Dilute Regime Research Articles

An inertial number regulated constitutive model for the gas-fluidization of binary mixtures across dilute and dense regimes

In this study, a constitutive model that is applicable to both dilute and dense regimes for simulating gas-particle flows with polydisperse particles is proposed. In dilute regime, solid stress is closed by kinetic theory of granular flow (KTGF). In dense regime, solid stress is closed by the η(Is) rheology. The transition from dilute to dense regimes is realized smoothly by the dynamic blending function χmix, which is a function of the inertial number of mixture Imix. The dynamic blending function χmix is also used to link the solid–solid drag force at different regimes. This new model was validated with the experimental data of polydisperse bubbling fluidized bed of Girimonte et al. (2018). When compared with the conventional baseline model, this new model improves the transition from dilute to dense regimes and the predictions in both density and size segregations.

Computational Analysis of Ion Clustering Statistics in Liquid Battery Electrolytes

The transport properties of liquid battery electrolytes critically depend on molecular-level solvation structures of ions. In contrast to the dilute regime where the majority of counterions are sufficiently separated spatially, in global or localized high-concentrated electrolytes (HCEs) counterions tend to form superstructures such as contact-ion pairs (CIPs) or aggregates (AGGs). The presence and relative population of such superstructures can drastically change the ionic transport behavior in liquid electrolytes. We have developed a computer program [1] that can efficiently perform analysis of ion clustering statistics from molecular dynamics simulations, complementary to experimental measurements. We have successfully applied our method to various systems, including (1) a prototypical concentrated liquid electrolyte (Li:EC:DEC) [2], and (2) a series of newly developed high-entropy liquid electrolytes [3]. In both cases, we find consistent agreements between our prediction and experimentally measured nanostructures and solvation strengths. We envision that our method would be widely applicable to generic structurally disordered systems.[1] Cohen et al., Journal of Open Source Software 8, 84 (2023)[2] Xie and Wang et al., Science Advances 9, eadc9721 (2023)[3] Kim and Wang et al. Nature Energy 8, 814–826 (2023)

Dynamics and sorting of run-and-tumble particles in fluid flows with transport barriers

We investigate the dynamics of individual run-and-tumble particles in a convective flow which is a prototype of fluid flows with transport barriers. We consider the most prevalent case of swimmers denser than the background fluid. As a result of gravity and the effects of the carrying flow, in the absence of swimming the particles either sediment or remain in a convective cell. When run-and-tumble also takes place, the particles may move to upper convective cells. We derive analytically the probability of uprise. Since that probability in a given fluid flow can vary strongly across species, our findings inspire a purely dynamical mechanism for species extraction in the dilute regime. Numerical simulations support our analytical predictions and demonstrate that a judicious choice of the fluid flow’s parameters can lead to particle sorting with an arbitrary degree of purity.

Quasiclassical Gluon Fields and Low's Soft Theorem at Small Momentum-Fraction x.

In the high-energy limit, soft gluons can be approximately described by quasiclassical gluon fields. It is well known that the gluon field is a pure gauge field on the transverse plane at eikonal order. We derived the complete next-to-eikonal order solutions of the classical Yang-Mills equations for soft gluons in the dense nuclear regime. Utilizing these solutions, it is shown that Low's soft theorem at small x can be obtained by considering off-diagonal matrix elements of quasiclassical chromoelectric field between single-gluon states in the dilute regime. We further propose on extending Low's soft theorem at small x to incorporate the effects of gluon saturation in the dense regime.

Synthesis, characterization and stability of crosslinked chitosan-maltodextrin pH-sensitive nanogels

Polysaccharide-based nanogels offer a wide range of chemical compositions and are of great interest due to their biodegradability, biocompatibility, non-toxicity, and their ability to display pH, temperature, or enzymatic response. In this work, we synthesized monodisperse and tunable pH-sensitive nanogels by crosslinking, through reductive amination, chitosan and partially oxidized maltodextrins, by keeping the concentration of chitosan close to its overlap concentration, i.e. in the dilute and semi-dilute regime. The chitosan/maltodextrin nanogels presented sizes ranging from 63 ± 9 to 279 ± 16 nm, showed quasi-spherical and cauliflower-like morphology, reached a ζ-potential of +36 ± 2 mV and maintained a colloidal stability for up to 7 weeks. It was found that the size and surface charge of nanogels depended both on the oxidation degree of maltodextrins and chitosan concentration, as well as on its degree of acetylation and protonation, the latter tuned by pH. The pH-responsiveness of the nanogels was evidenced by an increased size, owed to swelling, and ζ-potential when pH was lowered. Finally, maltodextrin-chitosan biocompatible nanogels were assessed by cell viability assay performed using the HEK293T cell line.

Structure of irradiated polymer in semi dilute regime concentration: roles of dose values

Structure of irradiated polymer in semi dilute regime concentration: roles of dose values

Comparison of viscoelastic flows in two- and three-dimensional serpentine channels.

Polymer solutions in the dilute regime play a significant role in industrial applications. Due to the intricate rheological properties of these highly viscoelastic fluids, especially in complex flow geometries, a thorough numerical analysis of their flow dynamics is imperative. In this research, we present a numerical investigation of purely elastic instability occurring in two- and three-dimensional serpentine channels under conditions where fluid inertia is negligible and across a broad spectrum of polymer relaxation times. Our findings reveal a strong qualitative agreement between the existing experimental results obtained from dilute solutions of flexible polymers in microfluidic devices and the numerical simulations conducted in two and three dimensions using the Oldroyd-B model. Spatial flow observations and statistical analysis of temporal flow features indicate that this purely elastic turbulent flow exhibits nonhomogeneous, non-Gaussian, and anisotropic characteristics across all scales. Additionally, our comparison of two- and three-dimensional simulations demonstrates that the elastic instability is primarily driven by the curvature of the streamlines induced by the flow geometry, rather than the weak secondary flow in the azimuthal direction. Therefore, our two-dimensional numerical simulations successfully replicate, at least qualitatively, the features observed in three-dimensional experiments. Furthermore, spectral analysis suggests that, in comparison to elastic turbulence in the dilute regime, the range of scales for the excited fluctuations is narrower.

Valence can control the nonexponential viscoelastic relaxation of multivalent reversible gels.

Gels made of telechelic polymers connected by reversible cross-linkers are a versatile design platform for biocompatible viscoelastic materials. Their linear response to a step strain displays a fast, near-exponential relaxation when using low-valence cross-linkers, while larger supramolecular cross-linkers bring about much slower dynamics involving a wide distribution of timescales whose physical origin is still debated. Here, we propose a model where the relaxation of polymer gels in the dilute regime originates from elementary events in which the bonds connecting two neighboring cross-linkers all disconnect. Larger cross-linkers allow for a greater average number of bonds connecting them but also generate more heterogeneity. We characterize the resulting distribution of relaxation timescales analytically and accurately reproduce stress relaxation measurements on metal-coordinated hydrogels with a variety of cross-linker sizes including ions, metal-organic cages, and nanoparticles. Our approach is simple enough to be extended to any cross-linker size and could thus be harnessed for the rational design of complex viscoelastic materials.

The properties of chitosan and paracetamol in acidic water mixture by viscosity study for pharmaceutical and biomedical applications

ABSTRACT Paracetamol (PC), the active ingredient in Efferalgan, is an effervescent tablet medicine used in the treatment of mild-to-moderate pain. Chitosan (CH) is a biopolymer having immense structural possibilities for chemical and mechanical modifications to generate novel properties, functions and applications, especially in the pharmaceutical area. The dynamic viscosities, η, have been measured, and investigated for the mixture of CH in acidic water and paracetamol aqueous solutions over the entire range of the temperature 20–65°C, and concentrations of 1–10 g/l. The main objective of our research described here was to determine the viscometric properties of biopolymer solutions of CH in PC, as a function of the volume fraction fCH in the CH and PC mixture and temperatures. We identified three main concentration regimes, corresponding to a dilute regime, a critical regime and the semi-dilute regime. Our results showed that the CH/PC is a promising mixture for use in pharmaceutical and antibacterial applications. The chitosan shows antimicrobial activity against various microorganisms, including bacteria.

Investigation and modeling of physicochemical properties of bentonite-chitosan composites versus the concentration of chitosan added by intercalation

This study attempts to understand the relationship between structural and physicochemical changes of clay modified by a chitosan intercalation, and to valorize the clay-chitosan composites for wastewater treatment purpose. More precisely, the main aim was a characterization and modeling of the physicochemical properties of clay-polymer composites (especially bentonite-chitosan composites) versus the concentration of chitosan added by intercalation. We first developed a method for intercalating chitosan into the clay matrix, with different polymer concentrations (6g/L, 12g/L, 18g/L, 24g/L, 27g/L and 30g/L). Second, we carried out a characterization of the structural and physicochemical properties of clay-polymer composites based on X-Rays Diffraction (XRD), Fourier Transform Infrared (FTIR) and Scanning Electric Microscopy (SEM). In particular, the results of these analyses show that the measured pores within the composites with chitosan concentrations 6g/L and 30g/L are of nanometric order, while the composite at 12g/L is rather of microscopic order. Third, XRD measurements allow the determination of the clay interlayer space (basal distance) and particles size versus the polymer concentration. For basal distance, we have demonstrated the existence of five regimes depending on the value of the polymer concentration, namely (i) dilute regime, (ii) first semi-dilute regime, (iii) well-regime, (iv) second semi-dilute regime and (v) saturation regime. Finally, these bentonite-chitosan composites may be used as kinds of filtration membranes for wastewater, retaining, for example, a dye whose adsorption quantity was found to be improved by an amount of 76.92 %.

Polyethylene Glycol Impacts Conformation and Dynamics of Escherichia coli Prolyl-tRNA Synthetase Via Crowding and Confinement Effects.

Polyethylene glycol (PEG) is a flexible, nontoxic polymer commonly used in biological and medical research, and it is generally regarded as biologically inert. PEG molecules of variable sizes are also used as crowding agents to mimic intracellular environments. A recent study with PEG crowders revealed decreased catalytic activity of Escherichia coli prolyl-tRNA synthetase (Ec ProRS), where the smaller molecular weight PEGs had the maximum impact. The molecular mechanism of the crowding effects of PEGs is not clearly understood. PEG may impact protein conformation and dynamics, thus its function. In the present study, the effects of PEG molecules of various molecular weights and concentrations on the conformation and dynamics of Ec ProRS were investigated using a combined experimental and computational approach including intrinsic tryptophan fluorescence spectroscopy, atomic force microscopy, and atomistic molecular dynamic simulations. Results of the present study suggest that lower molecular weight PEGs in the dilute regime have modest effects on the conformational dynamics of Ec ProRS but impact the catalytic function primarily via the excluded volume effect; they form large clusters blocking the active site pocket. In contrast, the larger molecular weight PEGs in dilute to semidilute regimes have a significant impact on the protein's conformational dynamics; they wrap on the protein surface through noncovalent interactions. Thus, lower-molecular-weight PEG molecules impact protein dynamics and function via crowding effects, whereas larger PEGs induce confinement effects. These results have implications for the development of inhibitors for protein targets in a crowded cellular environment.

Solidified water at room temperature hosting tailored fluidic channels by using highly anisotropic cellulose nanofibrils

Highly anisotropic cellulose nanofibrils can solidify liquid water, creating self-supporting structures by incorporating a tiny number of fibrils. These fibrillar hydrogels can contain as much as 99.99 wt% water. The structure and mechanical properties of fibrillar networks have so far not been completely understood, nor how they solidify the bulk water at such low particle concentrations. In this work, the mechanical properties of cellulose fibrillar hydrogels in the dilute regime from a wt% perspective have been studied, and an elastoplastic model describing the network structure and its mechanics is presented. A significant insight from this work is that the ability of the fibrils to solidify water is very dependent on particle stiffness and the number of contact points it can form in the network structure. The comparison between the experimental results and the theoretical model shows that the fibrillar networks in the dilute regime form via a non-stochastic process since the fibrils have the time and freedom to find contact points during network formation by translational and rotational diffusion. The formed, dilute fibrillar network deforms by sliding fibril contacts upon straining the network beyond its elastic limit. Our results also show that before macroscopic failure, the fibril contacts are restored once the load is released. The exceptional properties of this solidified water are exploited to host fluidic channels, allowing directed fluid transportation in water. Finally, the microfluidic channels formed in the hydrogels are tailored by the layer-by-layer technique to be interactive against external stimuli, a characteristic envisioned to be useful in biomedical applications.

Dynamical analysis of a ferrofluid subjected to oscillatory field and shear rates: Applications to magnetic hyperthermia

The dynamical magnetic response of a small sample of ferrofluid is investigated numerically. A validated in-house research code is used to compute the translational and rotational motion of the particles. This code uses a robust scheme of Ewald summation to compute long-range periodic torques and forces between the particles. The model of the ferrofluid consists in a suspension of magnetic hard spheres replicated in lattices to ensure the convergence of the system’s transport properties. Ensemble averages over several realizations for each simulation are carried out. The suspension is subjected to field and shear time-dependent excitations. The effect of magnetic hyperthermia is evaluated by the average rate of internal energy dissipation due to an oscillatory magnetic field and oscillatory shear rate. Nonlinear dynamical tools such as phase spaces and Poincaré maps are applied to interpret the dynamical behavior of the suspension. Results show that when the suspension reaches its saturation magnetization due to a delayed time-response of the particles the internal energy dissipation decreases with respect to the local applied field. The Poincaré maps reveal that in dilute regimes the system presents quasi-periodic behavior for an alternating magnetic field. Also, the application of an oscillatory shear-rate increases the bulk internal energy of the system due to the magneto-viscous effect, even though it diminishes the internal area of the hysteresis loop. This work brings a better understanding of the dynamical response of ferrofluids that could be useful in magnetic hyperthermia applications.

Colloid and nanoparticle-driven phase behavior in weakly perturbed nematic liquid crystals

We study theoretically weakly colloidal particle or nano-particle perturbed nematic liquid crystal (LC) order. We consider cases where the particles are essentially homogeneously dispersed in LC matrix. A mesoscopic Landau-de Gennes approach is used where we focus on the temperature variations in the amplitude order parameter field for different particle volume concentrations ϕ. In the dilute regime, we describe the nematic order in terms of interface and volume amplitude order fields. This modeling suggests that the temperature-driven order-disorder crossover exhibits a two-step transformation. In the case that particles locally support LC order, on decreasing temperature firstly the interface order is established, followed by the build-up of volume order. The latter appears via the 1st order phase transition. On the other hand, the interface order growth is formed either gradually or via the 1st order interfacial transition. In the regime of relatively high values of ϕ the system can be effectively described in terms of a single amplitude order parameter field. In this case, the temperature-driven phase behaviour is qualitatively similar to the interface order temperature response in the diluted regime. We analytically express phase transition temperatures and determine critical points in the temperature-interface interaction phase space. Key novelties of the paper are the determination of conditions for particle-LC interface-driven discontinuous change of nematic ordering and nanoparticle-enabled critical point condition in pressure-driven order-disorder phase transformations. Experimental evidences supporting the predicted phase behavior are presented.

Γ-convergence analysis of the nonlinear self-energy induced by edge dislocations in semi-discrete and discrete models in two dimensions

Abstract We propose nonlinear semi-discrete and discrete models for the elastic energy induced by a finite system of edge dislocations in two dimensions. Within the dilute regime, we analyze the asymptotic behavior of the nonlinear elastic energy, as the core-radius (in the semi-discrete model) and the lattice spacing (in the purely discrete one) vanish. Our analysis passes through a linearization procedure within the rigorous framework of Γ-convergence.

Direct Numerical Simulation of Vortex Breakdown in Evaporating Dilute Sprays

AbstractThe effects of different vortex breakdown states on the evaporation process characterizing air-acetone vapor swirling jets laden with liquid acetone droplets in the dilute regime are discussed based on results provided by direct numerical simulations. Adopting the point-droplet approximation, the carrier phase is solved using an Eulerian framework, whereas a Lagrangian tracking of the dispersed phase is used. Three test cases are investigated: one with fully-turbulent pipe inflow conditions and two with a laminar Maxworthy velocity profile at different swirl rates. Consequently, turbulent, bubble-type, and regular conical vortex breakdown states are established. Following phenomenological and statistical analyses of both phases, a significant enhancement of the overall droplet evaporation process due to the onset of the conical vortex breakdown is observed due to the strongest centrifugal forces driving the entire liquid drops towards the low-saturation mixing layer of the jet. The effects of droplet inertia on evaporation are isolated through an additional set of simulations where liquid droplets are treated as Lagrangian tracers. While it is found that inertial effects contribute to enhanced vaporization near the mixing layer under bubble vortex breakdown conditions, droplet inertia plays a secondary role under both turbulent and conical vortex breakdown due to intense turbulent mixing and high centrifugal forces, respectively.

Particle-based model of liquid crystal skyrmion dynamics.

Motivated by recent experimental results that reveal rich collective dynamics of thousands-to-millions of active liquid crystal skyrmions, we have developed a coarse-grained, particle-based model of the dynamics of skyrmions in a dilute regime. The basic physical mechanism of skyrmion motion is related to squirming undulations of domains with high director twist within the skyrmion cores when the electric field is turned on and off. The motion is not related to mass flow and is caused only by the reorientation dynamics of the director field. Based on the results of the "fine-grained" Frank-Oseen continuum model, we have mapped these squirming director distortions onto an effective force that acts asymmetrically upon switching the electrical field on or off. The resulting model correctly reproduces the skyrmion dynamics, including velocity reversal as a function of the frequency of a pulse width modulated driving voltage. We have also obtained approximate analytical expressions for the phenomenological model parameters encoding their dependence upon the cholesteric pitch and the strength of the electric field. This has been achieved by fitting coarse-grained skyrmion trajectories to those determined in the framework of the Frank-Oseen model.

Microrheology of gemini surfactants at interfaces and in solutions in the dilute and semidilute regimes.

Gemini surfactants are ideal systems to study a wide range of rheological behaviours in soft matter, showing fascinating analogies with living polymers and polyelectrolytes. By only changing the concentration, the shear viscosity can vary by 7 orders of magnitude in the bulk when transitioning through the semidilute regime. In order to elucidate on the intrinsic shear viscosity profile at the interface in soft matter systems manifesting various concentration regimes and morphological transitions, we performed microrheology and adsorption experiments under a wide range of experimental conditions. The surface shear viscosity has been characterized by passive microrheology, tracking Brownian particles trapped at the air-solution interface, under particle wetting conditions precisely characterized by interferometry. We observe that a steep increase in bulk shear viscosity as a function of the concentration does not translate at the interface, which may show a negative surface shear viscosity. By comparing macrorheology and microrheology, we measure significant differences both at the interface and in the bulk in the semidilute regime, where wormlike micelles start to entangle. The disparity in rheological measurements can be attributed to notable depletion effects near both the air-solution and particle-solution interfaces.

Pinching dynamics and extensional rheology of dense colloidal suspensions with depletion attractions

We study the extensional flow properties by characterizing the capillarity-driven pinching dynamics of dense colloidal suspensions at a constant volume fraction ϕ=0.40 with polymer-induced depletion interactions using a dripping-onto-substrate (DoS) protocol. Methacrylate copolymer particles with dimethylacrylamide copolymer brushes are suspended in a refractive-index- and density-matched mixture of 80 (w/w)% glycerol in water with NaCl added to screen the electrostatic repulsions. Depletion attractions between the colloids are introduced by adding polyacrylamide polymers of weight and dispersity. The addition of polymer delays and modifies the pinch-off dynamics of the dense suspensions, depending on the size and dispersity of the polymer. The extensional relaxation time λE of suspensions collapses as a function of the normalized free volume polymer concentration c/c∗ with the corresponding polymer solutions, indicating that the elastic properties of the polymer solutions control the extensional time scale. Following the results of our previous study [Soetrisno et al., Macromolecules 56, 4919–4928 (2023)], the polymer size determines the scaling exponent of λE for colloid-polymer mixtures in the dilute regime and high dispersity shifts the concentration where the scaling of λE transitions from power-law to linear. The filament lifespans tf of colloid-polymer mixtures and of polymer solutions collapse onto a master curve as a function of c/c∗ when normalized by the filament lifespan of the corresponding fluid without polymer tf,0. These results provide insight into the role of the polymer size in dictating the pinching dynamics and extensional rheology of colloid-polymer mixtures and further suggest that the shear and extensional responses of these mixtures can be separately tuned through the concentrations of the two constituents.

Sodium Carboxymethylcellulose Rheological Behavior in a Water Mixture for Pharmaceutical and Biomedical Applications

Sodium carboxymethylcellulose Na-CMC is derived from cellulose fibers. Knowledge of the behavior of Na-CMC as a charged polysaccharide in solution is important, because most of its biomedical industrial applications utilize its solutions. The main objective of our research described here was to determine the rheological and viscometric properties of polysaccharide solutions of Na-CMC in water (W), as a function of the cutting speed and the polymer concentration. The evolution of reduced dynamic viscosities and the effect of the Gibbs energy of Na-CMC in W were characterized as a function of the temperature (15 °C to 45 °C), and concentration (1 to 10) g/l. We identified three main concentration regimes, corresponding to a dilute regime, a critical regime, and the semi-dilute regime. The thermodynamic parameters of the viscosity also supported the obtained results. Our results show that Na-CMC/W is a promising solvent mixture for use in pharmaceutical and antibacterial applications.