institut für Theoretische Physik , Auf der Morgenstelle 14 , 72076 Tübingen , Germany.

The effective interactions between two hard spheres mediated by a square-well fluid is studied within the framework of classical density functional theory. The main focus of this study is the behavior of the effective interactions in the supercritical regime close to the Fisher-Widom line, where the decay of the bulk correlations of the solvent changes from monotonic to oscillatory, and close to the Widom line, which is the locus of (local) maxima in the bulk correlation length. The effective interactions are calculated by employing the potential distribution theorem.
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http://dx.doi.org/10.1021/acs.jpcb.7b10771DOI Listing
April 2018

Massively parallel GPU-accelerated minimization of classical density functional theory.

J Chem Phys 2017 Aug;147(6):064508

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

In this paper, we discuss the ability to numerically minimize the grand potential of hard disks in two-dimensional and of hard spheres in three-dimensional space within the framework of classical density functional and fundamental measure theory on modern graphics cards. Our main finding is that a massively parallel minimization leads to an enormous performance gain in comparison to standard sequential minimization schemes. Furthermore, the results indicate that in complex multi-dimensional situations, a heavy parallel minimization of the grand potential seems to be mandatory in order to reach a reasonable balance between accuracy and computational cost.
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http://dx.doi.org/10.1063/1.4997636DOI Listing
August 2017

Bubble gating in biological ion channels: A density functional theory study.

Phys Rev E 2017 Jun 12;95(6-1):062407. Epub 2017 Jun 12.

Institut für Theoretische Physik, Universität Tübingen, D-72076 Tübingen, Germany.

We study the behavior of a waterlike liquid inside the gate of a biological ion channel following the basic geometry of the well studied potassium channel KcsA. We calculate the three-dimensional density distribution ρ(r) of the liquid within the framework of classical density functional theory and observe the formation of a low density region (bubble) when the gate is narrow. This observation corresponds to a finite-size form of capillary evaporation and supports the so-called bubble-gate theory. From the density profile we also compute the energy landscape of the gate and the energy required to change the gate from a closed (narrow) to an open (wide) state and vice versa.
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http://dx.doi.org/10.1103/PhysRevE.95.062407DOI Listing
June 2017

Strong Isotope Effects on Effective Interactions and Phase Behavior in Protein Solutions in the Presence of Multivalent Ions.

J Phys Chem B 2017 02 13;121(7):1731-1739. Epub 2017 Feb 13.

Institut für Angewandte Physik, Universität Tübingen , Auf der Morgenstelle 10, 72076 Tübingen, Germany.

In this article, we have studied the influence of the isotopic composition of the solvent (HO or DO) on the effective interactions and the phase behavior of the globular protein bovine serum albumin in solution with two trivalent salts (LaCl and YCl). Protein solutions with both salts exhibit a reentrant condensation phase behavior. The condensed regime (regime II) in between two salt concentration boundaries (c* < c < c**) is significantly broadened by replacing HO with DO. Within regime II, liquid-liquid phase separation (LLPS) occurs. The samples that undergo LLPS have a lower critical solution temperature (LCST). The value of LCST decreases significantly with increasing solvent fraction of DO. The effective protein-protein interactions characterized by small-angle X-ray scattering demonstrate that although changing the solvent has negligible effects below c*, where the interactions are dominated by electrostatic repulsion, an enhanced effective attraction is observed in DO above c*, consistent with the phase behavior observed. As the LCST-LLPS is an entropy-driven phase transition, the results of this study emphasize the role of entropy in solvent isotope effects.
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http://dx.doi.org/10.1021/acs.jpcb.6b12814DOI Listing
February 2017

Structural relaxation and diffusion in a model colloid-polymer mixture: dynamical density functional theory and simulation.

J Phys Condens Matter 2016 11 9;28(45):455101. Epub 2016 Sep 9.

Institute for Theoretical Physics, University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen, Germany.

Within the Asakura-Oosawa model, we study structural relaxation in mixtures of colloids and polymers subject to Brownian motion in the overdamped limit. We obtain the time evolution of the self and distinct parts of the van Hove distribution function G(r,t) by means of dynamical density functional theory (DDFT) using an accurate free-energy functional based on Rosenfeld's fundamental measure theory. In order to remove unphysical interactions within the self part, we extend the recently proposed quenched functional framework (Stopper et al 2015 J. Chem. Phys. 143 181105) toward mixtures. In addition, we obtain results for the long-time self diffusion coefficients of colloids and polymers from dynamic Monte Carlo simulations, which we incorporate into the DDFT. From the resulting DDFT equations we calculate G(r, t), which we find to agree very well with our simulations. In particular, we examine the influence of polymers which are slow relative to the colloids-a scenario for which both DDFT and simulation show a significant peak forming at r  =  0 in the colloid-colloid distribution function, akin to experimental findings involving gelation of colloidal suspensions. Moreover, we observe that, in the presence of slow polymers, the long-time self diffusivity of the colloids displays a maximum at an intermediate colloid packing fraction. This behavior is captured by a simple semi-empirical formula, which provides an excellent description of the data.
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http://dx.doi.org/10.1088/0953-8984/28/45/455101DOI Listing
November 2016

Gyroid phase of fluids with spherically symmetric competing interactions.

Phys Rev E 2016 06 29;93(6):062146. Epub 2016 Jun 29.

Institut für Theoretische Physik, Universität Tübingen, D-72076 Tübingen, Germany.

We study the phase diagram of a fluid with spherically symmetric competing pair interactions that consist of a short-ranged attraction and a longer-ranged repulsion in addition to a hard core. To this end we perform free minimizations of three-dimensional triple periodic structures within the framework of classical density functional theory. We compare our results to those from Landau theory. Our main finding is that the double gyroid phase can exist as a thermodynamically stable phase.
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http://dx.doi.org/10.1103/PhysRevE.93.062146DOI Listing
June 2016

Cation-Induced Hydration Effects Cause Lower Critical Solution Temperature Behavior in Protein Solutions.

J Phys Chem B 2016 08 29;120(31):7731-6. Epub 2016 Jul 29.

Institut für Angewandte Physik, Universität Tübingen , Auf der Morgenstelle 10, 72076 Tübingen, Germany.

The phase behavior of protein solutions is important for numerous phenomena in biology and soft matter. We report a lower critical solution temperature (LCST) phase behavior of aqueous solutions of a globular protein induced by multivalent metal ions around physiological temperatures. The LCST behavior manifests itself via a liquid-liquid phase separation of the protein-salt solution upon heating. Isothermal titration calorimetry and zeta-potential measurements indicate that here cation-protein binding is an endothermic, entropy-driven process. We offer a mechanistic explanation of the LCST. First, cations bind to protein surface groups driven by entropy changes of hydration water. Second, the bound cations bridge to other protein molecules, inducing an entropy-driven attraction causing the LCST. Our findings have general implications for condensation, LCST, and hydration behavior of (bio)polymer solutions as well as the understanding of biological effects of (heavy) metal ions and their hydration.
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http://dx.doi.org/10.1021/acs.jpcb.6b04506DOI Listing
August 2016