Giant amplification of interfacially driven transport by hydrodynamic slip: diffusio-osmosis and beyond

Slip Opacity and Fast Osmotic Transport of Hydrophobes at Aqueous Interfaces with Two-Dimensional Materials

We investigate the interfacial transport of water and hydrophobic solutes on van der Waals bilayers and heterostructures formed by stacking graphene, hBN, and MoS2 using extensive ab initio molecular dynamics simulations. We compute water slippage and the diffusio-osmotic transport coefficient of hydrophobic particles at the interface by combining hydrodynamics and the theory of the hydrophobic effect. We find that slippage is dominated by the layer that is in direct contact with water and only marginally altered by the second layer, leading to a so-called "slip opacity". The screening of the lateral forces, where the liquid does not feel the forces coming from the second nearest layer, is one of the factors leading to the "slip opacity" in our systems. The diffusio-osmotic transport of small hydrophobes (with a radius below 2.5 Å) is also affected by the slip opacity, being dramatically enhanced by slippage. Furthermore, the direction of diffusio-osmotic flow is controlled by the solute size, with the flow in the opposite direction of the concentration gradient for smaller hydrophobes, and vice versa for larger ones. We connect our findings to the wetting properties of two-dimensional materials, and we propose that slippage and wetting can be controlled separately: whereas the slippage is mostly determined by the layer in closer proximity to water, wetting can be finely tuned by stacking different two-dimensional materials. Our study advances the computational design of two-dimensional materials and van der Waals heterostructures, enabling precise control over wetting and slippage properties for applications in coatings and water purification membranes.

Power-law intermittency in the gradient-induced self-propulsion of colloidal swimmers

Active colloidal microswimmers serve as archetypical active fluid systems, and as models for biological swimmers. Here, by studying in detail their velocity traces, we find robust power-law intermittency with system-dependent exponential cut off. We model the intermittent motion by an interplay of the field gradient-dependent active force, which depends on a fluid gradient and is reduced when the swimmer moves, and the locally fluctuating hydrodynamic drag, that is set by the wetting properties of the substrate. The model closely describes the velocity distributions of two disparate swimmer systems: AC field activated and catalytic swimmers. The generality is highlighted by the collapse of all data in a single master curve, suggesting the applicability to further systems, both synthetic and biological.

Chemical Micromotors Move Faster at Oil-Water Interfaces

Many real-world scenarios involve interfaces, particularly liquid-liquid interfaces, that can fundamentally alter the dynamics of colloids. This is poorly understood for chemically active colloids that release chemicals into their environment. We report here the surprising discovery that chemical micromotors─colloids that convert chemical fuels into self-propulsion─move significantly faster at an oil-water interface than on a glass substrate. Typical speed increases ranged from 3 to 6 times up to an order of magnitude and were observed for different types of chemical motors and interfaces made with different oils. Such speed increases are likely caused by faster chemical reactions at an oil-water interface than at a glass-water interface, but the exact mechanism remains unknown. Our results provide valuable insights into the complex interactions between chemical micromotors and their environments, which are important for applications in the human body or in the removal of organic pollutants from water. In addition, this study also suggests that chemical reactions occur faster at an oil-water interface and that micromotors can serve as a probe for such an effect.

Open Questions of Chemically Powered Nano- and Micromotors

Chemically powered nano- and micromotors are microscopic devices that convert chemical energy into motion. Interest in these motors has grown over the past 20 years because they exhibit interesting collective behaviors and have found potential uses in biomedical and environmental applications. Understanding how these motors operate both individually and collectively and how environments affect their operation is of both fundamental and applied significance. However, there are still significant gaps in our knowledge. This Perspective highlights several open questions regarding the propulsion mechanisms of, interactions among, and impact of confinements on nano- and micromotors driven by self-generated chemical gradients. These questions are based on my own experience as an experimentalist. For each open question, I describe the problem and its significance, analyze the status-quo, identify the bottleneck problem, and propose potential solutions. An underlying theme for these questions is the interplay among reaction kinetics, physicochemical distributions, and fluid flows. Unraveling this interplay requires careful measurements as well as a close collaboration between experimentalists and theoreticians/numerical experts. The interdisciplinary nature of these challenges suggests that their solutions could bring new revelations and opportunities across disciplines such as colloidal sciences, material sciences, soft matter physics, robotics, and beyond.

Non-equilibrium molecular dynamics of steady-state fluid transport through a 2D membrane driven by a concentration gradient

We use a novel non-equilibrium algorithm to simulate steady-state fluid transport through a two-dimensional (2D) membrane due to a concentration gradient by molecular dynamics (MD) for the first time. We confirm that, as required by the Onsager reciprocal relations in the linear-response regime, the solution flux obtained using this algorithm agrees with the excess solute flux obtained from an established non-equilibrium MD algorithm for pressure-driven flow. In addition, we show that the concentration-gradient-driven solution flux in this regime is quantified far more efficiently by explicitly applying a transmembrane concentration difference using our algorithm than by applying Onsager reciprocity to pressure-driven flow. The simulated fluid fluxes are captured with reasonable quantitative accuracy by our previously derived continuum theory of concentration-gradient-driven fluid transport through a 2D membrane [D. J. Rankin, L. Bocquet, and D. M. Huang, J. Chem. Phys. 151, 044705 (2019)] for a wide range of solution and membrane parameters, even though the simulated pore sizes are only several times the size of the fluid particles. The simulations deviate from the theory for strong solute-membrane interactions relative to thermal energy, for which the theoretical approximations breakdown. Our findings will be beneficial for a molecular-level understanding of fluid transport driven by concentration gradients through membranes made from 2D materials, which have diverse applications in energy harvesting, molecular separations, and biosensing.

Electric Fields in Liquid Water Irradiated with Protons at Ultrahigh Dose Rates

We study the effects of irradiating water with 3 MeV protons at high doses by observing the motion of charged polystyrene beads outside the proton beam. By single-particle tracking, we measure a radial velocity of the order of microns per second. Combining electrokinetic theory with simulations of the beam-generated reaction products and their outward diffusion, we find that the bead motion is due to electrophoresis in the electric field induced by the mobility contrast of cations and anions. This work sheds light on the perturbation of biological systems by high-dose radiations and paves the way for the manipulation of colloid or macromolecular dispersions by radiation-induced diffusiophoresis.

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

Numerical study supplemented with simplified model on electrophoresis of a hydrophobic colloid incorporating finite ion size effects and ion-solvent interactions

We consider a modified electrokinetic model to study the electrophoresis of a hydrophobic particle by considering the finite sized ions. The mathematical model adopted in this study incorporates the ion steric repulsion, ion-solvent interactions as well as Maxwell stress on the electrolyte. The dielectric permittivity and viscosity of the electrolyte is considered to vary with the local ionic volume fraction. Based on this modified model for the electrokinetics we have analyzed the electrophoresis in a single as well as mixture of electrolytes of monovalent and non- z : z $z:z$ electrolytes. The dependence of viscosity on local ionic volume fraction modifies the hydrodynamic drag as well as diffusivity of ions, which are ignored in existing studies on electrophoresis. A simplified model for electrophoresis of a hydrophobic particle incorporating the ion steric repulsion and ion-solvent interactions is developed based on the first-order perturbation on applied electric field. This simplified model is established to be efficient for a Debye layer thinner than the particle size and a smaller range of slip length. This model can be implemented for any number of ionic species as well as non- z : z $z:z$ electrolytes. It is established that the ion steric interactions and dielectric decrement creates a counterion saturation in the Debye layer leading to an enhanced mobility compared to the standard model. However, experimental data for non-dilute cases often under predicts the theoretically determined mobility. The present modified model fills this lacuna and demonstrate that the consideration of finite ion size modifies the medium viscosity and hence, ionic mobility, which in combination lowers the mobility value.

Determination of the swimming mechanism of Au@TiO2 active matter and implications on active-passive interactions

Non-equilibrium dynamic assembly attracts considerable attention due to the possibility of forming diverse structures that can potentially lead to functional materials. Despite significant progress in understanding and modelling, the complexity of the system implies that different phases of the assembly formation are governed by different interactions. It is clear that both, hydrodynamic and chemical interactions stem from the activity of the particle, but correlation to specific chemical species remains not yet understood. Here, we investigate the origin of the main driving forces for light-driven Au@TiO₂ micromotors and look at the implication this causes for the interactions between active and passive particles. We develop precision experimental measurements of the photochemical reaction rate, which are correlated with the observed speed of Au@TiO₂ micromotors. The comparison with two distinct models allows the conclusion that the dominant propulsion mechanism of the active particles is self-electrophoresis based on the self-generated H⁺ gradient. We verify this assumption by adding salt and confirm the dependence of the expected swimming behaviour on salt concentration and investigate the consequences for raft formation in COMSOL simulations.

Microscale Diffusiophoresis of Proteins

Living systems are characterized by their spatially highly inhomogeneous nature which is susceptible to modify fundamentally the behavior of biomolecular species, including the proteins that underpin biological functionality in cells. Spatial gradients in chemical potential are known to lead to strong transport effects for colloidal particles, but their effect on molecular scale species such as proteins has remained largely unexplored. Here, we improve on existing diffusiophoresis microfluidic technique to measure protein diffusiophoresis in real space. The measurement of proteins is made possible by two ameliorations. First, a label-free microscope is used to suppress label interference. Second, improvements in numerical methods are developed to meet the particular challenges posed by small molecules. We demonstrate that individual proteins can undergo strong diffusiophoretic motion in salt gradients in a manner which is sufficient to overcome diffusion and which leads to dramatic changes in their spatial organization on the scale of a cell. Moreover, we demonstrate that this phenomenon can be used to control the motion of proteins in microfluidic devices. These results open up a path towards a physical understanding of the role of gradients in living systems in the spatial organization of macromolecules and highlight novel routes towards protein sorting applications on device.

Generating Shear Flows without Moving Parts by Thermo-osmosis in Heterogeneous Nanochannels

Shear flows play critical roles in biological systems and technological applications and are achieved experimentally using moving parts. However, when the system size is reduced to micro- and nanoscale, fabrication of moving parts becomes exceedingly challenging. We demonstrate that a heterogeneous nanochannel composed of two parallel walls with different wetting behaviors can generate shear flow without moving parts. Molecular dynamics simulations show that shear flows can be formed inside such a nanochannel under a temperature gradient. The physical origin is that thermo-osmosis velocities with different rates and directions can be tuned by wetting behaviors. Our analysis reveals that thermo-osmosis is governed by surface excess enthalpy and nanoscale interfacial hydrodynamics. This finding provides an efficient method of generating controllable shear flows at micro- and nanoscale confinement. It also demonstrates the feasibility of using fluids to drive micromechanical elements via shear torques generated by harvesting energy from temperature differences.

Osmotic Transport at the Aqueous Graphene and hBN Interfaces: Scaling Laws from a Unified, First-Principles Description

Osmotic transport in nanoconfined aqueous electrolytes provides alternative venues for water desalination and "blue energy" harvesting. The osmotic response of nanofluidic systems is controlled by the interfacial structure of water and electrolyte solutions in the so-called electrical double layer (EDL), but a molecular-level picture of the EDL is to a large extent still lacking. Particularly, the role of the electronic structure has not been considered in the description of electrolyte/surface interactions. Here, we report enhanced sampling simulations based on ab initio molecular dynamics, aiming at unravelling the free energy of prototypical ions adsorbed at the aqueous graphene and hBN interfaces, and its consequences on nanofluidic osmotic transport. Specifically, we predicted the zeta potential, the diffusio-osmotic mobility, and the diffusio-osmotic conductivity for a wide range of salt concentrations from the ab initio water and ion spatial distributions through an analytical framework based on Stokes equation and a modified Poisson-Boltzmann equation. We observed concentration-dependent scaling laws, together with dramatic differences in osmotic transport between the two interfaces, including diffusio-osmotic flow and current reversal on hBN but not on graphene. We could rationalize the results for the three osmotic responses with a simple model based on characteristic length scales for ion and water adsorption at the surface, which are quite different on graphene and on hBN. Our work provides fundamental insights into the structure and osmotic transport of aqueous electrolytes on 2D materials and explores alternative pathways for efficient water desalination and osmotic energy conversion.

Miniaturized Salinity Gradient Energy Harvesting Devices

Harvesting salinity gradient energy, also known as "osmotic energy" or "blue energy", generated from the free energy mixing of seawater and fresh river water provides a renewable and sustainable alternative for circumventing the recent upsurge in global energy consumption. The osmotic pressure resulting from mixing water streams with different salinities can be converted into electrical energy driven by a potential difference or ionic gradients. Reversed-electrodialysis (RED) has become more prominent among the conventional membrane-based separation methodologies due to its higher energy efficiency and lesser susceptibility to membrane fouling than pressure-retarded osmosis (PRO). However, the ion-exchange membranes used for RED systems often encounter limitations while adapting to a real-world system due to their limited pore sizes and internal resistance. The worldwide demand for clean energy production has reinvigorated the interest in salinity gradient energy conversion. In addition to the large energy conversion devices, the miniaturized devices used for powering a portable or wearable micro-device have attracted much attention. This review provides insights into developing miniaturized salinity gradient energy harvesting devices and recent advances in the membranes designed for optimized osmotic power extraction. Furthermore, we present various applications utilizing the salinity gradient energy conversion.

Nanofluidic Membranes to Address the Challenges of Salinity Gradient Power Harvesting

Salinity gradient power (SGP) has been identified as a promising renewable energy source. Reverse electrodialysis (RED) and pressure retarded osmosis (PRO) are two membrane-based technologies for SGP harvesting. Developing nanopores and nanofluidic membranes with excellent water and/or ion transport properties for applications in those two membrane-based technologies is considered viable for improving power generation performance. Despite recent efforts to advance power generation by designing a variety of nanopores and nanofluidic membranes to enhance power density, the valid pathways toward large-scale power generation remain uncertain. In this review, we introduce the features of ion and water transport in nanofluidics that are potentially beneficial to power generation. Subsequently, we survey previous efforts on nanofluidic membrane synthesis to obtain high power density. We also discuss how the various membrane properties influence the power density in RED and PRO before moving on to other important aspects of the technologies, i.e., system energy efficiency and membrane fouling. We analyze the importance of system energy efficiency and illustrate how the delicately designed nanofluidic membranes can potentially enhance energy efficiency. Previous studies are reviewed on fabricating antifouling and antimicrobial membrane for power generation, and opportunities are presented that can lead to the design of nanofluidic membranes with superior antifouling properties using various materials. Finally, future research directions are presented on advancing membrane performance and scaling-up the system. We conclude this review by emphasizing the fact that SGP has the potential to become an important renewable energy source and that high-performance nanofluidic membranes can transform SGP harvesting from conceptual to large-scale applications.

Reactive Flows in Porous Media: Challenges in Theoretical and Numerical Methods

We review theoretical and computational research, primarily from the past 10 years, addressing the flow of reactive fluids in porous media. The focus is on systems where chemical reactions at the solid-fluid interface cause dissolution of the surrounding porous matrix, creating nonlinear feedback mechanisms that can often lead to greatly enhanced permeability. We discuss insights into the evolution of geological forms that can be inferred from these feedback mechanisms, as well as some geotechnical applications such as enhanced oil recovery, hydraulic fracturing, and carbon sequestration. Until recently, most practical applications of reactive transport have been based on Darcy-scale modeling, where averaged equations for the flow and reactant transport are solved. We summarize the successes and limitations of volume averaging, which leads to Darcy-scale equations, as an introduction to pore-scale modeling. Pore-scale modeling is computationally intensive but offers new insights as well as tests of averaging theories and pore-network models. We include recent research devoted to validation of pore-scale simulations, particularly the use of visual observations from microfluidic experiments.

Nano-pump based on exothermic surface reactions

We present simulations indicating that it should be possible to construct a switchable nano-scale fluid pump, driven by exothermic surface reactions. Such a pump could, for instance, be controlled electro-chemically. In our simulations we explore a simple illustration of such a pump. We argue that the simplicity of the pump design could make it attractive for micro/nano-fluidics applications.

Net Unidirectional Fluid Transport in Locally Heated Nanochannel by Thermo-osmosis

Thermo-osmosis driven by temperature gradients generally requires two liquid reservoirs at different temperatures connected by porous bodies or capillaries. We demonstrate, by molecular dynamics simulation, a new phenomenon toward nanoscale thermo-osmosis. Upon heating at a certain region of a nanochannel, multiple nanoscale convective layers are formed and can be manipulated to generate a net fluid transport from one reservoir to another, even without a temperature difference between them. A net unidirectional fluid transport with different rates can be achieved by precisely controlling location of the heated region. The net fluid transport can be enhanced further by tuning liquid-wall interactions. The demonstrated phenomenon provides a strategy for enhancing fluid mixing, which is often inefficient in nanoscale flows. Our finding is promising for chip-level cooling. The heat generated by chips can be employed to produce asymmetric temperature gradients in channels through proper configuration. Coolant liquids can thus be circulated without extra pumps.

Viscoelasticity-Induced Onset of Slip at the Wall for Polymer Fluids

The progressive onset of slip at the wall, which corresponds to a slip length increasing with the solicitation time before reaching a plateau, has been investigated for model viscoelastic polymer solutions, allowing one to vary the longest relaxation time while keeping constant solid-fluid interactions. A hydrodynamic model based on a Maxwell fluid and the classical Navier's hypothesis of a linear response for the friction stress at the interface fully accounts for the data. In the limit of the linear viscoelasticity of the fluid, we could postulate a Newtonian response for the interfacial friction coefficient, reflecting the local character of solid-liquid friction mechanisms. Deviations between the experiments and our model are observed when the fluid is far from its linear viscoelastic behavior.

Liquid-Solid Slip on Charged Walls: The Dramatic Impact of Charge Distribution

Nanofluidic systems show great promise for applications in energy conversion, where their performance can be enhanced by nanoscale liquid-solid slip. However, efficiency is also controlled by surface charge, which is known to reduce slip. Combining molecular dynamics simulations and analytical developments, we show the dramatic impact of surface charge distribution on the slip-charge coupling. Homogeneously charged graphene exhibits a very favorable slip-charge relation (rationalized with a new theoretical model correcting some weaknesses of the existing ones), leading to giant electrokinetic energy conversion. In contrast, slip is strongly affected on heterogeneously charged surfaces, due to the viscous drag induced by counterions trapped on the surface. In that case slip should depend on the detailed physical chemistry of the interface controlling the fraction of bound ions. Our numerical results and theoretical models provide new fundamental insight into the molecular mechanisms of liquid-solid slip, and practical guidelines for searching new functional interfaces with optimal energy conversion properties, e.g., for blue energy or waste heat harvesting.

Pt-SiO2 Janus Particles and the Water/Oil Interface: A Competition between Motility and Thermodynamics

Various aspects of the behavior of Janus particles near liquid/liquid interfaces have been studied through different experimental and theoretical realizations, but the effect of motility on the behavior of Janus particles near liquid/liquid interfaces has not been investigated, yet. Here, we demonstrate the ability to engineer the behavior of highly interfacial active Janus particles near a water/oil interface by introducing motility to the system. Passive, i.e., nonmotile, platinum-capped 8 μm silica (Pt-SiO₂) Janus particles exhibit a strong tendency to attach to water/oil interfaces with the Pt-cap facing the oil and the SiO₂ side facing the water phase. In contrast, we show that active, i.e., motile, 8 μm Pt-SiO₂ Janus particles approach the interface, orient in a sideways fashion with the Janus boundary perpendicular to the interface, and then swim in the vicinity of the interface similar to observations reported near solid/liquid interfaces. Active Pt-SiO₂ Janus particles near the water/oil interface show motility as a result of adding H₂O₂ to the particle solution. The decomposition of H₂O₂ into O₂ and H₂O creates a nonuniform gradient of O₂ around the particle that hydrodynamically interacts with the water/decalin boundary. The interaction enables rotation of the particle within the swimming plane that is parallel to the interface but restricts rotation in and out of the swimming plane, thereby preventing adsorption to the liquid/liquid interface.

Nanoconfined Fluids: What Can We Expect from Them?

Nanoconfined fluids (NCFs), which are confined in nanospaces, exhibit distinctive nanoscale effects, including surface effects, small-size effects, quantum effects, and others. The continuous medium hypothesis in fluid mechanics is not valid in this context because of the comparable characteristic length of spaces and molecular mean free path, and accordingly, the classical continuum theories developed for the bulk fluids usually cannot describe the mass and energy transport of NCFs. In this Perspective, we summarize the nanoscale effects on the thermodynamics, mass transport, flow dynamics, heat transfer, phase change, and energy transport of NCFs and highlight the related representative works. The applications of NCFs in the fields of membrane separation, oil and gas production, energy harvesting and storage, and biological engineering are especially indicated. Currently, the theoretical description framework of NCFs is still missing, and it is expected that this framework can be established by adopting the classical continuum theories with the consideration of nanoscale effects.

Local and global force balance for diffusiophoretic transport

Electro- and diffusio- phoresis of particles correspond respectively to the transport of particles under electric field and solute concentration gradients. Such interfacial transport phenomena take their origin in a diffuse layer close to the particle surface, and the motion of the particle is force-free. In the case of electrophoresis, it is further expected that the stress acting on the moving particle vanishes locally as a consequence of local electroneutrality. But the argument does not apply to diffusiophoresis, which takes its origin in solute concentration gradients. In this paper we investigate further the local and global force balance on a particle undergoing diffusiophoresis. We calculate the local tension applied on the particle surface and show that, counter-intuitively, the local force on the particle does not vanish for diffusiophoresis, in spite of the global force being zero as expected. Incidentally, our description allows to clarify the osmotic balance in diffusiophoresis, which has been a source of debates in the recent years. We explore various cases, including hard and soft interactions, as well as porous particles, and provide analytic predictions for the local force balance in these various systems. The existence of local stresses may induce deformation of soft particles undergoing diffusiophoresis, hence suggesting applications in terms of particle separation based on capillary diffusiophoresis.

Rapid characterization of neutral polymer brush with a conventional zetameter and a variable pinch of salt

The fundamental and practical importance of particle stabilization has motivated various characterization methods for studying polymer brushes on particle surfaces. In this work, we show how one can perform sensitive measurements of neutral polymer coating on colloidal particles using a commercial zetameter and salt solutions. By systematically varying the Debye length, we study the mobility of the polymer-coated particles in an applied electric field and show that the electrophoretic mobility of polymer-coated particles normalized by the mobility of non-coated particles is entirely controlled by the polymer brush and independent of the native surface charge, here controlled with pH, or the surface-ion interaction. Our result is rationalized with a simple hydrodynamic model, allowing for the estimation of characteristics of the polymer coating: the brush length L, and the Brinkman length ξ, determined by its resistance to flows. We demonstrate that the Debye layer provides a convenient and faithful probe to the characterization of polymer coatings on particles. Because the method simply relies on a conventional zetameter, it is widely accessible and offers a practical tool to rapidly probe neutral polymer brushes, an asset in the development and utilization of polymer-coated colloidal particles.

Studying polymer diffusiophoresis with non-equilibrium molecular dynamics

We report a numerical study of the diffusiophoresis of short polymers using non-equilibrium molecular dynamics simulations. More precisely, we consider polymer chains in a fluid containing a solute that has a concentration gradient and examine the variation of the induced diffusiophoretic velocity of the polymer chains as the interaction between the monomer and the solute is varied. We find that there is a non-monotonic relation between the diffusiophoretic mobility and the strength of the monomer-solute interaction. In addition, we find a weak dependence of the mobility on the length of the polymer chain, which shows clear difference from the diffusiophoresis of a solid particle. Interestingly, the hydrodynamic flow through the polymer is much less screened than for pressure driven flows.

Diffusio-osmosis and wetting on solid surfaces: a unified description based on a virtual work principle

In order to account for diffusio-osmosis, Derjaguin proposed long ago that there is an excess pressure confined within a layer of typically a few nanometers in the vicinity of a solid surface immersed in a liquid and resulting from the interaction between the liquid and the surface. In the presence of a composition gradient in the liquid a confined pressure gradient parallel to the surface is therefore responsible for the diffusio-osmotic flow. This picture appears in contradiction with the contact theorem of colloidal science according to which such excess pressure does not exist. We propose a theoretical description for calculating hydrodynamic flows in inhomogeneous liquids in the vicinity of solid interfaces which is consistent with the contact theorem. This approach is based on a Gibbs free energy and a virtual work principle for calculating the driving forces in the liquid due to inhomogeneous composition along a capillary and to the interaction with the solid interfaces. Our approach allows us to show that the physics at play is the same in wetting or in diffusio-osmosis experiments, as one can go continuously from the latter to the former by making composition gradients sharper. We obtain an explicit expression for the diffusio-osmotic mobility which depends on the Gibbs free energy density in the vicinity of the interface and its dependance on the solute concentration in the liquid beyond the interfacial region, and which is inversely proportional to the liquid viscosity.

Active rotational dynamics of a self-diffusiophoretic colloidal motor

The dynamics of a spherical chemically-powered synthetic colloidal motor that operates by a self-diffusiophoretic mechanism and has a catalytic domain of arbitrary shape is studied using both continuum theory and particle-based simulations. The motor executes active rotational motion when self-generated concentration gradients and interactions between the chemical species and colloidal motor surface break spherical symmetry. Local variations of chemical reaction rates on the motor catalytic surface with catalytic domain sizes and shapes provide such broken symmetry conditions. A continuum theoretical description of the active rotational motion is given, along with the results of particle-based simulations of the active dynamics. From these results a detailed description of the factors responsible for the active rotational dynamics can be given. Since active rotational motion often plays a significant part in the nature of the collective dynamics of many-motor systems and can be used to control motor motion in targeted cargo transport, our results should find applications beyond those considered here.

Slip Length Dependent Propulsion Speed of Catalytic Colloidal Swimmers near Walls

Catalytic colloidal swimmers that propel due to self-generated fluid flows exhibit strong affinity for surfaces. Here, we report experimental measurements of a significant dependence of such microswimmers' speed on the nearby substrate material. We find that speeds scale with the solution contact angle θ on the substrate, which relates to the associated hydrodynamic substrate slip length, as V∝(cosθ+1)^{-3/2}. We show that such dependence can be attributed to osmotic coupling between swimmers and substrate. Our work points out that hydrodynamic slip at nearby walls, though often unconsidered, can significantly impact microswimmer self-propulsion.

Large effect of lateral box size in molecular dynamics simulations of liquid-solid friction

Molecular dynamics simulations are a powerful tool to characterize liquid-solid friction. A slab configuration with periodic boundary conditions in the lateral dimensions is commonly used, where the measured friction coefficient could be affected by the finite lateral size of the simulation box. Here we show that for a very wetting liquid close to its melting temperature, strong finite size effects can persist up to large box sizes along the flow direction, typically ∼30 particle diameters. We relate the observed decrease of friction in small boxes to changes in the structure of the first adsorbed layer, which becomes less commensurable with the wall structure. Although these effects disappear for lower wetting cases or at higher temperatures, we suggest that the possible effect of the finite lateral box size on the friction coefficient should not be automatically set aside when exploring unknown systems.

Giant Thermoelectric Response of Nanofluidic Systems Driven by Water Excess Enthalpy

Nanofluidic systems could in principle be used to produce electricity from waste heat, but current theoretical descriptions predict a rather poor performance as compared to thermoelectric solid materials. Here we investigate the thermoelectric response of NaCl and NaI solutions confined between charged walls, using molecular dynamics simulations. We compute a giant thermoelectric response, 2 orders of magnitude larger than the predictions of standard models. We show that water excess enthalpy-neglected in the standard picture-plays a dominant role in combination with the electro-osmotic mobility of the liquid-solid interface. Accordingly, the thermoelectric response can be boosted using surfaces with large hydrodynamic slip. Overall, the heat harvesting performance of the model systems considered here is comparable to that of the best thermoelectric materials, and the fundamental insight provided by molecular dynamics suggests guidelines to further optimize the performance, opening the way to recycle waste heat using nanofluidic devices.

Determining phoretic mobilities with Onsager's reciprocal relations: Electro- and thermophoresis revisited

We use a hydrodynamic reciprocal approach to phoretic motion to derive general expressions for the electrophoretic and thermophoretic mobility of weakly charged colloids in aqueous electrolyte solutions. Our approach shows that phoretic motion can be understood in terms of the interfacial transport of thermodynamic excess quantities that arises when a colloid is kept stationary inside a bulk fluid flow. The obtained expressions for the mobilities are extensions of previously known results as they can account for different hydrodynamic boundary conditions at the colloidal surface, irrespective of how the colloid-fluid interaction range compares to the colloidal radius.

Osmosis, from molecular insights to large-scale applications

Osmosis is a universal phenomenon occurring in a broad variety of processes and fields. It is the archetype of entropic forces, both trivial in its fundamental expression - the van 't Hoff perfect gas law - and highly subtle in its physical roots. While osmosis is intimately linked with transport across membranes, it also manifests itself as an interfacial transport phenomenon: the so-called diffusio-osmosis and -phoresis, whose consequences are presently actively explored for example for the manipulation of colloidal suspensions or the development of active colloidal swimmers. Here we give a global and unifying view of the phenomenon of osmosis and its consequences with a multi-disciplinary perspective. Pushing the fundamental understanding of osmosis allows one to propose new perspectives for different fields and we highlight a number of examples along these lines, for example introducing the concepts of osmotic diodes, active separation and far from equilibrium osmosis, raising in turn fundamental questions in the thermodynamics of separation. The applications of osmosis are also obviously considerable and span very diverse fields. Here we discuss a selection of phenomena and applications where osmosis shows great promises: osmotic phenomena in membrane science (with recent developments in separation, desalination, reverse osmosis for water purification thanks in particular to the emergence of new nanomaterials); applications in biology and health (in particular discussing the kidney filtration process); osmosis and energy harvesting (in particular, osmotic power and blue energy as well as capacitive mixing); applications in detergency and cleaning, as well as for oil recovery in porous media.

Surface charge-dependent hydrodynamic properties of an electroosmotic slip flow

The electroosmosis effects at the interface of an aqueous NaCl solution and a charged silicon surface are studied using a molecular dynamics (MD) method. Considering a plug-like electroosmotic flow, we identified a thin interfacial layer in the immediate vicinity of the charged surface, where the flow velocity experiences almost linear spatial variations. The thickness of this interfacial layer is found to be linearly dependent on the surface charge density, with a negative slope which slightly depends on the surface hydrophobicity while being independent of the salt concentration, electric field strength, and orientation of the surface lattice. It is also found that upon increasing the surface charge density, the effective slip length first increases up to a maximum amount and then follows an almost linear reduction. We found that increasing the salt concentration drastically reduces the surface charge at which the effective slip length reaches its maximum amount. For highly concentrated solutions, therefore, the effective slip length could be assumed to change linearly in the whole range of the surface charge density, with a slope which is proportional to the square root of the electric field strength divided by the depth of the potential well assigned to the surface atoms εwall. Also, in a wide range of the surface charge density, the slip velocity is found to be a constant fraction of the electroosmotic velocity, which could be measured experimentally. Finally, by comparing the electroosmotic velocities calculated from the Stokes equation (considering both the slip and no-slip boundary conditions) with our MD results, we found that the no-slip boundary condition, which is normally used in analytical calculations, leads to a very inaccurate result for the studied system.

Fluctuating chemohydrodynamics and the stochastic motion of self-diffusiophoretic particles

The propulsion of active particles by self-diffusiophoresis is driven by asymmetric catalytic reactions on the particle surface that generate a mechanochemical coupling between the fluid velocity and the concentration fields of fuel and product in the surrounding solution. Because of thermal and molecular fluctuations in the solution, the motion of micrometric or submicrometric active particles is stochastic. Coupled Langevin equations describing the translation, rotation, and reaction of such active particles are deduced from fluctuating chemohydrodynamics and fluctuating boundary conditions at the interface between the fluid and the particle. These equations are consistent with microreversibility and the Onsager-Casimir reciprocal relations between affinities and currents and provide a thermodynamically consistent basis for the investigation of the dynamics of active particles propelled by diffusiophoretic mechanisms.

Theory of diffusioosmosis in a charged nanochannel

We probe the diffusioosmotic transport in a charged nanofluidic channel in the presence of an applied tangential salt concentration gradient. Ionic salt gradient driven diffusioosmosis or ionic diffusioosmosis (IDO) is characterized by the generation of an induced tangential electric field and a diffusioosmotic velocity (DOSV) that is a combination of an electroosmotic velocity (EOSV) triggered by this electric field and a chemiosmotic velocity (COSV) triggered by an induced tangential pressure gradient. We explain that unlike the existing theories on IDO, it is more appropriate to apply the zero net current conditions (formulation F2) and not more restrictive zero net local flux conditions (formulation F1) particularly for the case where one considers a nanochannel connected to two reservoirs. We pinpoint limitations in the existing literature in correctly predicting the diffusioosmotic behavior even for the case where formulation F1 is used. We address these limitations and establish that (a) the induced electric field is an interplay of the differences in ionic diffusivity, the EDL-induced imbalance in ion concentrations, and the advection effects, (b) formulation F1 may overpredict or underpredict the electric field and the EOSV leading to an overprediction/underprediction of the DOSV and (c) formulation F2 demonstrates remarkable fluid physics of localized backflows owing to a dominant local influence of the COSV, which is missed by formulation F1. We anticipate that our theory will provide the first rigorous understanding of nanofluidic IDO with applications in multiple areas of low Reynolds number transport such as biofluidics, microfluidic separation, and colloidal transport.

Dramatic pressure-sensitive ion conduction in conical nanopores

Ion transporters in Nature exhibit a wealth of complex transport properties such as voltage gating, activation, and mechanosensitive behavior. When combined, such processes result in advanced ionic machines achieving active ion transport, high selectivity, or signal processing. On the artificial side, there has been much recent progress in the design and study of transport in ionic channels, but mimicking the advanced functionalities of ion transporters remains as yet out of reach. A prerequisite is the development of ionic responses sensitive to external stimuli. In the present work, we report a counterintuitive and highly nonlinear coupling between electric and pressure-driven transport in a conical nanopore, manifesting as a strong pressure dependence of the ionic conductance. This result is at odds with standard linear response theory and is akin to a mechanical transistor functionality. We fully rationalize this behavior on the basis of the coupled electrohydrodynamics in the conical pore by extending the Poisson-Nernst-Planck-Stokes framework. The model is shown to capture the subtle mechanical balance occurring within an extended spatially charged zone in the nanopore. The pronounced sensitivity to mechanical forcing offers leads in tuning ion transport by mechanical stimuli. The results presented here provide a promising avenue for the design of tailored membrane functionalities.

Adsorption-induced slip inhibition for polymer melts on ideal substrates

Hydrodynamic slip, the motion of a liquid along a solid surface, represents a fundamental phenomenon in fluid dynamics that governs liquid transport at small scales. For polymeric liquids, de Gennes predicted that the Navier boundary condition together with polymer reptation implies extraordinarily large interfacial slip for entangled polymer melts on ideal surfaces; this Navier-de Gennes model was confirmed using dewetting experiments on ultra-smooth, low-energy substrates. Here, we use capillary leveling—surface tension driven flow of films with initially non-uniform thickness—of polymeric films on these same substrates. Measurement of the slip length from a robust one parameter fit to a lubrication model is achieved. We show that at the low shear rates involved in leveling experiments as compared to dewetting ones, the employed substrates can no longer be considered ideal. The data is instead consistent with a model that includes physical adsorption of polymer chains at the solid/liquid interface.

When modeling fluid flow over a solid surface, one must determine the slip velocity at the boundary. Here Ilton et al. perform experiments to quantify the slip length of polymer melts at a nearly ideal solid surface and capture them in a model involving the density of physically adsorbed polymer chains.

What Controls Thermo-osmosis? Molecular Simulations Show the Critical Role of Interfacial Hydrodynamics

Thermo-osmotic and related thermophoretic phenomena can be found in many situations from biology to colloid science, but the underlying molecular mechanisms remain largely unexplored. Using molecular dynamics simulations, we measure the thermo-osmosis coefficient by both mechanocaloric and thermo-osmotic routes, for different solid-liquid interfacial energies. The simulations reveal, in particular, the crucial role of nanoscale interfacial hydrodynamics. For nonwetting surfaces, thermo-osmotic transport is largely amplified by hydrodynamic slip at the interface. For wetting surfaces, the position of the hydrodynamic shear plane plays a key role in determining the amplitude and sign of the thermo-osmosis coefficient. Finally, we measure a giant thermo-osmotic response of the water-graphene interface, which we relate to the very low interfacial friction displayed by this system. These results open new perspectives for the design of efficient functional interfaces for, e.g., waste-heat harvesting.

Fluid-driven metamorphism of the continental crust governed by nanoscale fluid flow

The transport of fluids through the Earth's crust controls the redistribution of elements to form mineral and hydrocarbon deposits, the release and sequestration of greenhouse gases, and facilitates metamorphic reactions that influence lithospheric rheology. In permeable systems with a well-connected porosity, fluid transport is largely driven by fluid pressure gradients. In less permeable rocks, deformation may induce permeability by creating interconnected heterogeneities, but without these perturbations, mass transport is limited along grain boundaries or relies on transformation processes that self-generate transient fluid pathways. The latter can facilitate large-scale fluid and mass transport in nominally impermeable rocks without large-scale fluid transport pathways. Here, we show that pervasive, fluid-driven metamorphism of crustal igneous rocks is directly coupled to the production of nanoscale porosity. Using multi-dimensional nano-imaging and molecular dynamics simulations, we demonstrate that in feldspar, the most abundant mineral family in the Earth's crust, electrokinetic transport through reaction-induced nanopores (10-100 nm) can potentially be significant. This suggests that metamorphic fluid flow and fluid-mediated mineral transformation reactions can be considerably influenced by nanofluidic transport phenomena.

Molecular Simulation of Thermo-osmotic Slip

Thermo-osmotic slip-the flow induced by a thermal gradient along a surface-is a well-known phenomenon, but curiously there is a lack of robust molecular-simulation techniques to predict its magnitude. Here, we compare three different molecular-simulation techniques to compute the thermo-osmotic slip at a simple solid-fluid interface. Although we do not expect the different approaches to be in perfect agreement, we find that the differences are barely significant for a range of different physical conditions, suggesting that practical molecular simulations of thermo-osmotic slip are feasible.