Diffusioosmotic Flow Research Articles

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.

Manipulation of artificial and living small objects by light driven diffusioosmotic flow

Here we report on light-triggered generation of local flow utilizing a bio-compatible non-ionic photo-active surfactant. The mechanism is based on diffusioosmotic phenomenon, where the gradient of relative concentration with respect to different chemical species near a surface leads to an osmotic pressure gradient driving liquid flow along the surface. The application of a photo-responsive surfactant allows for easy and reversible changes in concentration gradient by positioning a light source at the desired place. Along with the so-inscribed concentration gradient one can change reversible the direction and strength of the flow even in a closed system. The phenomenology of light-driven diffusioosmotic flow (LDDO) can be used in a rather flexible way: colloids can be gathered or dispersed and bio-compatibility extends the range of colloid types also to living microorganisms such as soil bacterium Pseudomonas putida. We show that DO flow can be considered a versatile method to set hydrodynamic conditions along the sample for investigating the motility of living cells. Further advantages of employing LDDO are the flexibility of flow generation in a reversible way and with spatiotemporal control, without the need to either change the channel geometry by loading a different device, or the periphery of pumps and connectors.

Diffusioosmotic flow reversals due to ion-ion electrostatic correlations.

Existing theories of diffusioosmosis have neglected ion-ion electrostatic correlations, which are important in concentrated electrolytes. Here, we develop a mathematical model to numerically compute the diffusioosmotic mobilities of binary symmetric electrolytes across low to high concentrations in a charged parallel-plate channel. We use the modified Poisson equation to model the ion-ion electrostatic correlations and the Bikerman model to account for the finite size of ions. We report two key findings. First, ion-ion electrostatic correlations can cause a unique reversal in the direction of diffusioosmosis. Such a reversal is not captured by existing theories, occurs at ≈ 0.4 M for a monovalent electrolyte, and at a much lower concentration of ≈ 0.003 M for a divalent electrolyte in a channel with the same surface charge. This highlights that diffusioosmosis of a concentrated electrolyte can be qualitatively different from that of a dilute electrolyte, not just in its magnitude but also its direction. Second, we predict a separate diffusioosmotic flow reversal, which is not due to electrostatic correlations but the competition between the underlying chemiosmosis and electroosmosis. This reversal can be achieved by varying the magnitude of the channel surface charge without changing its sign. However, electrostatic correlations can radically change how this flow reversal depends on the channel surface charge and ion diffusivity between a concentrated and a dilute electrolyte. The mathematical model developed here can be used to design diffusioosmosis of dilute and concentrated electrolytes, which is central to applications such as species mixing and separation, enhanced oil recovery, and reverse electrodialysis.

Complexes of photosensitive surfactant and fluorescent dye for light-induced manipulation of colloids

Light-driven diffusioosmosis is a membrane-free method for manipulating colloidal ensembles at solid–liquid interfaces based on photo-sensitive molecules inducing fluid flows along solid surfaces. In this study, we present our findings on porous colloids settled at a solid wall in an aqueous solution comprising a photo-sensitive azobenzene-containing cationic surfactant and a cyanine-based dye, capable of ionically binding to each other. The surfactant acts as an activation agent for diffusioosmotic flow. When exposed to modulated light, it undergoes photo-isomerization from a hydrophobic trans-state to a more hydrophilic cis-state, creating a concentration gradient near the irradiated area of the wall. The resulting osmotic pressure gradient sets the flow in motion. Porous colloids actively participate in flow generation by readily incorporating the surfactant molecules in the trans-state and releasing them in the cis-state, creating a constant source of diffusioosmotic flow. Under UV illumination, an excess of cis-isomers near the porous colloids elicits long-range repulsive interactions, tenfold the diameter of a particle. The dye acts as a sensor for the surfactant filling or emptying the pores of the colloids. It forms a complex with the trans-isomer and diffuses into the pores, where photoisomerization to cis-state destroys the complex and causes both the dye and the surfactant to leave the pores, altering the luminescence brightness within the colloids. We demonstrated that the presence of the dye affects cis-trans isomer ratios of the surfactant at photo-stationary states, thereby influencing the process of diffusioosmosis. This process enables the manipulation of colloidal particles and remote control of the interaction potential between them, facilitating the formation of well-ordered surface aggregates.

Diffusioosmotic dispersion of solute in a long narrow channel

Solute–surface interactions have garnered considerable interest in recent years as a novel control mechanism for driving unique fluid dynamics and particle transport with potential applications in fields such as biomedicine, the development of microfluidic devices and enhanced oil recovery. In this study, we will discuss dispersion induced by the diffusioosmotic motion near a charged wall in the presence of a solute concentration gradient. Here, we introduce a plug of salt with a Gaussian distribution at the centre of a channel with no background flow. As the solute diffuses, the concentration gradient drives a diffusioosmotic slip flow at the walls, which results in a recirculating flow in the channel; this, in turn, drives an advective flux of the solute concentration. This effect leads to cross-stream diffusion of the solute, altering the effective diffusivity of the solute as it diffuses along the channel. We derive theoretical predictions for the solute dynamics using a multiple-time-scale analysis to quantify the dispersion driven by the solute–surface interactions. Furthermore, we derive a cross-sectionally averaged concentration equation with an effective diffusivity analogous to that from Taylor dispersion. In addition, we use numerical simulations to validate our theoretical predictions.

Photosensitive Spherical Polymer Brushes: Light-Triggered Process of Particle Repulsion

We report on a light-triggered process at which repulsive interactions between microparticles with a polyelectrolyte (PE) brush coating can be remotely controlled. The spherical polyelectrolyte brushes are loaded with photosensitive azobenzene containing surfactant which can undergo reversible photo-isomerization from trans to cis state. The surfactant hydrophilicity is altered by illumination with light of an appropriate wavelength, at which a dynamic exchange of the more surface-active trans isomer in comparison to the more water soluble cis isomer with the PE brush generates a concentration gradient of the cis isomers near a solid surface where the particle is sedimented. In this way, each spherical brush produces its local lateral diffusioosmotic flow pointing outside in a radial direction resulting in mutual long-range repulsive interactions. We demonstrate that a PE layer has a higher tendency to absorb surfactant in comparison to plain silica particles, yielding a larger flow strength. This correlation holds true up to a critical intensity, where the dynamic exchange is adsorption limited with respect to trans isomers and especially pronounced for the PE-coated particles.

Directional motion of charged particles near filter membrane

Membrane has many applications in the fields of filtration and separation, but due to the attraction or repulsion exerted by the membrane, the particles will experience a directional motion. As a result, two totally opposite effects, i.e. particle enrichment and exclusion zone, take place in the vicinity of the membrane, and the underlying reason is still not clear. In this work, colloidal particles with negative surface charge are used as a model substance, with the advantages of monitoring the particle concentration in a real time and <i>in situ</i> way, to investigate the influence of cellulose membrane on the movement of particles. The experimental results show that the particles are enriched in the vicinity of the membrane. The diffusiophoresis effect originating from tiny number of ions released by the film is the main reason of the directional movement of the charged particles. Based on the two mechanisms of diffusiophoresis and diffusion, we construct a model and make relevant numerical calculation, and the numerical results are qualitatively consistent with the experimental results. Moreover, in addition to the longitudinal motion of the particles towards the filter membrane, diffusio-osmotic flow and particles lateral diffusion also result in the migration of particles towards the container wall, and further increasing the particle number near the wall.

Influence of viscoelectric effect on diffusioosmotic transport in nanochannel.

We have investigated the role of viscoelectric effect on diffusioosmotic flow (DOF) through a nanochannel connected with two reservoirs. The transport equations governing the flow dynamics are solved numerically using the finite element technique. We have extensively analyzed the variation of induced field due to electric double layer (EDL) phenomenon, relative viscosity as modulated by the viscoelectric effect as well as reservoir's concentration difference, and their eventual impact on the underlying flow characteristics. It is revealed that the induced electric field in the EDL enhances fluid viscosity substantially near the charged wall at a higher concentration. We have shown that neglecting viscoelectric effect in the paradigm of diffusioosmotic transport overestimates the net throughput, particularly at a higher concentration difference. Furthermore, we show that pertaining to chemiosmosis dominated regime, the average flow velocity modifies with the increase in concentration difference up to a critical value. In comparison, the rise in the strength of resistive electroosmotic actuation by the accumulation of anions in the upstream reservoir reduces the average flow velocity at a higher concentration difference. We have reported a reduction in critical concentration with the increase in viscoelectric effect. The inferences of this analysis are deemed pertinent to reveal the bearing of viscoelectric effect as a flow control mechanism pertaining to DOF at nanoscale.

Interplay of diffusio- and thermo-osmotic flows generated by single light stimulus

Flow control is a highly relevant topic for micromanipulation of colloidal particles in microfluidic applications. Here, we report on a system that combines two-surface bound flows emanating from thermo-osmotic and diffusio-osmotic mechanisms. These opposing flows are generated at a gold surface immersed into an aqueous solution containing a photo-sensitive surfactant, which is irradiated by a focused UV laser beam. At low power of incoming light, diffusio-osmotic flow due to local photo-isomerization of the surfactant dominates, resulting in a flow pattern oriented away from the irradiated area. In contrast, thermo-osmotic flow takes over due to local heating of the gold surface at larger power, consequently inducing a flow pointing toward the hotspot. In this way, this system allows one to reversibly switch from outward to inward liquid flow with an intermittent range of zero flow at which tracer particles undergo thermal motion by just tuning the laser intensity only. Our work, thus, demonstrates an optofluidic system for flow generation with a high degree of controllability that is necessary to transport particles precisely to desired locations, thereby opening innovative possibilities to generate advanced microfluidic applications.

Unraveling the physiochemical nature of colloidal motion waves among silver colloids.

Traveling waves are common in biological and synthetic systems, including the recent discovery that silver (Ag) colloids form traveling motion waves in H2O2 and under light. Here, we show that this colloidal motion wave is a heterogeneous excitable system. The Ag colloids generate traveling chemical waves via reaction-diffusion, and either self-propel through self-diffusiophoresis (“ballistic waves”) or are advected by diffusio-osmotic flows from gradients of neutral molecules (“swarming waves”). Key results include the experimental observation of traveling waves of OH− with pH-sensitive fluorescent dyes and a Rogers-McCulloch model that qualitatively and quantitatively reproduces the key features of colloidal waves. These results are a step forward in elucidating the Ag-H2O2-light oscillatory system at individual and collective levels. In addition, they pave the way for using colloidal waves either as a platform for studying nonlinear phenomena, or as a tool for colloidal transport and for information transmission in microrobot ensembles.

Diffusioosmosis-driven dispersion of colloids: a Taylor dispersion analysis with experimental validation

Diffusiophoresis refers to the movement of colloidal particles in the presence of a concentration gradient of a solute and enables directed motion of colloidal particles in geometries that are inaccessible, such as dead-end pores, without imposing an external field. Previous experimental reports on dead-end pore geometries show that, even in the absence of mean flow, colloidal particles moving through diffusiophoresis exhibit significant dispersion. Existing models of diffusiophoresis are not able to predict the dispersion and thus the comparison between the experiments and the models is largely qualitative. To address these quantitative differences between the experiments and models, we derive an effective one-dimensional equation, similar to a Taylor dispersion analysis, that accounts for the dispersion created by diffusioosmotic flow from the channel sidewalls. We derive the effective dispersion coefficient and validate our results by comparing them with direct numerical simulations. We also compare our model with experiments and obtain quantitative agreement for a wide range of colloidal particle sizes. Our analysis reveals two important conclusions. First, in the absence of mean flow, dispersion is driven by the flow created by diffusioosmotic wall slip such that spreading can be reduced by decreasing the channel wall diffusioosmotic mobility. Second, the model can explain the spreading of colloids in a dead-end pore for a wide range of particle sizes. We note that, while the analysis presented here focuses on a dead-end pore geometry with no mean flow, our theoretical framework is general and can be adapted to other geometries and other background flows.

Generation of Local Diffusioosmotic Flow by Light Responsive Microgels.

Here we show that microgels trapped at a solid wall can issue liquid flow and transport over distances several times larger than the particle size. The microgel consists of cross-linked poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-AA) polymer chains loaded with cationic azobenzene-containing surfactant, which can assume either a trans- or a cis-state depending on the wavelength of the applied irradiation. The microgel, being a selective absorber of trans-isomers, responds by changing its volume under irradiation with light of appropriate wavelength at which the cis-isomers of the surfactant molecules diffuse out of the particle interior. Together with the change in particle size, the expelled cis-isomers form an excess of the concentration and subsequent gradient in osmotic pressure generating a halo of local light-driven diffusioosmotic (l-LDDO) flow. The direction and the strength of the l-LDDO depends on the intensity and irradiation wavelength, as well as on the amount of surfactant absorbed by the microgel. The flow pattern around a microgel is directed radially outward and can be maintained quasi-indefinitely under exposure to blue light when the trans-/cis-ratio is 2/1, establishing a photostationary state. Irradiation with UV light, on the other hand, generates a radially transient flow pattern, which inverts from inward to outward over time at low intensities. By measuring the displacement of tracer particles around neutral microgels during a temperature-induced collapse, we can exclude that a change in particle shape itself causes the flow, i.e., just by expulsion or uptake of water. Ultimately, it is its ability to selectively absorb two isomers of photosensitive surfactant under different irradiation conditions that leads to an effective pumping caused by a self-induced diffusioosmotic flow.

Numerical Investigation of Diffusioosmotic Flow in a Tapered Nanochannel.

Diffusioosmosis concerns ionic flow driven by a concentration difference in a charged nano-confinement and has significant applications in micro/nano-fluidics because of its nonlinear current-voltage response, thereby acting as an active electric gating. We carry out a comprehensive computation fluid dynamics simulation to investigate diffusioosmotic flow in a charged nanochannel of linearly varying height under an electrolyte concentration gradient. We analyze the effects of cone angle (), nanochannel length (l) and tip diameter (), concentration difference ( = 0–1 mM), and external flow on the diffusioosmotic velocity in a tapered nanochannel with a constant surface charge density (). External flow velocity (varied over five orders of magnitude) shows a negligible influence on the diffusioosmotic flow inside the tapered nanochannel. We observed that a cone angle causes diffusioosmotic flow to move towards the direction of increasing gap thickness because of stronger local electric field caused by the overlapping of electric double layers near the smaller orifice. Moreover, the magnitude of average nanoflow velocity increases with increasing . Flow velocity at the nanochannel tip increases when is smaller or when l is greater. In addition, the magnitude of diffusioosmotic velocity increases with increasing . Our numerical results demonstrate the nonlinear dependence of tapered, diffusioosmotic flow on various crucial control parameters, e.g., concentration difference, cone angle, tip diameter, and nanochannel length, whereas an insignificant relationship on flow rate in the low Peclet number regime is observed.

How to Make a Surface Act as a Micropump

AbstractIn this paper, the phenomenon of light‐driven diffusioosmotic (DO) long‐range attractive and repulsive interactions between micro‐sized objects trapped near a solid wall is investigated. The range of the DO flow extends several times the size of microparticles and can be adjusted to point towards or away from the particle by varying irradiation parameters such as intensity or wavelength of light. The “fuel” of the light‐driven DO flow is a photosensitive surfactant which can be photo‐isomerized between trans and cis‐states. The trans‐isomer tends to accumulate at the interface, while the cis‐isomer prefers to stay in solution. In combination with a dissimilar photo‐isomerization rate at the interface and in bulk, this yields a concentration gradient of the isomers around single particles resulting in local light‐driven diffusioosmotic (l‐LDDO) flow. Here, the extended analysis of the l‐LDDO flow as a function of irradiation parameters by introducing time‐dependent development of the concentration excess of isomers near the particle surface is presented. It is also demonstrated that the l‐LDDO can be generated at any solid/liquid interface being more pronounced in the case of strongly absorbing material. This phenomenon has plenty of potential applications since it makes any type of surface act as a micropump.

Diffusion of a chemically active colloidal particle in composite channels

Diffusion of colloidal particles in microchannels has been extensively investigated, where the channel wall is either a no-slip or a slip-passive boundary. However, in the context of active fluids, driving boundary walls are ubiquitous and are expected to have a substantial effect on the particle dynamics. By mesoscale simulations, we study the diffusion of a chemically active colloidal particle in composite channels, which are constructed by alternately arranging the no-slip and diffusio-osmotic boundary walls. In this case, the chemical reaction catalyzed by the active colloidal particle creates a local chemical gradient along the channel wall, which drives a diffusio-osmotic flow parallel to the wall. We show that the diffusio-osmotic flow can significantly change the spatial distribution and diffusion dynamics of the colloidal particle in the composite channels. By modulating the surface properties of the channel wall, we can achieve different patterns of colloidal position distribution. The findings thus propose a novel possibility to manipulate colloidal diffusion in microfluidics, and highlight the importance of driving boundary walls in dynamics of colloidal particles in microchannels.

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.

Diffusiophoresis and diffusioosmosis in tandem: Two-dimensional particle motion in the presence of multiple electrolytes

We develop a model of time-dependent diffusiophoretic compaction of colloids in a two-dimensional pore due to the gradient of a multivalent solute in tandem with a diffusioosmotic slip-driven background flow field. The model is simulated for various surface charges and particle diffusivities. We also conduct experiments varying the initial ion combinations and total solute concentration. Our results indicate that diffusiophoretic compaction can be increased or decreased by manipulating electrolyte and total solute concentrations, wall charge, and particle diffusivity; each effect can modify the particle velocity, with varying strength, in unison or in opposition to the other effects.

Isotropically active colloids under uniform force fields: from forced to spontaneous motion

Abstract

Light-induced manipulation of passive and active microparticles

We consider sedimented at a solid wall particles that are immersed in water containing small additives of photosensitive ionic surfactants. It is shown that illumination with an appropriate wavelength, a beam intensity profile, shape and size could lead to a variety of dynamic, both unsteady and steady state, configurations of particles. These dynamic, well-controlled and switchable particle patterns at the wall are due to an emerging diffusio-osmotic flow that takes its origin in the adjacent to the wall electrostatic diffuse layer, where the concentration gradients of surfactant are induced by light. The conventional nonporous particles are passive and can move only with already generated flow. However, porous colloids actively participate themselves in the flow generation mechanism at the wall, which also sets their interactions that can be very long ranged. This light-induced diffusio-osmosis opens novel avenues to manipulate colloidal particles and assemble them to various patterns. We show in particular how to create and split optically the confined regions of particles of tunable size and shape, where well-controlled flow-induced forces on the colloids could result in their crystalline packing, formation of dilute lattices of well-separated particles, and other states.Graphic

Competition between electroosmotic and chemiosmotic flow in charged nanofluidics

In electrolyte solutions, charged nanoscale pores or channels with overlapping electrical double layers are charge selective, thereby benefiting a wide range of applications such as desalination, bio-sensing, membrane technology, and renewable energy. As an important forcing mechanism, a gradient of electrolyte concentration along a charged nano-confinement can drive flow without an external electrical field or applied pressure difference. In this paper, we numerically investigate such a diffusioosmotic nanoflow, particularly for dilute electrolyte concentrations (0.01 mM–1 mM), and calculate the corresponding electrical and concentration fields in a charged nanochannel connecting two reservoirs of different salt concentrations—a typical fluidic configuration for a variety of experimental applications. Under a wide range of parameters, the simulation results show that the flow speed inside the nanochannel is linearly dependent on the concentration difference between the two reservoir solutions, Δc, whereas the flow direction is primarily influenced by three key parameters: nanochannel length (l), height (h), and surface charge density (σ). Through a comparison of the chemiosmotic (due to ion-concentration difference) and electroosmotic (as a result of the induced electric field) components of this diffusioosmotic flow, a non-dimensional number (C=h/lλGC) has been identified to delineate different nanoscale flow directions in the charged nanochannel, where λGC is a characteristic (so-called Gouy–Chapman) length associated with surface charge and inversely proportional to σ. This critical dimensionless parameter, dependent on the above three key nanochannel parameters, can help in providing a feasible strategy for flow control in a charged nanochannel.