Arben Jusufi

• PhD Physics
• Researcher at ExxonMobil Research & Engineering

We develop and harness a phase field simulation method to study liquid filling on grooved surfaces. We consider both short-range and long-range liquid–solid interactions, with the latter including purely attractive and repulsive interactions as well as those with short-range attraction and long-range repulsion. This allows us to capture complete, p...

We develop and harness a phase field simulation method to study liquid filling on grooved surfaces. We consider both short-range and long-range liquid-solid interactions, with the latter including purely attractive and repulsive interactions, as well as those with short-range attraction and long-range repulsion. This allows us to capture complete,...

Capillary rise within rough structures is a wetting phenomenon that is fundamental to survival in biological organisms, deterioration of our built environment, and performance of numerous innovations, from 3D microfluidics to carbon capture. Here, to accurately predict rough capillary rise, we must couple two wetting phenomena: capillary rise and h...

Economical CO2 capture demands low-energy separation strategies. We use a liquid-infused surface (LIS) approach to immobilize reactive liquids, such as amines, on a textured and thermally conductive solid substrate with high surface-area to volume ratio (A/V) continuum geometry. The infused, micrometer-thick liquid retains that high A/V and directl...

The ability to predict liquid transport rates on textured surfaces is key to the design and optimization of devices and processes such as oil recovery, coatings, reaction-separation, high-throughput screening, and thermal management. In this work we develop a fully analytical model to predict the propagation coefficients for liquids hemiwicking thr...

Reactive molecular dynamics simulations of MoS2 crystallization from amorphous precursor materials showed that crystal domain size decreased because of excess S or O, relative to the stoichiometric case. Simulation results were corroborated by comparison of calculated limiting domain sizes to experimental measurements of MoS2 crystals grown from th...

Mo- and S-based lubricant additives reduce friction in boundary lubrication through the formation of molybdenum disulfide (MoS2) during operation. However, the fundamental mechanisms of MoS2 formation are still not fully understood, in part because direct experimental measurement is challenging during the crystallization process. Previously, reacti...

Development of low friction surface materials matched with low friction lubricants is an important route to improving engine efficiency and reducing emissions. Diamond-like carbon (DLC) is a material of interest because it combines the durability of diamond with the low friction properties of graphite. Consequently, various forms of DLC have been s...

Improved friction performance is an important objective of equipment manufacturers for meeting improved energy efficiency demands. The addition of friction reducing additives, or friction modifiers (FMs), to lubricants are a key part of the strategy. The performance of these additives is related to their surface activity and their ability to form a...

The quartz crystal microbalance (QCM) is frequently used to measure the adsorption of molecules from a liquid bulk phase onto surfaces. However, determining the true mass adsorbed onto the surface from the raw frequency shift values often involves various correction factors. In this paper, we highlight yet another correction factor that arises from...

We present a newly-developed Monte Carlo scheme to predict bulk surfactant concentrations and surface tensions at the air-water interface for various surfactant interfacial coverages. Since the concentration regime of these systems of interest are typically very dilute (< 10^-5 mol. frac.), Monte Carlo simulations with the use of insertion/deletion...

A molecular modeling approach is presented with a focus on quantitative predictions of the surface tension of aqueous surfactant solutions. The approach combines classical Molecular Dynamics (MD) simulations with a molecular-thermodynamic theory (MTT) [Y. J. Nikas, S. Puvvada, D. Blankschtein, Langmuir 1992, 8, 2680]. The MD component is used to ca...

A capillary desaturation curve (CDC) depicts the relationship between residual oil saturation, Sor, (i.e. oil left behind in a well-swept permeable medium) and capillary number. A CDC is one of the most fundamental curves of oil recovery as it reveals flow conditions required for good oil displacement in porous media. Despite the importance of this...

In this article, we focus on simulation methodologies to obtain the critical micelle concentration (cmc) and equilibrium distribution of aggregate sizes in dilute surfactant solutions. Even though it is now relatively easy to obtain micellar aggregates in simulations starting from a fully dispersed state, several major challenges remain. In particu...

We report on two recent developments in molecular simulations of self-assembly processes of amphiphilic solutions. We focus on the determination of micelle formation of ionic surfactants which exhibit the archetype of self-assembling compounds in solution. The first approach is centred on the challenge in predicting micellisation properties through...

We present a study of self-assembly of charged surfactants by a combination of molecular simulations and anomalous small-angle x-ray scattering (ASAXS). Solvent-free Grand Canonical Monte Carlo simulations are used to obtain the equilibrium structure of tetradecyltrimethylammonium bromide (TTAB) micelles. Subsequent Molecular Dynamics simulations o...

We investigate the complexation of star-shaped polyelectroltyes (PEs) on oppositely charged colloids. Using Molecular Dynamics simulations, we study the chain adsorption on the colloidal surface at fixed size aspect ratio between star and colloid of around 0.6. Depending on the charge ratio between the colloid and the PE star, we observe a variety...

A hydrophobic theory is combined with a Debye-Hückel approximation to calculate surfactant micellization properties such as the critical micelle concentration (cmc) and concentration effects. The predictive power of the theory is validated by comparison with experimental data of various ionic surfactant types in presence of salt. The theory is also...

Due to the relatively long time scales inherent to ionic surfactant self-assembly (>μs), an aggressive computational approach is needed to obtain converged data on micellar solutions. This work presents a study of micellization using a coarse-grained (CG) model of aqueous ionic surfactants in replicated molecular dynamics (MD) simulations run on gr...

We have investigated micellization properties of surfactants using a recently developed implicit-solvent model and grand canonical Monte Carlo simulations. The original model had been parametrized for ionic surfactants at a single temperature; it is extended here to aqueous solutions of nonionic surfactants and given an explicit temperature depende...

We consider the interaction of colloidal spheres in the presence of mono-, di-, and trivalent ions. The colloids are stabilized by electrostatic repulsion due to surface charges. The repulsive part of the interaction potential Ψ(d) is deduced from precise measurements of the rate of slow coagulation. These "microsurface potential measurements" allo...

The transfer of various nano-scale fullerenes into lipid bilayers has been studied using all-atom (AA) and coarse-grained (CG) molecular dynamics (MD) simulations. The free energy change, when C60, C180, and C540fullerenes are transferred from water to the interior of a lipid [dioleoylphosphatidylcholine (DOPC)] bilayer, has been calculated. Upon e...

The transfer of various nano-scale fullerenes into lipid bilayers has been studied using all-atom (AA) and coarse-grained (CG) molecular dynamics (MD) simulations. The free energy change, when C-60, C-180, and C-540 fullerenes are transferred from water to the interior of a lipid [dioleoylphosphatidylcholine (DOPC)] bilayer, has been calculated. Up...

Recently, we reported new coarse grain (CG) force fields for lipids and phenyl/fullerene based molecules. Here, we developed the cross parameters necessary to unite those force fields and then applied the model to investigate the nature of benzene and C(60) interactions with lipid bilayers. The interaction parameters between the phenyl and lipid CG...

In this paper we discuss the stability behavior of spherical polyelectrolyte brushes (SPB) in the presence of trivalent lanthanum counterions. Stability behavior is measured through the rate of coagulation of the SPB as a function of the lanthanum concentration using simultaneous static and dynamic light scattering. As the counterion concentration...

Recent progress in the field of a versatile and common system in soft matter physics, namely, star-shaped polyelectrolytes, is reviewed. These charged macromolecules combine in their properties aspects of polymer physics, colloidal science, and the rich physics of charged matter, rendering them into versatile building blocks for new materials as we...

We consider the zeta-potential and the effective charge of spherical polyelectrolyte brushes (SPBs) in aqueous solution in the presence of trivalent europium ions. The SPB consists of a polystyrene core of ca. 250 nm diameter onto which long chains of the strong polyelectrolyte poly(styrene sulfonate) are grafted (contour length: 82 nm). At low con...

We have used atomistic simulations to study the role of electrostatic screening and charge correlation effects in self-assembly processes of ionic surfactants into micelles. Specifically, we employed grand canonical Monte Carlo simulations to investigate the critical micelle concentration (cmc), aggregation number, and micellar shape in the presenc...

We propose a method for parametrization of implicit solvent models for the simulation of the self-assembly of ionic surfactants into micelles. The parametrization is carried out in two steps. The first step involves atomistic molecular dynamics simulations of headgroups and counterions with explicit solvent to determine structural properties. An im...

We propose a new way to determine weak repulsive forces operative between colloidal particles by measuring the rate of slow coagulation. The rate of slow coagulation is directly related to the competition of the repulsion with thermal motion. Since the thermal forces are weak, measurements of the coagulation rate can lead to precise information on...

On basis of a coarse-grained model, we investigate the conformational behavior of a spherical polyelectrolyte brush (SPB) in a solution containing oppositely charged linear polyelectrolytes. Our results obtained from Brownian dynamics (BD) simulations show that with increasing amount of linear polyelectrolytes the SPB undergoes the process of swell...

We propose an implicit-solvent model for the simulation of self-assembly of alkali metal dodecyl sulfate micelles. Association of amphiphilic molecules entails multiple time- and length scales. Spontaneous self-assembly of ionic surfactants at low concentrations is currently computationally intractable for atomistic models with explicit solvent. In...

We consider the interaction of multivalent counterions with spherical polyelectrolyte brushes (SPBs). SPBs result if linear polyelectrolyte chains (contour length 60 nm) are densely grafted to colloidal spheres of 116 nm in diameter. When dispersed in water the surface layer, consisting of chains of the strong polyelectrolyte poly(styrene sulfonic...

By definition, soft matter systems react sensitively upon external mechanical perturbations. This material class includes mesoscopic complex fluids such as colloids, polymers and membranes. It is a major challenge to understand the fascinating properties of soft matter from first principles, i.e., by deriving its properties from the microscopic int...

We report on the synthesis and characterization of star-shaped strong polyelectrolytes and their precursor stars with up to 24 arms. To achieve this we polymerized 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) by atom transfer radical polymerization employing a core-first attempt. Sugar-based scaffolds as well as silsesquioxane nanoparticles wer...

Implicit water potentials are developed for the study of thermodynamic and structural properties of solutions of NaCl, LiCl, and KCl. The interaction potential between cations and anions is parametrized from the ionic crystal potential. Two short-range corrections were added to the system to account for the water solvent. The first is due to dielec...

We consider the interaction of multivalent counterions with spherical polyelectrolyte brushes (SPB). The SPB result if linear polyelectrolyte (PE) chains (contour length: 60 nm) are densely grafted to colloidal spheres of 116 nm in diameter. Dispersed in water, the surface layer consisting of chains of the strong PE poly(styrene sulfonic acid) (PSS...

We review recent structural investigations done by anomalous small-angle X-ray scattering (ASAXS). ASAXS uses the dependence of the scattering length of a given element if the energy of the incident X-ray beam is near the absorption edge of this element. The analysis of the ASAXS data leads to three partial intensities. We show that the comparison...

We performed molecular dynamics simulation of a charged colloidal particle with explicit counterions. Our work provides a direct comparison between simulations and ASAXS‐experiments, offering insight into the counterion distribution of charged colloidal suspensions. We give a detailed constitution of the appearing scattering terms with their physic...

We perfomed Molecular Dynamics simulations of a charged macroion surrounded by counterions. Various scattering contributions result from correlations between counterions, macroion, and between both of them. Experimentally, these scattering contributions can be measured by anomalous small-angle X-ray scattering (ASAXS) experiments. As a result the c...

We performed molecular-dynamics simulation to investigate fluctuation effects of monomers and counterions in starlike polyelectrolyte systems. We further study correlations between monomers and counterions. These quantities are systematically studied by variation of two parameters: the functionality f and the degree of dissociation alpha. Fluctuati...

Large-scale Monte Carlo simulations of a phase-separating colloid-polymer mixture are performed and compared to recent experiments. The approach is based on effective interaction potentials in which the central monomers of self-avoiding polymer chains are used as effective coordinates. By incorporating polymer nonideality together with soft colloid...

A review is given on recent studies of charged colloidal suspensions and polyelectrolytes both in static and non-equilibrium situations. As far as static equilibrium situations are concerned, we discuss three different problems: 1) Sedimentation density profiles in charged suspensions are shown to exhibit a stretched non-bariometric wing at large h...

We consider the electrosteric repulsion of colloidal particles whose surface carries a dense layer of long polyelectrolyte chains (spherical polyelectrolyte brushes). The theory of electrosteric repulsion of star polyelectrolytes developed recently is augmented to include particles with a finite core radius. It is shown that most of the counterions...

A brief review is given on recent studies of charged soft matter solutions, as modelled by the 'primitive' approach of strongly coupled Coulomb systems, where the solvent just enters as a dielectric background. These include charged colloids, biological macromolecules such as proteins and DNA, polyelectrolytes and polyelectrolyte stars. Also some o...

In this work, we first briefly review recent results regarding effective interactions of charged star polymers (polyelectrolyte stars) and then we present new results for many-body systems obtained by employing this interaction. The dominant role is played by the trapped counterions that bring about an entropic repulsion between the centres of thes...

Understanding how macroscopic properties depend on intermolecular interactions for complex fluid systems is an enormous challenge in statistical mechanics. This issue is of particular importance for designing optimal industrial fluid formulations such as responsive oilfield fluids, based on viscoelastic surfactant solutions. We have carried out ext...

We examine the conformations and effective interactions of star-branched polyelectrolytes with and without added salt, by employing monomer-resolved molecular dynamics simulations and an analytical theory. The simulations take into account the excluded-volume and Coulomb interactions between the individual monomers, as well as the counter-and coion...

Combining monomer-resolved molecular dynamics simulations with a theory based on a variational free energy, we calculate the conformational properties and the effective interactions of star-branched polyelectrolytes for a large variety of arm numbers, degrees of polymerization, and charge fractions, with and without added salt. We find quantitative...

Recent results in theory and simulation of star-polymer--colloid mixtures are reviewed. We present the effective interaction between hard, colloidal particles and star polymers in a good solvent derived by monomer-resolved Molecular Dynamics simulations and theoretical arguments. The relevant parameters are the size ratio q between the stars and th...

We examine the demixing transition in star-polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-polymer-colloid size ratios 0.18< or =q< or =0.50. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecu...

Recent progress in the theory and computer simulation of effective interactions and phase transitions of colloidal dispersions is reviewed. Particular emphasis is put on the role of the discrete solvent in determining the effective interaction between charged colloids, bulk fluid-fluid phase separation of star-polymer-colloid mixtures, and on inter...

Using monomer-resolved Molecular Dynamics simulations and theoretical arguments based on the radial dependence of the osmotic pressure in the interior of a star, we systematically investigate the effective interactions between hard, colloidal particles and star polymers in a good solvent. The relevant parameters are the size ratio q between the sta...

We examine the demixing transition in star polymer-colloid mixtures for star arm numbers f=2,6,16,32 and different star-colloid size ratios. Theoretically, we solve the thermodynamically self-consistent Rogers-Young integral equations for binary mixtures using three effective pair potentials obtained from direct molecular computer simulations. The...

We analyze the effective triplet interactions between the centers of star polymers in a good solvent. Using an analytical short-distance expansion inspired by scaling theory, we deduce that the triplet part of the three-star force is attractive but only 11% of the pairwise part even for a close approach of three star polymers. We have also performe...

We analyze the effect of polydispersity in the arm number on the effective interactions, structural correlations and the phase behavior of star polymers in a good solvent. The effective interaction potential between two star polymers with different arm numbers is derived using scaling theory. The resulting expression is tested against monomer-resol...

We review recent work on fluid-solid and solid-solid phase transitions in soft matter systems such as colloidal suspensions and star polymer solutions. Starting from a given interparticle pair potential we predict the corresponding phase diagrams using computer simulations, cell theory, and density functional theory. When possible, the results are...

We analyze the effective triplet interactions between the centers of star polymers in a good solvent. Using an analytical short distance expansion inspired by scaling theory, we deduce that the triplet part of the three-star force is attractive but only 11% of the pairwise part even for a close approach of three star polymers. We have also performe...

Possibilities of fluid-solid and solid-solid phase transformations in colloidal suspensions and star polymer solutions are reviewed. We start from given interparticle pair potentials and predict the corresponding phase diagrams using computer simulations and density functional theory. When possible, the results are compared with experimental data....

We derive the short distance interaction of star polymers in a colloidal solution. We calculate the corresponding force between two stars with arbitrary numbers of legs f_1 and f_2. We show that a simple scaling theory originally derived for high f_1, f_2 nicely matches with the results of elaborated renormalization group analysis for f_1 + f_2 =<...

The distance-resolved effective interaction between two star polymers in a good solvent is calculated by Molecular Dynamics computer simulations. The results are compared with a pair potential proposed recently by Likos et al. [Phys. Rev. Lett. 1998, 80, 4450] which is exponentially decaying for large distances and crosses over, at the corona diame...

Die Arbeit befasst sich mit der Bestimmung der effektiven Wechselwirkung in komplexen Systemen, verdeutlicht an zwei Beispielen aus der aktuellen Forschung auf dem Gebiet der weichen Materie. Zum einen berechnen wir das effektive Wechselwirkungspotential zwischen zwei sternfoermigen Polyelektrolyten (PE-Sterne), zum anderen bestimmen wir dieselbe G...