Bulk Fluid Research Articles

Synchronization and metachronal waves of elastic cilia caused by unsteady viscous flow

Hydrodynamic coordination of cilia is ubiquitous in biology. It is commonly modeled using the steady Stokes equations. The flow around ciliated cells, however, exhibits finite-time vorticity diffusion, requiring a dynamical description. We present a model of elastic cilia coupled by unsteady viscous flow in the bulk fluid. Therein, vorticity diffusion impacts cilia coordination qualitatively and quantitatively. In particular, pairs of cilia synchronize in antiphase for long diffusion times. Moreover, metachronal waves occur in cilia chains larger than the viscous penetration depth, whereas global synchronization occurs in Stokes flow. Published by the American Physical Society 2025

Unveiling Nonlinear Modes of Induced-Charge Electro-Osmosis in AC Electric Fields.

Induced-charge electro-osmosis (ICEO) is prevalent in electrochemistry and micro/nanofluidics, even when subjected to an alternating current (AC) electric field. Previous studies suggest that ICEO primarily occurs near electrodes or ion-selective membranes within a range of tens of microns. In this experimental study, we reveal the existence of additional modes in ICEO driven by an AC electric field. We demonstrate that nonlinear electroosmotic flow (EOF) can occur between two electrodes situated 5 mm apart through long-range electroconvection. Notably, we observe a soliton-like wave of ion concentration fluctuation traveling from the working electrode to the ground electrode, causing significant disruption to the electric triple layer (ETL) on the ground electrode. Consequently, both the ETL structure on the electrodes and the bulk fluid are profoundly influenced by an external electric field. These findings are vital for enhancing the understanding of intrinsic dynamics in electrochemical and energy systems and for the development of novel micro/nanofluidic devices.

Analytical protocol for measuring micro-molar quantities of sulfur volatile species in experimental high pressure and temperature fluids

Validating thermodynamic models is essential in experimental geosciences for exploring increasingly complex systems and developing analytical protocols. However, investigating solid–fluid equilibria in mm3-sized experimental capsules poses several challenges, particularly in sulfur-bearing chemical systems. These include maintaining bulk fluid composition and performing quantitative analysis with extremely low amounts of synthesized fluid. We present an innovative methodology for measuring ultra-low amounts of sulfur volatiles (H2S and SO2) generated during experimental runs at high pressure and temperature conditions of 3 GPa and 700 °C. Using solid sulfides (FeS + FeS2) and water as reactants, we performed redox-controlled syntheses employing a piston cylinder apparatus. We demonstrate that ex-situ measurements of these fluids by quadrupole mass spectrometry ensure accurate and precise analysis, confirming predicted thermodynamic compositions. This methodology allows in-depht investigation of sulfide solid–fluid equilibria, shedding light on sulfur volatiles behavior and geochemical cycles under high P–T conditions characteristic of the Earth’s interior.

Density Functional Study on the Condensation and Evaporation Heats of the Nanoconfined Fluids.

Within the framework of classical density functional theory, the isosteric heat and condensation/evaporation heat of the confined methane are studied. First, the theoretical expression for condensation/evaporation heat is derived on the basis of the first law of thermodynamics. The method for computation is also proposed. With the proposed method, the dependences of the condensation/evaporation heat on temperature, pore width, and surface strength are studied and analyzed in detail. First of all, it is found that the condensation/evaporation heat of the confined fluid decreases monotonically with the temperature. Moreover, it is also found that at the same temperature the condensation heat is usually larger than the corresponding evaporation heat, which confirms the irreversibility of the condensation and evaporation from an exothermic point of view. Furthermore, our results reveal that the condensation/evaporation heat of confined fluid in pores decreases with the width of the pore. With the further increase in the pore width, the condensation/evaporation heat tends gradually to that of the corresponding bulk fluids. In addition, an interesting result is found that the condensation/evaporation heat is insensitive to variation in surface strength.

The bcc coating of Lennard-Jones crystal nuclei vanishes with a change of local structure detection algorithm.

Since the influential work of ten Wolde, Ruiz-Montero, and Frenkel [Phys. Rev. Lett. 75, 2714 (1995)], crystal nucleation from a Lennard-Jones fluid has been regarded as a paradigmatic example of metastable crystal ordering at the surface of a critical nucleus. We apply seven commonly used local structure detection algorithms to characterize crystal nuclei obtained from transition path sampling simulations. The polymorph composition of these nuclei varies significantly depending on the algorithm used. Our results indicate that one should be very careful when characterizing the local structure near solid-solid and solid-fluid interfaces. Particles near such interfaces exhibit a local structure distinct from that of bulk fluid or bulk crystal phases. We argue that incorporating outlier detection into the local structure detection method is beneficial, leading to greater confidence in the classification results. Interestingly, the bcc coating nearly disappears when adopting a machine learning method with outlier detection.

Starting Electroosmosis in a Fibrous Porous Medium with Arbitrary Electric Double-Layer Thickness

The transient electroosmotic response in a charged porous medium consisting of a uniform array of parallel circular cylindrical fibers with arbitrary electric double layers filled with an electrolyte solution, for the stepwise application of a transverse electric field, is analyzed. The fluid momentum conservation equation is solved for each cell by using a unit cell model, where a single cylinder is surrounded by a coaxial shell of the electrolyte solution. A closed-form expression for the transient electroosmotic velocity of the bulk fluid in the Laplace transform is obtained as a function of the ratio of the cylinder radius to the Debye screening length and the porosity of the fiber matrix. The effect of the fiber matrix porosity on the continuous growth of the electroosmotic velocity over time is substantial and complicated. For a fiber matrix with larger porosity, the bulk fluid velocity takes longer to reach a certain percentage of its final value. Although the final value of the bulk fluid velocity generally increases with increasing porosity, early velocities may decrease with increasing porosity. For a given fiber matrix porosity, the transient electroosmotic velocity is a monotonically increasing function of the ratio of the cylinder radius to the Debye length.

Hydrodynamic modes in nano-channels

This paper investigates the local hydrodynamics of a dense fluid confined in nanoscale slit-pores with different heights. Using non-equilibrium molecular dynamics simulations of the fluid system, we induce a steady-state sinusoidal velocity profile across the channel having a characteristic wavelength, thus, probing the fluid response to a specific Fourier mode. As expected, for sufficiently large channel heights and wavelengths there is an excellent agreement between the hydrodynamic predictions and simulation data. As the wavelength decreases to around 5 molecular diameters, the classical hydrodynamics fails to predict the steady-state velocity profile; we attribute this to the non-local nature of the fluid response and the presence of density gradients in the wall–fluid interfacial region. Using generalized hydrodynamics and the Fourier spectrum of the density profile, we derive the strain rate amplitude and shear pressure corrections due to these two effects. The local relaxation from the steady-state to the zero flow situation is tracked for different channel heights and wavelengths. The relaxation is in general visco-elastic in the wall–fluid region, and we argue that this phenomenon is the mechanism behind the “enhanced viscosity” used in the literature. We also report a surprising dynamics for the fluid located between the wall–fluid region and bulk region, which cannot be explained by classical hydrodynamics; here, an initial exponential relaxation abruptly transitions into a linear relaxation. The work highlights the many different physical mechanisms present in nano-confined fluids, and that the fluid response is in general position and wavelength dependent.

Effect of foulant on temperature and steam quality profiles in once-through steam generator tubes

Once-through steam generators (OTSGs) are commonly used in the oil sands industry for steam generation forin-situthermal recovery of extra heavy oil. These are expensive operations with challenges associated with the deposition of foulants, arising from solids and hydrocarbons in the boiler feed water, on the inner tube wall that leads to a decreasedheat transfer rate. The foulant deposition also causes elevated tube-wall temperatures, which may lead to tube failure. This study examines the impact of foulant on the tube-wall temperature profile and vapor-to-liquid ratio in an OTSG. A three-dimensional finite volume model of multiphase flow and heat transfer in a single pass of a field-scale OTSG unit is described. Detailed flow behavior and heat transfer characteristics of water and steam from the CFD model were compared to field data. Thereafter, fouling was added to the model to explore overheating and erosion zones in the OTSG tubes. Velocity, temperature distribution, and steam generation in the OTSG tubes in the convection section and radiant section were investigated for areas prone overheating. It was found that the CFD model simulation results closely matched the bulk fluid temperature of the field unit. In the presence of fouling, the outer skin temperature from the model was well within the range of the thermocouple data from the field unit, enabling the application of the model to identify the foulant thickness based on the outer skin temperature. Lastly, the model demonstrated that the bends are the most likely sites for erosion within the pass.

Mass transport modelling of two partially miscible, multicomponent fluids in nanoporous media

High-pressure fluid transport in nanoporous media such as shale formations requires further understanding because conventional continuum approaches become inadequate due to their ultralow permeability and complexity of transport mechanisms. We propose a species-based approach for modelling two partially miscible, multicomponent fluids in nanoporous media – one that does not rely on conventional bulk fluid transport frameworks but on species movement. We develop a numerical model for species transport of partially miscible, non-ideal fluid mixtures using the chemical potential gradient as the driving force. The model considers the binary friction concept to include the friction between fluid molecules as well as between fluid molecules and pore walls, and incorporates the key multicomponent transport mechanisms – Knudsen, viscous and molecular diffusion. Under single-phase conditions, the system under consideration is quantified by introducing multicomponent Sherwood number (Sh), Péclet number (Pe) and fluid–solid friction modulus (φ). Despite the complexity of fluid transport in nanopores, the steady-state single-phase transport results reveal the contribution of diffusion in nanopores, where all parameters collapse on a set of master curves for the multicomponent Sh with a dependence on multicomponent Pe and φ. Unsteady state, two-phase transport modelling of the codiffusion process shows that light and intermediate alkanes are produced much higher than heavy alkanes when the vapour phase appears. We demonstrate that the pressure gradient is also crucial in promoting CO2 and alkane mixing during counterdiffusion processes. These results stress the need for a paradigm shift from classical bulk flow modelling to species-based transport modelling in nanoporous media.

Machine learning-inspired study of dynamical parameters of single vapor bubble under nucleate flow boiling regime

Heat transfer performance of nucleate boiling is interlinked with the departure dynamics of vapor bubbles, which also serves as the basis for heat transfer partitioning schemes and bubble growth models. Given the fast transients and multiple bubble ebullition cycles, boiling experiments lead to voluminous data that are to be analysed for determining the bubble dynamic parameters, and subsequently, the boiling heat transfer rates. Conventional approach of manual handling of such temporal and spatially-resolved experimental data is inherently time-consuming and error-prone. In the backdrop of these limitations, importance of machine learning algorithms gets highlighted in making the entire process automated and accurate as they minimize the need of any human intervention. The present work explores the potential of machine learning techniques towards quantifying the bubble departure characteristics and heat transfer partitioning during nucleate flow boiling. Water-based experiments have been conducted in a vertical channel under varying subcooling levels (ΔTsub = 2, 5, and 8 K) and flow rates (Re = 2400, and 3600). Mask R-CNN machine learning (ML) model is employed to evaluate the temporal variations of dynamical parameters of vapor bubbles and departure characteristics. The bubble departure characteristics as well as the corresponding evaporative and convective heat transfer rates, as retrieved through ML-generated masks, showed a strong dependence on the level of bulk fluid subcooling and flow rates. Heat transfer partitioning analysis revealed a competing interplay between the evaporative and condensation components of overall boiling heat transfer rates. These findings demonstrate the potential of ML techniques towards optimizing the thermal performance of boiling phenomena that have significant implications in a range of critical engineering applications.

Analytical model of small- and large-amplitude drop oscillation dynamics

The mechanical energy balance over a bulk of fluid that oscillates between prolate and oblate shapes in another immiscible fluid is solved using a general spheroidal coordinate system. The drop shape is described by a unique parameter that may continuously vary over the time, making the implementation of the model rather simple. Potential flow is assumed, and inertial and viscous effects are accounted for in both the inner and outer flow fields. The characteristics of drop oscillation for small and large amplitudes are studied, and the results are compared with theoretical, experimental, and numerical data from the open literature. The rather satisfactory validation of the model over a large variety of operating conditions allows its extension to include other physical phenomenon, like evaporation and its effect on drop oscillation.

Thermophysical Properties of the Hydrogen Carrier System Based on Aqueous Solutions of Isopropanol or Acetone

One concept for the safe storage and transport of molecular hydrogen (H2) is the use of hydrogen carrier systems which can bind and release hydrogen in repeating cycles. In this context, the liquid system based on isopropanol and its dehydrogenated counterpart acetone is particularly interesting for applications in direct isopropanol fuel cells that are operated with an excess of water. For a comprehensive characterization of diluted aqueous solutions of isopropanol or acetone with technically relevant solute amount fractions between 0.02 and 0.08, their liquid density, liquid viscosity, and interfacial tension were investigated using various light scattering and conventional techniques as well as equilibrium molecular dynamics (EMD) simulations between (283 and 403) K. Polarization-difference Raman spectroscopy (PDRS) was used to monitor the liquid-phase composition during surface light scattering (SLS) experiments on viscosity and interfacial tension. For comparison purposes and to expand the database, capillary viscometry and dynamic light scattering (DLS) from bulk fluids with dispersed particles were also applied to determine the viscosity while the pendant-drop (PD) method allowed access to the interfacial tension. By adding isopropanol or acetone to water, density and, in particular, interfacial tension decrease significantly, while viscosity shows a pronounced increase. The behavior of viscosity and interfacial tension is closely related to the strong hydrogen bonding between the unlike mixture components and the pronounced enrichment of both solutes at the vapor–liquid interface, as revealed by EMD simulations. For an aqueous solution with an isopropanol amount fraction of 0.04, minor variations in interfacial tension and viscosity were found in the presence of pressurized H2 up to 7.5 MPa. Overall, the results from this study contribute to an extended database for diluted aqueous solutions of isopropanol or acetone, especially at temperatures above 323 K.

Reproducing hydrodynamics and elastic objects: A hybrid mesh-free model framework for dynamic multi-phase flows

Reproducing Hydrodynamics and Elastic Objects (RHEO) is a new meshfree modeling framework for simulating complex multi-phase flows of fluids and solids. RHEO is implemented within the open-source Large-Scale Atomic/Molecular Massively Parallel Simulator particle dynamics code and couples a reproducing kernel smoothed particle hydrodynamics scheme for modeling fluid flow and heat transfer with a bonded particle model for modeling breakable elastic bodies. The resulting scheme provides a robust framework for simulating multi-phase material systems with complex and evolving boundaries and interfaces. RHEO collects many advanced mesh-free modeling features into a centralized, modular, and easily extensible package, including heat transport, reversible melting and solidification, particle distribution regularization, and user adjustable kernel accuracy. We report comprehensive tests of RHEO's accuracy and convergence for common benchmark flows of bulk fluids, boundary driven flows, and complex fluid/solid systems. A series of multi-phase systems are highlighted, including a bouncing water balloon, the fracture and flow of a brittle egg, melting of a free-standing solid beam, and conductive cooling and solidification of an extruded polymer during 3D printing.

Current Analytical Methods and Their Limitations in Quantifying Antidiabetic Drug Nateglinide and Repaglinide in Pharmaceutical Formulations and Biological Fluids

Pharmaceutical growth dominated a transformation in human health. These drugs need to attend to their target only, so they must be free from impurities and appropriately controlled. Due to that, diverse instrumental techniques were advanced at steady intervals to accomplish their intention to quantify the limits per the regulatory. NTG and RPG pharmaceuticals might generate impurities during the development phases, packing, and shipping, which could be risky to administer. Hence, detecting and quantifying them using various analytical techniques at multiple stages is necessary. This review highlights the function of different analytical methods, including UV–Vis, HPLC, HPTLC, UPLC, HPLC/MS and UPLC/MS, in quantifying drugs, impurities and metabolites in bulk, pharmaceutical formulations and biological fluids. Also, it discussed the specific advantages and limitations of individual techniques. It compared them regarding sensitivity, specificity, cost, time consumption, efficacy, and the practical challenges of implementing these analytical techniques in real-world settings to determine pharmaceuticals.

Synthesis of Co/Co3O4 Heterostructure in N-Doped Porous, Amorphous Carbon: A Superior Electrochemical Sensor for Sensitive Determination of Alectinib in Various Fluids.

Highly crystallized Co and Co3O4 nanoparticles embedded in an N-doped amorphous carbon matrix have been successfully fabricated by the molten-salt-assisted method using KClO3 and zeolitic imidazolate framework-12 (ZIF-12). Pyrolysis of ZIF-12 with different concentrations of KClO3 leads to embedded Co and Co3O4 nanoparticles in a conductive amorphous carbon network. The impact of salt concentration on the morphology and electrochemical performance of these composites was investigated for electrochemical sensor applications. By employing a straightforward and efficient technique, Co/Co3O4 heterostructures were successfully synthesized in N-doped porous amorphous carbon. The Co/Co3O4 carbon heterostructures were optimized by varying the salt concentration, resulting in a significant electrochemical sensor performance for detecting ALC in both bulk and biological fluids. The sensor demonstrates excellent sensitivity (62.97 nmol/L) and selectivity toward ALC, with a wide linear range (0.2-2 μM) and a low detection limit (18.89 nM). Furthermore, it displays remarkable stability and reproducibility, positioning it as a strong contender for practical use in pharmaceutical analysis and biomedical research. This study presents a significant advancement in electrochemical sensing technology and underscores the potential of Co/Co3O4 heterostructures in the development of high-performance sensors for detecting bioactive compounds in complex matrices.

The Role of the Working Fluid and Non-Ideal Thermodynamic Effects on Performance of Gas Lubricated Bearings

Abstract Small-scale turbomachinery operating at high rotational speed is a key technology for increasing the power density of energy and propulsion systems. A notable example is the turbine of an organic Rankine cycle turbogenerator for thermal recuperation from prime engines and industrial processes. Such systems typically operate with organic compounds characterized by complex molecular structures, to allow the design of efficient fluid machinery and flexibility in matching the heat source and sink temperature profiles. Gas bearings are considered advantageous compared to traditional oil-lubricated rolling element bearings for supporting the turbine rotor, enabling greater machine compactness and reduced complexity, and to avoid contamination of the working fluid. In certain operating conditions, however, the lubricant of the bearing is in thermodynamic states near the saturated vapor line or in the vicinity of the fluid critical point whereby non-ideal effects are relevant and may affect bearing performance. This work investigates the physics of thin film flows in gas bearings operating with fluids made by complex molecules. The influence of non-ideal thermodynamic effects on gas bearing performance is discussed by analysis of the fluid bulk modulus. Reduced values of the non-dimensional bulk modulus near the critical point or saturated vapor line decrease bearing performance. The main parameter characterizing the influence of molecular complexity on bearing performance is shown to be the acentric factor. For complex fluids with large acentric factors, the impact of non-ideal thermodynamic effects on non-dimensional bearing load capacity and rotor-dynamic characteristics is less pronounced.

High thermohydraulic performance of solar air collector utilizing high turbulence deflectors and porous media

In this research investigation, high thermohydraulic performance of a solar air heater is obtained utilizing a deflector attached to the back plate and inserted porous media in the channel that generators high turbulence. The optimum values of the geometrical parameters such as fin pitch 0.02m≤P≤0.04m,fin height 0.005m≤H≤0.02m and channel porosity 80%≤ϕ≤100% and flow parameter, i.e., Reynolds number 2000≤Re≤11000 are obtained using numerical investigation. The obtained results present impressive facts through contours of velocity and temperature, and ensured the manifestation of high heat transfer to the flowing air in the investigated design. It is revealed that due to the use of deflectors, the bulk fluid got deflected towards the hot absorber plate, while the both deflector plus porous media in the channel causes high turbulence and consequently high thermal and thermohydraulic (THPP) performance is achieved. Numerical results presented interesting results of occurring the wavy flow inside the design of porous solar air heater with deflector. The THPP of the deflector plus porous media design is obtainted as 15, while it is about 1.2 for the design consisting of only deflectors. The optimum value of the combined deflector plus porous media design is obtained as P = 0.04 m, H = 0.02 m, ϕ=95% and Re = 8000, while it is P = 0.06 m, H = 0.02 m and Re = 8000 for the only simple deflector design.

COMPOSITES BASED ON OYMASH SALT

In recent years, an urgent task is to find materials of domestic origin and create on their basis affordable and cheap, easy-to-use, reliable composite compositions for the preparation of well killing fluid. Nowadays, the need for process fluids that help preserve and restore reservoir properties always remains. Available and inexpensive natural salt from the Oimash deposit (Mangistau region) can create the required density in the well killing fluid and be its main component. The aim of the work is to develop compositions based on Oimash salt for the preparation of well killing fluids, which include all necessary additives. Methodology: The physicochemical characteristics of the composite compositions for the preparation of well killing fluid (bulk density, mass fraction of insoluble substances in water, mass fraction of moisture, solubility) and using standard methods corrosion activity studied. Results. The physicochemical parameters of Oimash salt showed that the salt is not prone to caking, the proportion of insoluble substances in sea water is 0.45%, and in waste water - 0.54%. The solubility of the developed compositions depending on the types of water was studied. All compositions are soluble in various types of water. The solubility of all compositions in seawater is 96.64-97.56%, and in wastewater from 95.59% to 95.97%. The optimal composition of the salt composition is the composition of sample No. 1 with the percentage content of components TPPF: NTF: PAA = 0.5: 0.5: 0.1:30, with a bulk density of 0.7055 t/m3, a mass fraction of insoluble substances in water - 1.4334%, well soluble in sea and waste water 97.56 and 95.66%. As the results of samples No. 1 showed, the corrosive environment is slightly aggressive in terms of aggressiveness.

Confined fluid dynamics in a viscoelastic, amorphous, and microporous medium: Study of a kerogen by molecular simulations and the generalized Langevin equation.

We combine the use of molecular dynamics simulations and the generalized Langevin equation to study the diffusion of a fluid adsorbed within kerogen, the main organic phase of shales. As a class of microporous and amorphous materials that can exhibit significant adsorption-induced swelling, the dynamics of the kerogen's microstructure is expected to play an important role in the confined fluid dynamics. This role is investigated by conducting all-atom simulations with or without solid dynamics. Whenever the dynamics coupling between the fluid and solid is accounted for, we show that the fluid dynamics displays some qualitative differences compared to bulk fluids, which can be modulated by the amount of adsorbed fluid owing to adsorption-induced swelling. We highlight that working with the memory kernel, the central time correlation function of the generalized Langevin equation, allows the fingerprint of the dynamics of the solid to appear on that of the fluid. Interestingly, we observe that the memory kernels of fluid diffusion in kerogen qualitatively behave as those of tagged particles in supercooled liquids. We emphasize the importance of reproducing the velocity-force correlation function to validate the memory kernel numerically obtained as confinement enhances the numerical instabilities. This route is interesting as it opens the way for modeling the impact of fluid concentration on the diffusion coefficient in such ultra-confining cases.

Computational fluid dynamics simulation of cryogenic vertical upflow boiling under Earth gravity

This study presents numerical simulations and their validation for flow boiling of liquid nitrogen (LN2) in a vertical upflow orientation, with a primary aim to understand the complex two-phase flow and heat transfer phenomena important to space applications. The computational fluid dynamics (CFD) model utilized the coupled level set volume of fluid (CLSVOF) method, incorporating additional source terms for bubble collision dispersion force and shear lift force in the momentum conservation equation to enhance simulation accuracy. The simulations were conducted for two mass velocities (G = 526 and 804 kg/m2s) and three different heat flux levels (approximately 10%, 30%, and 70% of critical heat flux (CHF) under Earth gravity. The model was validated against measured wall temperature data acquired from the authors’ previous experimental studies, demonstrating average deviations of less than 2.8 K across all operating conditions. The simulated two-phase flow contours illustrated various flow patterns, including bubbly, slug, churn, and annular. Both mass velocity and heat flux were observed to impact the onset of nucleate boiling (ONB), bubble nucleation, growth, and coalescence, and overall vapor structure. The simulations also offered insight into axial and radial void fraction and velocity profiles, revealing local flow acceleration trends synchronized with void fraction development. A comparison between predicted and measured bulk fluid temperature profiles showed excellent agreement, further validating the CFD model’s accuracy and practical usefulness for two-phase cryogenic flow boiling simulations in space applications.