Dilute Concentration Regime Research Articles

Inertial effects in sedimenting suspensions of solid spheres in a liquid

Particle-resolved Direct Numerical Simulations have been performed on the gravitational settling of mono-disperse solid spheres in a viscous fluid and triply periodic domain. In a comprehensive study, the bulk solid volume concentration was varied from ϕ=0.5 to 30%. To study the effect of inertia, three different Galileo numbers were considered in the inertial regime, Ga=144, 178 and 210, for which a single settling sphere exhibits distinctly different wake and path characteristics. The particle/fluid mass density ratio was fixed at 1.5. We find that for ϕ=2−30% the suspension microstructure and dynamics depend predominantly on the bulk concentration. In qualitative agreement with previous studies in literature, three different sedimentation regimes can be distinguished: (1) the dilute concentration regime for ϕ≲2% with preferential settling of particles in vertical trains, (2) the moderate concentration regime for 2%≲ϕ≲10% with preferential settling of particles in horizontal pairs with an interparticle distance of ∼ 1.5 particle diameters, and (3) the dense concentration regime for ϕ≳10% with a nearly random (“hard-sphere”) distribution of the particles in space. The clustering of particles is dictated by, respectively, trapping of particles in the wake of other particles, a drafting–kissing–tumbling (DKT) instability by which two vertically aligned particles quickly reorient themselves into a horizontally aligned particle pair, and short-range multiparticle interactions through viscous lubrication and to a lesser extent collisions between particles. In all cases, hindered settling at a reduced speed is observed as compared to a single settling sphere. The well-known Richardson–Zaki relation for the mean sedimentation velocity appears valid only for the dense concentration regime. We provide ample evidence that in the dense regime the characteristic velocity and time scales of particle motion are proportional to gDp and Dp/g, respectively, with g the gravitational acceleration and Dp the particle diameter. We also observe an ω−3 scaling of the particle velocity spectra for ωDp/g≳0.4 and we propose a model to explain this scaling behavior, based on the inertial response of the particles to small-scale flow perturbations. Kinematic waves, i.e., vertically propagating plane waves in the local concentration field, are observed in all cases, though unrelated particle motions are responsible for significant loss of the spatio-temporal coherence of the waves. The wave speed was determined from repeated space–time autocorrelations of the local concentration field and appears in reasonable agreement with Kynch sedimentation theory using the Richardson–Zaki relation. The passage of kinematic waves causes perturbations in the particle velocity at a frequency that matches well with peak frequencies in the particle velocity spectra for concentrations up to ϕ≈10%. The time-lagged cross-correlation of the vertical and horizontal particle velocity suggests that kinematic waves may trigger DKT instabilities, while conversely DKT instabilities may be responsible for the onset of kinematic waves. Finally, we suggest that obstruction and perturbation of the particle wake by neighboring particles could offer an explanation for the small influence of the Galileo number on the suspension behavior for ϕ=2−30%.

The Zircon Structure As a Prototype Host for Fast Monovalent and Di-Valent Ionic Conduction

"Beyond Li-ion" energy storage solutions based on ions such as Na, Mg, Ca and Zn-ion have attracted increasing attention due to growing concerns about the cost, resource availability and safety of the currently dominant Li-ion batteries. One of the greatest challenges for beyond-Li systems, especially multivalent ones, is the lack of materials with high ionic mobility.In this study, we find that zircon-type framework presents a unique structural environment that enables superior conduction of multiple species including Na+, Ca2+, Mg2+ and Zn2+, even in the dilute carrier concentration regime. Nudged elastic band (NEB) calculations show very low diffusion barriers for Na/Ca/Mg/Zn in pristine YPO4 and ab initio molecular dynamics (AIMD) simulations confirm the 1D percolating diffusion channel in the zircon-type framework. These 1D percolating channels of adjacent, distorted octahedral sites result in a smoothly varying coordination environment thus correspondingly low activation barriers. The phase stability of doped compositions is examined when both carrier ion and subvalent dopants are introduced into the system and confirm good stability through low decomposition energy of compositions with different carrier-dopant combinations. Among these compositions, we found Na0.0625YSi0.0625P0.9375O4 exhibiting a high Na+ conductivity of 0.99 mS/cm at 300 K with an activation energy of 220 meV. Finally, we identify dopant-carrier association in doped YPO4 as a key challenge for long-range diffusion in this structure family.

Colloid thermophoresis in the dilute electrolyte concentration regime: from theory to experiment.

Colloid thermophoresis in aqueous media is vital for numerous applications in nanoscience and life sciences. To date, a general description of colloid thermophoresis in DI water has not been determined. Here, we describe a theoretical model within the framework of the Fokker-Planck formalism and the flickering cluster concept to describe the hydration entropy effect on the thermophoretic behaviour of colloids suspended in DI water and compare this to new experimental results. We built an experimental platform to allow for rapid and robust temperature control and investigate the thermophoretic behaviour of silica microspheres with different sizes at various background temperatures for comparison. In this work, the ionic shielding effect is accounted for by using the well-known Duhr-Dhont's model, and the hydration layer effect is determined using the developed theoretical model. For the latter, our model reveals that the sign of the Soret coefficient is governed by the interplay between the binding energy and the chemical potential of water molecules, which were found to be in the same order of magnitude. We show that our analysis accurately describes the experimental behaviour of colloidal particles that opens a new avenue for developing versatile trapping and separation techniques for various colloidal particles in aqueous systems according to their size and background temperature.

Depletion interaction mediated by semiflexible polymers.

We present a simple mean-field theory to describe the polymer-mediated depletion attraction between colloidal particles that accounts for the polymer's chain stiffness. We find that for fixed polymer radius of gyration and volume fraction, the strength of this attraction increases with increasing chain stiffness in both dilute and semidilute concentration regimes. In contrast, the range of attraction monotonically decreases with chain stiffness in the dilute regime, while it attains a maximum in the semidilute regime. The obtained analytical expressions for the depletion interaction were compared with numerical self-consistent field lattice computations and shown to be in quantitative agreement. From the interaction potential between two spheres, we calculated the second osmotic virial coefficient B2, which appears to be a convex function of chain stiffness. A minimum of B2 as a function of chain stiffness was observed both in the numerical self-consistent field computations and the analytical theory. These findings help explain the general observation that semiflexible polymers are more effective depletants than flexible polymers and give insight into the phase behavior of mixtures containing spherical colloids and semiflexible polymers.

Orientation dynamics of anisotropic and polydisperse colloidal suspensions

Small-angle light scattering (SALS) is employed to determine in situ microstructural changes of polydisperse colloidal suspensions under capillary flow. Titanium dioxide (rod-like) bundles and kaolinite nanoclay (platelet) flake-like stacks of particles are suspended in a Newtonian fluid to explore the effects of shape anisotropy. Subjecting the systems to a startup test in a flow cell in Stokes's regime, the evolution of the anisotropy factor (AF), and the average orientation angle (χ) of particles vs strain is probed at different averaged Péclet (Pe¯) numbers. Dilute and semi-dilute concentration regimes are explored in a capillary flow. Moving in the vorticity direction, we show significant changes in χ due to the gradient in shear rate in the capillary flow, while no cross-sectional flow migration was exhibited. In these polydisperse colloidal suspensions, two characteristic stages are observed: initial particle alignment and subsequent orientational demixing. Probing the velocity–vorticity (xy) plane, an initial particle alignment in the flow direction at high Pe¯ (> 1) occurs, is demonstrated by an elliptical SALS pattern, and then, an increase in AF is observed due to particles' phase mixing. This behavior is then followed by a breakdown in structure and loss of particle alignment due to orientational demixing. The evolution of the average orientation angle of particles in the xy plane can be clearly observed through these two stages by the help of high-resolution SALS contours. These experimental findings provide novel insights into the flow–microstructure relationship of polydisperse colloidal suspensions for the optimization of many industrial processing schemes.

Reclamation of nutrients, carbon, and metals from compromised surface waters fated to the Salton Sea: Biomass production and ecosystem services using an attached periphytic algae flow-way

The Salton Sea in the Imperial Valley of California is a threatened ecosystem. To address the challenges related to trace element accumulation and eutrophication, a 270 m long attached periphytic algae flow-way was deployed at the Alamo River Wetlands, a tributary of the Salton Sea. To assess opportunities for efficient generation of biomass from reclamation of run-off-derived nutrients, the quantities of available and biosorpbed nutrients, organics, and trace elements were monitored. Over the course of 2 years, persistent algal biomass production was achieved at an average ash-free biomass productivity of 5.8 ± 2.7 g/m2/day for the full flow-way length. Overall, harvested biomass consisted of high ash (76.3 ± 5.4%) and low lipid (1.1 ± 1.0%) content, which are typical of periphytic algal biomass. Nitrogen removal rates of 530 ± 190 mg N/m2/day, phosphorous removal rates of 14 ± 6 mg P/m2/day, and 73 ± 25% of BOD (Biological Oxygen Demand) removal rates were achieved. Furthermore, spatial variations of the biomass productivity along with nutrient removal rates were observed to have a decreasing trend over the length of the flow-way, while N and P contents of the biomass showed increasing trend, indicating variable nutrient utilization efficiency as an important factor for system scaling. Temperature and solar irradiation were found to be key environmental factors for biomass productivity and nutrient removal rates. However, stable uptake of nutrients, organics, and metals in the biomass, despite intermittent variation of the analyte concentrations in the source water, indicate the resilience of attached periphytic algae biomass production at dilute nutrient concentration regimes. Trace metal analysis of the water from the surrounding area revealed levels exceeding federal toxicity guidelines for Selenium and Copper. Significant bioaccumulation of these and other metals contaminants were also identified in the harvested biomass, including Nickel, Chromium, Cadmium and Lead. Together these findings demonstrate several potential value propositions for attached algae cultivation from agricultural runoff-impacted surface waters, including remediation of N/P nutrients, organics, and common toxic metals, concomitant with biomass production.

Role of physicochemical characteristics of poly(N,N-diethylacrylamide) on the polymer thermal responsivity and interfacial properties in aqueous solution: All-atom simulation study

Molecular dynamic behaviors of poly(N,N-diethylacrylamide) (PDEA) as well as its interfacial properties in water were studied measuring the polymer single chain in a dilute concentration regime via molecular dynamics simulation. The investigation of chain length and temperature impacts on the rate of affinity variation of PDEA to water through calculating non-bonded interactions between them showed that the increment of two mentioned items reduced the polymer hydrophilicity in water. The interactional variation altered the PDEA diffusivity in the solution so that the decrement of PDEA tendency to water enhanced the chain movements because of reducing the interfacial friction between the chain and media, particularly at the transition zone. The chains spatial dimensions, conformation and shape were determined as a function of temperature to evaluate the chain hydrodynamic features. A particular order parameter for the PDEA backbone bonds and distribution of their dihedral angles were calculated to consider entropy of the chains with temperatures. The results indicated that PDEA tended to have a rod conformation with less entropy at lower temperatures and chain lengths. Finally, a novel parameter, <Pθφ(T) >, based on the interfacial structure of water was presented to quantitate the relationship between the thermodynamics of PDEA and its structure and hydrophilicity variation with temperature.

Interparticle interactions and rheological signatures of Ti3C2Tz MXene dispersions

HypothesisWe hypothesize that dispersed Ti3C2Tz MXene particle interactions are reflected in the bulk viscoelastic properties of the dispersions and can be analyzed using classical colloidal theory for anisotropic particles. The relevant kinetic theory for Brownian anisotropic particles is given by the Doi and Edwards (D-E) Model, and the Maxwell Model is used to fit the relaxation times as a function of frequency. Such behavior is relevant to a variety of MXene processing techniques, particularly printing and coating. ExperimentsSmall oscillatory shear tests were performed for dilute Ti3C2Tz MXene aqueous dispersions as a function of their concentration and temperature. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), ζ potential measurements, Dynamic Light Scattering (DLS) were used to characterize the Ti3C2Tz MXene nanoparticles. FindingsTi3C2Tz dispersions show gel-like and viscous-like behavior at low and high temperatures, respectively. Experimental relaxation times fitted to the Maxwell model are found to be close to the theoretical values. However, at high temperatures, relaxation time values differ due to the inter-particle interactions, even in the dilute concentration regime. For Ti3C2Tz dispersions, aggregation, and clustering can have dramatic consequences for dispersion rheology, including gelation, as the sample transitions from liquid-like to solid-like behavior.

Determination of a critical separation concentration for associative polymers in porous media based on quantification of dilute and semi-dilute concentration regimes

Hydrophobic interactions are an inherent property of associative polymers which results in the formation of molecular aggregates. The loss of hydrophobic interactions under reservoir conditions have been reported, and this results in increased fluid – rock interaction effect such as adsorption. However, existing adsorption studies only reports the interaction of individual molecules with rock surface without taking into account mechanically retained molecular aggregates in narrow pores. The implication of this is a reduction in associative effect between polymer molecules during transport in a porous media. In this work, the minimization of this phenomenon and sustenance of the interaction network was studied through a theoretically defined dimensionless parameter for the quantification of molecular interactions in the various polymer concentration regimes. It was established in this work that associative polymers exhibit a critical separation concentration during propagation in a porous media under given reservoir conditions. This critical separation concentration marks the concentration beyond which large molecular aggregates constituting the hydrophobic network is sterically excluded and retained in narrow pore spaces. This separation phenomenon was predicted to occur when the proportion of molecular interactions arising from hydrophobic and intramolecular interactions are equal. Thus, the balance in the molecular interactions in the semi – dilute concentration regime was identified as key in minimizing the loss of hydrophobic interactions to aggregate retention and optimizing its sustainability in a porous media. Consequently, a novel approach was developed based on this knowledge for the transport of associative polymers at which these hydrophobic interactions are sustained. It was demonstrated that propagating associative polymers using this approach ensured the sustainability at concentrations below the critical separation concentration tend to maximize the hydrophobic interaction network with minimal loss of polymer chains retention mechanisms.

Frequency-dependent force between ac-voltage-biased plates in electrolyte solutions.

We analyze the frequency dependence of the force between ac-voltage-biased plates in electrolyte solutions. To this end we solve analytically the Poisson-Nernst-Planck transport model in the dilute concentration and low voltage regime for a 1:1 symmetric electrolyte with blocking electrodes under a dc+ac applied voltage. The total force, which is the resultant of the electric and osmotic forces, shows a complex dependence on plate separation, frequency, ion concentration, and compact layer properties, different from that predicted from electrostatic current models or equivalent circuit models, due to the relevance of the osmotic force contribution in almost the whole range of frequencies. For the total dc force, we show that it decays at fixed ion concentration, linearly with plate separation for separations larger than a few times the Debye screening length. This linear dependence is due to the assumption about the conservation of the number of ions in the system. Moreover, the 1ω and 2ω ac harmonics of the total force show a broad peak at intermediate frequencies; it is centered at about the inverse of the charging time of the double layer capacitance, and covers the frequency range between the inverse of the diffusion time and the inverse of the electrolyte dielectric relaxation time. Finally, the 1ω ac harmonic component attains its high frequency asymptotic value at frequencies much higher than the inverse of the electrolyte dielectric relaxation time due to the very slow relaxation of the osmotic 1ωharmonic component at high frequencies. The derived analytical expressions for the total force remain valid up to voltages of the order of the thermal voltage, as has been assessed by means of numerical calculations. The numerical calculations are also used to explore the onset of higher force harmonics for larger applied voltages. Understanding the frequency dependence of the force acting on voltage-biased plates in electrolyte solutions can be of relevance for electrical actuation strategies in microelectromechanical systems and for the interpretation of some emerging electric scanning probe force microscopy techniques operating in electrolyte solutions.

Oscillating grid turbulence in shear-thinning polymer solutions

Oscillating grid apparatuses are well known and convenient tools for the fundamental study of turbulence and its interaction with other phenomena since they allow to generate turbulence supposedly homogeneous, isotropic, and free of mean shear. They could, in particular, be used to study turbulence and mass transfer near the interface between non-Newtonian liquids and a gas, as already done in air-water situations. Although frequently used in water and Newtonian fluids, oscillating grid turbulence (OGT) generation has yet been rarely applied and never characterized in non-Newtonian media. The present work consists of a first experimental characterization of the flow properties of shear-thinning polymer (Xanthan Gum, XG) solutions stirred by an oscillating grid. Various polymer concentrations are tested for a single grid stirring condition. The dilute and semidilute entanglement concentration regimes are considered. Liquid phase velocities are measured by Particle Image Velocimetry. The existing mean flow established in the tank is described and characterized, as well as turbulence properties (intensity, decay rate, length scales, isotropy, etc.). OGT in dilute polymer solutions induces an enhanced mean flow compared to water, a similar decay behavior with yet different decay rates, and enhanced turbulence large scales and anisotropy. In the semidilute regime of XG, turbulence and mean flows are essentially damped by viscosity. The evolution of mean flow and turbulence indicators leads to the definition of several polymer concentration subregimes, within the dilute one. Critical concentrations around 20 ppm and 50 ppm are found, comparable to drag reduction characteristic concentrations.

Structure of polyvinylpyrrolidone aqueous solution in semi-dilute regime: Roles of polymer-surfactant complexation

The effect of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) addition on the structure of polyvinylpyrrolidone (PVP) aqueous solution, in dilute and semi-dilute regimes, has been investigated at 25 °C. The intrinsic viscosity, Huggins constant, Flory exponent and the Mark-Houwink-Sakurada coefficient were deduced by viscosimetry. The structure of PVP (MW = 40.000 g/mol) aqueous solution in semi-dilute regime was described according the blob model. The monomer size, the density of monomer, the correlation length and the number of monomer in each blob were determined. The PVP-AOT interaction in water was studied in dilute and semi-dilute concentration regime of the polymer by electrical conductivity. A complexation occurs between polymer and surfactant independently on the concentration regime, where the complex nature depends on surfactant concentration. The overlapping concentration of PVP-AOT aqueous solution increases with increasing AOT concentration from 0 to 0.6 g/dl. The Flory exponent decreases from 0.545 to 0.509 and the Mark-Houwink-Sakurada coefficient decreases from 0.636 to 0.528 g/dl. The effect of AOT complexation with PVP in water was also discussed according the blob model. At the overlapping concentration, the density of monomers and the correlation length decrease with increasing surfactant concentration. The density of monomers increases from its value at the overlapping concentration as the polymer concentration increases at the semi-dilute regime. However, the monomer size, the correlation length and the number of monomer in each blob decrease from their values at the overlapping concentration as the polymer concentration increases.

Polycation-globular protein complex: Ionic strength and chain length effects on the structure and properties

AbstractThe effect of high salt concentration and pH on the binding of globular protein to polycation at different molar masses has been investigated by dynamic light scattering (DLS), turbidimetry and electrostatic modeling for the protein. In dilute concentration regime, DLS and pH titrations showed three remarkable pH transitions: pHc, the pH where soluble complexes of bovine serum albumin (BSA) and linear synthetic Polyethylenemine (PEI) are formed, pHc’ presents the end of primary soluble complex and pHφ presents the first appearance of microcoacervate droplets. All pH transitions increase with increasing NaCl concentration. The Adaptive Poisson-Boltzmann Solver (APBS) identify with precision the functional sites at the surface of BSA and shows that the strength of electrostatic interactions depends hugely on the variation of pH and ionic strength. At a very high concentration of salt, no remarkable effect on a mixture formed of a long chain of polycation and globular protein.

Characterizing droplet breakup rates of shear-thinning dispersed phase in microreactors

A two-phase flow predictive model with the integration of conservative level-set method (LSM) and Carreau-Yasuda constitutive equation was developed herein. The LSM was chosen as a potential interface capturing scheme for elucidating the interfacial phenomena including insight into the mechanism of shear-thinning droplets. In present paper, the dynamics of shear-dependent droplet emergence, growth, detachment and translocation in a Newtonian microsystem were examined via computational fluid dynamics (CFD) analysis. Dilute sodium carboxymethylcellulose (Na-CMC) solution was treated as dispersed phase (70 mPa.s < ηo < 10.2644 Pa.s) whereas the olive oil (68 mPa.s) was designated as continuous phase. Visualisation experiments were carried out and these laboratory data were used to validate the simulation results. Detailed 2D simulations were presented to examine systematically the impact of fluid properties on the droplet breakup rate at predefined flow rate ratio, Q of 0.05. The results yielded an inflection point in the dependence of droplet breakup rate on Na-CMC concentration was found in between the dilute and semi-dilute concentration regimes. This inflection point displays a non-monotonic profile which is mainly caused by the considerable viscosity effect of Na-CMC polymer when its concentration increases above a critical value (C > C*˜0.40 wt%). This striking behaviour highlights the importance of rheological effects in flows with a shear-dependent fluid under various flow conditions. The viscous effect of Na-CMC fluids substantially affects the manipulation over the droplet pinch-off time and production rate. Thus, it necessitate the control of the shear rate by adjusting the flow conditions and aspect ratio of microchannels.

Steady shear viscosity of semi-dilute bubbly suspensions

The steady shear viscosity of bubbly suspensions is known to depend upon the suspending fluid viscosity, the bubble volume fraction and a dimensionless number (the capillary number) accounting for the deformability of the bubbles under a viscous stress. However, experimental data on bubbly suspensions in the literature are scarce and present two main shortcomings: (i) the studied systems are polydisperse, which leads to arbitrariness in the definition of a capillary number, and (ii) there is a lack of data for capillary numbers of order unity, where bubbles are slightly deformable. This leads to the absence of clear conclusion on the relevance of the existing theoretical models. In order to make significant progress, an original device is designed to produce monodisperse suspensions of bubbles in a highly viscous Newtonian fluid. The steady shear viscosity of the bubbly suspensions is measured in the dilute and semi-dilute concentration regimes (volume fraction of bubbles between 2% and 18%) using a Couette geometry with a 20-bubbles-wide gap, for capillary numbers ranging between 0.01 and 10. The new obtained data are shown to be much more accurate than data previously reported in the literature. Experimental data are in very good agreement with the model of Frankel and Acrivos (1970), originally developed for dilute suspensions, in the whole range of studied volume fractions.

Coarse-Grained Molecular Dynamics Force-Field for Polyacrylamide in Infinite Dilution Derived from Iterative Boltzmann Inversion and MARTINI Force-Field.

We present a mesoscale model of aqueous polyacrylamide in the infinitely dilute concentration regime, by combining an extant coarse-grained (CG) force-field, MARTINI, and the Iterative Boltzmann Inversion protocol (IBI). MARTINI force-field was used to retain the thermodynamics of solvation of the polymer in water, whereas the structural properties and intrapolymer interactions were optimized by IBI. Atomistic molecular dynamics simulations of polymer in water were performed to benchmark the mesoscale simulations. Our results from the CG model show excellent agreement in structure with the atomistic system. We also studied the dynamical behavior of our CG system by computing the shear viscosity and compared it with the standard IBI model. The viscosity trends of our model were similar to the atomistic system, whereas the standard IBI model was highly dissimilar as expected. In summary, our hybrid CG model sufficiently mimics an infinitely dilute system, and is superior to both MARTINI and IBI in representing the structure and thermodynamics of the atomistic system, respectively. Our hybrid coarse-graining strategy promises applicability in large-scale simulations of polymeric/biological systems where the structure needs to be replicated accurately while preserving the thermodynamics of a smoother surrounding.

Self-Assembly of Mixtures of Telechelic and Monofunctional Amphiphilic Polymers in Water: From Clusters to Flowerlike Micelles

We study the self-assembly of mixtures of n-alkyl mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the dilute concentration regime. The monofunctional PEOs were prepared by living anionic polymerization with varying n-alkyl length (n = 14, 16, 22, 28) and constant PEO molecular weight of 5 kg/mol. The difunctional materials were obtained through end-to-end coupling of two of the monofunctionalized PEOs via their terminal hydroxyl groups. The chosen synthetic pathway yields well-defined model compounds with narrow molecular weight distribution and complete end-group functionalization. By using both small-angle neutron scattering (SANS) and dynamic light scattering (DLS) combined with theoretical data modeling, we have systematically investigated both the global and inner structure of the self-assembled micellar structures. For short n-alkyl chain-ends, we find a formation of clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike ...

Osmotic and diffusio-osmotic flow generation at high solute concentration. II. Molecular dynamics simulations.

In this paper, we explore osmotic transport by means of molecular dynamics (MD) simulations. We first consider osmosis through a membrane and investigate the reflection coefficient of an imperfectly semi-permeable membrane, in the dilute and high concentration regimes. We then explore the diffusio-osmotic flow of a solute-solvent fluid adjacent to a solid surface, driven by a chemical potential gradient parallel to the surface. We propose a novel non-equilibrium MD (NEMD) methodology to simulate diffusio-osmosis, by imposing an external force on every particle, which properly mimics the chemical potential gradient on the solute in spite of the periodic boundary conditions. This NEMD method is validated theoretically on the basis of linear-response theory by matching the mobility with their Green-Kubo expressions. Finally, we apply the framework to more realistic systems, namely, a water-ethanol mixture in contact with a silica or a graphene surface.

Polymer concentration regulated aging in aqueous Laponite suspensions

By rheologically examining poly(vinyl alcohol)-water-Laponite multicomponent suspensions, we report evidence of concentration of adsorbing polymer strictly regulating the aging dynamics in multicomponent Laponite suspensions. The study is performed in the dilute polymer concentration regime. We analyze the observed aging pattern by fixing the non-monotonic transitions observed between temporally achieved suspension elastic moduli and polymer concentration, as the border of a sub-regime. This divides the aging pattern into three sub-regimes, across which the suspension elastic modulus shows an oscillatory relationship with polymer concentration. We also observe that suspensions belonging to first two sub-regimes remained colloidally stable during the timescale of measurement. In the third sub-regime, macrophase separation occurred, while suspensions were aging. For an explanation to this observation, we conducted three complementary measurements on select suspensions from each sub-regime, after they attained structurally arrested state. They are (a) small-angle neutron scattering, (b) structure recovery upon shear melting, and (c) zeta potential estimation. The collective evidences are analyzed to reach a possible explanation. The observation and analysis highlight the effect of concentration of adsorbing polymer on aging clay suspensions and have direct applications in stabilization, rheology modification, as well as polymer nanocomposite preparation.

Ionic Liquid Surfactant Mediated Structural Transitions and Self-Assembly of Bovine Serum Albumin in Aqueous Media: Effect of Functionalization of Ionic Liquid Surfactants.

The self-assembly of globular protein bovine serum albumin (BSA) has been investigated in aqueous solutions of ionic liquid surfactants (ILSs), 1-dodecyl-3-methyl imidazolium chloride, [C12mim][Cl], and its amide, [C12Amim][Cl], and ester, [C12Emim][Cl], functionalized counterparts. Dynamic light scattering (DLS) has provided insights into the alterations in hydrodynamic radii (D(h)) of BSA as a function of concentration of ILSs establishing the presence of different types of BSA-ILS complexes in different concentration regimes of ILSs. Isothermal titration calorimetry (ITC) has been exploited to quantify the ILSs interacting with BSA in dilute concentration regime of ILSs. The zeta-potential measurements shed light on changes in the charged state of BSA. The morphology of various self-assembled structures of BSA in different concentration regimes of ILSs have been explored using confocal laser scanning microscopy (CLSM) and scanning electron microscopy. The structural variations in ILSs have been found to produce remarkable effect on the nature and morphology of self-assembled structures of BSA. The presence of nonfunctionalized [C12mim][Cl] IL at all investigated concentrations has led to the formation of unordered large self-assembled structures of BSA. On the other hand, in specific concentration regimes, ordered self-assembled structures such as long rods and right-handedly twisted helical amyloid fibers have been observed in the presence of functionalized [C12Amim][Cl] and [C12Emim][Cl] ILSs, respectively. The nature of the formed helical fibers as amyloid ones has been confirmed using FTIR spectroscopy. Steady-state fluorescence and circular dichroism (CD) spectroscopy have provided insights into folding and unfolding of BSA as fashioned by interactions with ILSs in different concentration regimes supporting the observations made from other studies.