A study of the anisotropy of stress in a fluid confined in a nanochannel

A Review of Laser-Induced Crystallization from Solution

Crystallization abounds in nature and industrial practice. A plethora of indispensable products ranging from agrochemicals and pharmaceuticals to battery materials are produced in crystalline form in industrial practice. Yet, our control over the crystallization process across scales, from molecular to macroscopic, is far from complete. This bottleneck not only hinders our ability to engineer the properties of crystalline products essential for maintaining our quality of life but also hampers progress toward a sustainable circular economy in resource recovery. In recent years, approaches leveraging light fields have emerged as promising alternatives to manipulate crystallization. In this review article, we classify laser-induced crystallization approaches where light-material interactions are utilized to influence crystallization phenomena according to proposed underlying mechanisms and experimental setups. We discuss nonphotochemical laser-induced nucleation, high-intensity laser-induced nucleation, laser trapping-induced crystallization, and indirect methods in detail. Throughout the review, we highlight connections among these separately evolving subfields to encourage the interdisciplinary exchange of ideas.

Hydrophobic Nanoconfinement Enhances CO2 Conversion to H2CO3

Understanding the formation of H₂CO₃ in water from CO₂ is important in environmental and industrial processes. Although numerous investigations have studied this reaction, the conversion of CO₂ to H₂CO₃ in nanopores, and how it differs from that in bulk water, has not been understood. We use ReaxFF metadynamics molecular simulations to demonstrate striking differences in the free energy of CO₂ conversion to H₂CO₃ in bulk and nanoconfined aqueous environments. We find that nanoconfinement not only reduces the energy barrier but also reverses the reaction from endothermic in bulk water to exothermic in nanoconfined water. Also, charged intermediates are observed more often under nanoconfinement than in bulk water. Stronger solvation and more favorable proton transfer with increasing nanoconfinement enhance the thermodynamics and kinetics of the reaction. Our results provide a detailed mechanistic understanding of an important step in the carbonation process, which depends intricately on confinement, surface chemistry, and CO₂ concentration.

Nanoconfinement facilitates reactions of carbon dioxide in supercritical water

The reactions of CO2 in water under extreme pressure-temperature conditions are of great importance to the carbon storage and transport below Earth's surface, which substantially affect the carbon budget in the atmosphere. Previous studies focus on the CO2(aq) solutions in the bulk phase, but underground aqueous solutions are often confined to the nanoscale, and nanoconfinement and solid-liquid interfaces may substantially affect chemical speciation and reaction mechanisms, which are poorly known on the molecular scale. Here, we apply extensive ab initio molecular dynamics simulations to study aqueous carbon solutions nanoconfined by graphene and stishovite (SiO2) at 10 GPa and 1000 ~ 1400 K. We find that CO2(aq) reacts more in nanoconfinement than in bulk. The stishovite-water interface makes the solutions more acidic, which shifts the chemical equilibria, and the interface chemistry also significantly affects the reaction mechanisms. Our findings suggest that CO2(aq) in deep Earth is more active than previously thought, and confining CO2 and water in nanopores may enhance the efficiency of mineral carbonation.

History-Dependent Stress Relaxation of Liquids under High-Confinement: A Molecular Dynamics Study

When liquids are confined into a nanometer-scale slit, the induced layering-like film structure allows the liquid to sustain non-isotropic stresses and thus be load-bearing. Such anisotropic characteristics of liquid under confinement arise naturally from the liquids’ wavenumber dependent compressibility, which does not need solidification to take place as a prerequisite. In other words, liquids under confinement can still retain fluidity with molecules being (sub-)diffusive. However, the extensively prolonged structural relaxation times can cause hysteresis of stress relaxation of confined molecules in response to the motions of confining walls and thereby rendering the quasi-static stress tensors history-dependent. In this work, by means of molecular dynamics, stress tensors of a highly confined key base-oil component, i.e., 1-decene trimer, are calculated after its relaxation from being compressed and decompressed. A maximum of 77.1 MPa normal stress discrepancy has been detected within a triple-layer boundary film. Analyses with respect to molecular morphology indicate that among the effects (e.g., confinement, molecular structure, and film density) that can potentially affect confined stresses, the ordering status of the confined molecules plays a predominant role.

Effect of charge inversion on nanoconfined flow of multivalent ionic solutions

A comprehensive understanding of fluid dynamics of dilute electrolyte solutions in nanoconfinement is essential to develop more efficient nanofluidic devices. In nanoconduits, the electrical double layer can occupy a considerable...

Stratification Mechanism in the Bidisperse Colloidal Film Drying Process: Evolution and Decomposition of Normal Stress Correlated with Microstructure

The evolution of the normal stress and microstructure in the drying process of bidisperse colloidal films is studied using the Brownian dynamics simulation. Here, we show that the formation process of small-on-top stratification can be explained by normal stress development. At high Pe_L's, a stratified layer with small particles is formed near the interface. The accumulated particles near the interface induce the localization of normal stress so that the normal stress at the interface increases from the beginning of drying. We analyze this stress development from two points of view, on the global length scale and particle length scale. On the global length scale, the localization of normal stress is quantified by the scaled normal stress difference between the interface and substrate. For all Pe_L's tested in this study, the scaled normal stress difference increases until the accumulation region reaches the substrate. After the maximum, the stress difference remains at the maximum at lower Pe_L's, while it decreases at higher Pe_L's. The microstructural analysis shows that this stress development is explained through the evolution of the particle contact number distribution at the interface and substrate. On the particle length scale, we derive the scaled local force applied to each type of particle by decomposing the local normal stress. At high Pe_L's, the scaled local force for the large particle is large compared to that for the small particle near the interface, indicating that the large particles are strongly pushed away from the interface. Associating the volume fraction profile with the local force field, we suggest that the strong scaled force for the large particle is attributed to the significant increase in the average number of small particles in contact with large ones. This study has significance in probing the drying mechanism of bidisperse colloidal films and the stratification mechanism.

Machine Learning Techniques for Fluid Flows at the Nanoscale

Simulations of fluid flows at the nanoscale feature massive data production and machine learning (ML) techniques have been developed during recent years to leverage them, presenting unique results. This work facilitates ML tools to provide an insight on properties among molecular dynamics (MD) simulations, covering missing data points and predicting states not previously located by the simulation. Taking the fluid flow of a simple Lennard-Jones liquid in nanoscale slits as a basis, ML regression-based algorithms are exploited to provide an alternative for the calculation of transport properties of fluids, e.g., the diffusion coefficient, shear viscosity and thermal conductivity and the average velocity across the nanochannels. Through appropriate training and testing, ML-predicted values can be extracted for various input variables, such as the geometrical characteristics of the slits, the interaction parameters between particles and the flow driving force. The proposed technique could act in parallel to simulation as a means of enriching the database of material properties, assisting in coupling between scales, and accelerating data-based scientific computations.

Surface Nanostructure-Wettability Coupling Leads to Unique Topological Evolution Dictating Water Transport over Nanometer Scales

Surface nanostructure, either designed or generated as an artifact of the fabrication procedure, is known to influence interfacial phenomena intriguingly. While surface roughness-wettability coupling over nanometer scales has been addressed to some extent, the explicit interplay of hydrodynamics and confinement toward dictating the underlying characteristics for practically relevant material interfaces remains unexplored. Here, we bring out unique roles of surface nanostructures toward altering flow of water in a copper nanochannel, by capturing an exclusive interplay of confinement, roughness, wettability and flow dynamics. Toward this, non-equilibrium molecular dynamics (NEMD) simulations are performed to examine the effect of nanoscale triangular roughness. The width and height of the triangular microgroove are varied along with different driving forces at the channel inlet, and the results are compared with those corresponding to smooth-walled nanochannels. We also unveil the nontrivial characteristics of the interfacial topology as a consequence of spontaneous phase separation at the fluid-solid interface. For a constant driving force, we show that the interface may exhibit concave or convex topology, depending on the nanogroove geometry. Our results provide new vistas on how designed nanoscale roughness structures can be harnessed toward controlling the transport of water in a practically engineered nanosystem, as demanded by the specific application on hand.

Prediction of fluid slip in cylindrical nanopores using equilibrium molecular simulations

We introduce an analytical method to predict the slip length (L _s) in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations, following the approach proposed by Sokhan and Quirke for planar channels [39]. Using this approach, we determined the slip length of water in carbon nanotubes (CNTs) of various diameters. The slip length predicted from our method shows excellent agreement with the results obtained from nonequilibrium molecular dynamics (NEMD) simulations. The data show a monotonically decreasing slip length with an increasing nanotube diameter. The proposed EMD method can be used to precisely estimate slip length in high slip cylindrical systems, whereas, L _s calculated from NEMD is highly sensitive to the velocity profile and may cause large statistical errors due to large velocity slip at the channel surface. We also demonstrated the validity of the EMD method in a BNNT-water system, where the slip length is very small compared to that in a CNT pore of similar diameter. The developed method enables us to calculate the interfacial friction coefficient directly from EMD simulations, while friction can be estimated using NEMD by performing simulations at various external driving forces, thereby increasing the overall computational time. The EMD analysis revealed a curvature dependence in the friction coefficient, which induces the slip length dependency on the tube diameter. Conversely, in flat graphene nanopores, both L _s and friction coefficient show no strong dependency on the channel width.

Molecular dynamics simulation of nanofluidics

This review reports the progress on the recent development of molecular dynamics simulation of nanofluidics. Molecular dynamics simulations of nanofluidics in nanochannel structure, surface roughness of nanochannel, carbon nanotubes, electrically charged, thermal transport in nanochannels and gases in nanochannels are illustrated and discussed. This paper will provide an expedient and valuable reference to designers who intend to research molecular dynamics simulation of nanofluidic devices.

Water flow in carbon nanotubes: The effect of tube flexibility and thermostat

Although the importance of temperature control in nonequilibrium molecular dynamics simulations is widely accepted, the consequences of the thermostatting approach in the case of strongly confined fluids are underappreciated. We show the strong influence of the thermostatting method on the water transport in carbon nanotubes (CNTs) by considering simulations in which the system temperature is controlled via the walls or via the fluid. Streaming velocities and mass flow rates are found to depend on the tube flexibility and on the thermostatting algorithm, with flow rates up to 20% larger when the walls are flexible. The larger flow rates in flexible CNTs are explained by a lower friction coefficient between water and the wall. Despite the lower friction, a larger solid-fluid interaction energy is found for flexible CNTs than for rigid ones. Furthermore, a comparison of thermostat schemes has shown that the Berendsen and Nosé-Hoover thermostats result in very similar transport rates, while lower flow rates are found under the influence of the Langevin thermostat. These findings illustrate the significant influence of the thermostatting methods on the simulated confined fluid transport.

Direct visualization of fluid dynamics in sub-10 nm nanochannels

Optical microscopy is the most direct method to probe fluid dynamics at small scales. However, contrast between fluid phases vanishes at ∼10 nm lengthscales, limiting direct optical interrogation to larger systems. Here, we present a method for direct, high-contrast and label-free visualization of fluid dynamics in sub-10 nm channels, and apply this method to study capillary filling dynamics at this scale. The direct visualization of confined fluid dynamics in 8-nm high channels is achieved with a conventional bright-field optical microscope by inserting a layer of a high-refractive-index material, silicon nitride (Si₃N₄), between the substrate and the nanochannel, and the height of which is accurately controlled down to a few nanometers by a SiO₂ spacer layer. The Si₃N₄ layer exhibits a strong Fabry-Perot resonance in reflection, providing a sharp contrast between ultrathin liquid and gas phases. In addition, the Si₃N₄ layer enables robust anodic bonding without nanochannel collapse. With this method, we demonstrate the validity of the classical Lucas-Washburn equation for capillary filling in the sub-10 nm regime, in contrast to the previous studies, for both polar and nonpolar liquids, and for aqueous salt solutions.

Effects of surface roughness on shear viscosity

This paper investigates the effect of surface roughness on fluid viscosity using molecular dynamics simulations. The three-dimensional model consists of liquid argon flowing between two solid walls whose surface roughness was modeled using fractal theory. In tandem with previously published experimental work, our results show that, while the viscosity in smooth channels remains constant across the channel width, in the presence of surface roughness it increases close to the walls. The increase of the boundary viscosity is further accentuated by an increase in the depth of surface roughness. We attribute this behavior to the increased momentum transfer at the boundary, a result of the irregular distribution of fluid particles near rough surfaces. Furthermore, although the viscosity in smooth channels has previously been shown to be independent of the strength of the solid-liquid interaction, here we show that in the presence of surface roughness, the boundary viscosity increases with the solid's wettability. The paper concludes with an analytical description of the viscosity as a function of the distance from the channel walls, the walls' surface roughness, and the solid's wetting properties. The relation can potentially be used to adjust the fluid dynamics equations for a more accurate description of microfluidic systems.

Dynamics of colloids confined in microcylinders

We studied both global and local effects of cylindrical confinement on the diffusive behavior of hard sphere (HS) colloids. Using confocal scanning laser microscopy (CSLM) and particle tracking, we measured the mean squared displacement (MSD) of 1 micron sized silica particles in water-glycerol. This combination of fluid and setup allowed us to measure MSDs in a 4-dimensional parameter space, defined by the HS volume fraction (Φ: 0.05-0.39), cylinder radius (R: 2.5-20 micron), distance to the wall (z) and lagtime (τ: 0.03-60 s). MSDs for the entire cylinder confirm earlier findings that both narrowing the cylinder and populating it cause a slower dynamics. Additionally a decrease in R was found to cause a stronger ordering of the fluid. The effect of confinement on dynamics was further examined as a function of (z) location. For the largest cylinder (with minor curvature), we found that the strong decrease in MSD near the wall, becomes much less pronounced for higher Φ. Analyzing the radial (r) and azimuthal (θ) components, we found pronounced differences in the z-dependence that were 'hidden' in the total MSD. Near the wall, the r-MSD shows a much steeper z-dependence while at larger z, it shows a remarkable anti-correlation with the (peaked) density n(z). Also the dependence of the r-MSD on lagtime correlates with n(z): diffusive in between layers, but subdiffusive inside layers. These observations bring earlier findings together, while also shedding new light on the diffusive dynamics of concentrated colloids in narrow capillaries.

Ion-specific adsorption and electroosmosis in charged amorphous porous silica

Aqueous electrolyte solutions (NaCl, KCl, CsCl, and SrCl₂) confined in a negatively charged amorphous silica slit pore.

Boundary-controlled barostats for slab geometries in molecular dynamics simulations

Molecular dynamics simulation barostat schemes are derived for achieving a given normal pressure for a thin liquid or solid layer confined between two parallel walls. This work builds on the boundary-controlled barostat scheme of Lupkowski and van Swol [J. Chem. Phys. 93, 737 (1990)]. Two classes of barostat are explored, one in which the external load is applied to a virtual regular lattice to which the wall atoms are bound using a tethering potential. The other type of barostat applies the external force directly to the wall atoms, which are not tethered. The extent to which the wall separation distribution is Gaussian is shown to be an effective measure of the quality of the barostat. The first class of barostat can suffer from anomalous dynamical signatures, even resonances, which are sensitive to the effective mass of the virtual lattice, whose value lacks any rigorous definition. The second type of barostat performs much better under equilibrium and wall-sliding nonequilibrium conditions and in not being so prone to resonance instabilities in the wall separation and does not require so many largely arbitrary parameters. The results of exploratory simulations which characterize the dynamical response of the model systems for both dry and wet or lubricated systems using the different barostats are presented. The barostats which have an inherent damping mechanism, such as the ones analogous to a damped harmonic oscillator, reduce the occurrence of large fluctuations and resonances in the separation between the two walls, and they also achieve a new target pressure more quickly. Near a nonequilibrium phase boundary the attributes of the barostat can have a marked influence on the observed behavior.

Local shear viscosity of strongly inhomogeneous dense fluids: from the hard-sphere to the Lennard-Jones fluids

This work aims at providing a tractable approach to model the local shear viscosity of strongly inhomogeneous dense fluids composed of spherical molecules, in which the density variations occur on molecular distance. The proposed scheme, which relies on the local density average model, has been applied to the quasi-hard-sphere, the Week-Chandler-Andersen and the Lennard-Jones fluids. A weight function has been developed to deal with the hard-sphere fluid given the specificities of momentum exchange. To extend the approach to the smoothly repulsive potential, we have taken into account that the non-local contributions to the viscosity due to the interactions of particles separated by a given distance are temperature dependent. Then, using a simple perturbation scheme, the approach is extended to the Lennard-Jones fluids. It is shown that the viscosity profiles of inhomogeneous dense fluids deduced from this approach are consistent with those directly computed by non-equilibrium molecular dynamics simulations.

Elastic properties of a confined fluid

Monte Carlo computer simulation is employed to determine the local, wave-number-dependent, high-frequency elastic properties of a Lennard-Jones fluid that is confined between two walls. In particular, the elastic constants are calculated from coarse-grained stress correlation functions and then related to the local fluid structure via planar radial distribution functions. We find that local fluid properties correlate with the inhomogeneous fluid density and the strength of the wall-fluid interaction. Finally, we discuss the utility of this analysis in the interpretation of experiments involving the characterization of confined fluids.