arXiv subjects

Evgeny S. Asmolov

Publications and source records attributed to Evgeny S. Asmolov.

At least 19 recordsLinked to original sources

arXiv2026

Reciprocal theorem for ion-releasing colloidal particles

We describe a generalization of the reciprocal theorem for particles suspended in electrolyte solutions and subjected to an electric field that could be either applied or emerged spontaneously. Attention is focused on catalytic colloids that release ions. The power of the generalization is to capture the effect of formation of a secondary cloud around a catalytic particle, which is equivalent to accounting for an excess charge $Q$ of a system. Our results show that the propulsion speed of catalytic particles has an extra contribution proportional to $Q$ and an external field $E_{\infty}$. The derived equation for $Q$ reveals that its sign is defined by the difference in the ion diffusivity and the magnitude is controlled by the average flux of ions from the surface. We demonstrate the application of the generalized theorem to electro- and diffusiophoresis of homogeneously releasing ions passive particles, as well as to a self-propulsion of inhomogeneous active particles (microswimmers). It is shown that whilst in some situations the extra term in the reciprocal theorem vanishes or has a little effect on the particle mobility, in many others it may dramatically change its magnitude, and even sign. In addition, the relevance of our results for microswimmer interactions is discussed briefly.

cond-mat.soft↗

arXiv2026

Diffusioosmosis of electrolyte solutions in axisymmetric channels

We present a theory of a flow of salt solutions in long axisymmetric channels induced by concentration and pressure drops between their ends. The consideration is restricted to thin, compared to the local radius, electrostatic diffuse layers, but remains valid even when the concentration drop is quite large. We show that the magnitude of the diffusio-osmotic fluid flow rate $Q_{DO}$ in the cylinder is the same as in the slit of equal to its diameter thickness, but channels of variable cross-sections could either retard or enhance it, depending on their geometry. The application of the pressure drop $Δp \neq 0$ results in an extra contribution $Q_{P}$ to the total flow rate of fluid $Q$, but does not affect $Q_{DO}$. We calculate the curves $Δp (Q)$ for several axisymmetric channels and conclude that they are nearly linear, with the sensitive to the shape slopes. This leads to the possibility of introducing a simple, but rather accurate, cylinder approximation, where the radius of the imaginary cylinder is related to a hydrodynamic resistivity of the real channel and can be easily determined, if its geometry is known. We also derive an equation relating the ionic flux with the total flow rate of fluid and demonstrate that both the sign and magnitude of ionic flux could be tuned by using the appropriate channel shape. Our analysis provides a framework for interpreting experimental and numerical data, as well as may guide the design of micro- and nanofluidic devices.

physics.flu-dyn↗

arXiv2026

Tuning diffusioosmosis of electrolyte solutions by hydrostatic pressure

When two reservoirs of a distinct salinity are connected by channels or pores, a fluid flow termed diffusio-osmotic is generated. This article investigates the flow emerging in an uniformly charged long slit whose thickness exceeds the local Debye screening length. Attention is focussed on the role of hydrostatic pressure drop $Δp$ in establishing diffusioosmosis at a finite concentration difference. For a thick slit we recover the known formula for a local diffusio-osmotic slip over a single wall, which is determined by the surface potential, salt concentration and its gradient. An equation for the global fluid flow rate $\mathcal{Q}$ is presented as a sum of the diffusio-osmotic and pressure-driven contributions. Although the diffusio-osmotic term itself remains unaffected by $Δp$, the nonlinear concentration and surface potential profiles along the slit, and consequently, the local slip velocity are dramatically modified. We present an equation relating the local concentration to $\mathcal{Q}$ and employ it to derive an expression describing the surface potential variation in the slit. Since $\mathcal{Q}$ can easily be tuned by $Δp$, the variety of possible concentration and surface potential profiles becomes very rich. Our theory provides a simple explanation of recent flow rate measurements and shows that experimental data provide rather direct information about concentration and surface potential profiles in the uniformly charged slit. The relevance of our results for sensing the salt dependence of surface potentials is discussed briefly.

physics.flu-dyn↗

arXiv2025

Diffusioosmosis of electrolyte solutions in uniformly charged channels

When the concentration of electrolyte solution varies along the channel the forces arise that drag the fluid toward the higher or lower concentration region inducing a flow termed diffusio-osmotic. This article investigates a flow that emerges in channels with constant density of surface charge σ and thin compared to their thickness electrostatic diffuse layers. An equation for the fluid flow rate Q is derived and used to describe analytically the flux of ions, and local potentials and concentrations. This equation, which allows to treat the diffusio-osmotic problems without tedious and time consuming computations, clarifies that the global flow rate is controlled only by the surface charge and concentration drop between the channel ends, and indicates that there always exist two different values of σ that correspond to a particular Q. Our theory provides a simple explanation of the directions of the fluid flow rate and ionic flux depending on the surface charge and diffusivity of ions, predicts a non-linear concentration distribution along the channel caused by convection, and relates it to the local potential changes by a compact formula. We also present and interpret the variations of the diffusio-osmotic velocity profiles and the apparent slip velocity along the channel and show that the latter is highly non-uniform and could even becomes alternating. The relevance of our results for diffusio-osmotic experiments and for some electrochemistry and membrane science issues is discussed briefly.

cond-mat.soft↗

arXiv2025

Enhanced zeta potentials caused by surface ion mobilities

The electro-hydrodynamics near conducting walls is revisited. Attention is focused on the impact of an explicit diffuse Stern layer, which permittivity and viscosity differ from the bulk values, on the velocity of an electro-osmotic plug flow. To solve this problem we propose an approach of mapping the flow in the Stern layer to the surface dividing the Stern and diffuse layer, where an effective electro-hydrodynamic slip boundary condition is imposed. The latter implies that an effective surface charge is responding to the applied field and characterized by a mobility parameter $μ\geq 1$. We derive analytic equations for $μ$ and demonstrate that it is determined only by electrostatic properties of the electric double layer. These equations are then used to calculate electrokinetic (zeta) potentials of surfaces. We show that the zeta potential generally exceeds the surface one, which implies an amplification of the electro-osmotic flow. This effect is most pronounced if the hydrodynamic slip length is large and/or in concentrated solutions.

physics.flu-dyn↗

arXiv2024

Electrophoretic velocity of ion-releasing colloidal particles

By means of a matched asymptotic expansions approach the electrophoretic velocity and zeta potential of a catalytic particle that uniformly releases ions have been investigated. Attention is focused on large, compared to diffuse layer, particles characterized, beside the surface potential $Φ_s$, by the Damköhler number Da that represents the ratio of the surface reaction rate to the diffusive transfer one. For vanishing Da, we recover the classical Smoluchowski formula for the electrophoretic velocity which states that the zeta potential of the particle is equal to $Φ_s$ and that the migration direction is determined by its sign. For small values of Da we show that the migration velocity is controlled mostly by $Φ_s$ and affected by an ion release only slightly. However, even small Da can induce the electrophoresis of electro-neutral particles that would be immobile if inert. For larger Da the direction of migration and the sign of zeta potential become independent on $Φ_s$ and are solely determined by the difference in diffusivity of released cations and anions. Still, the surface potential affects the magnitude of the particle velocity.

cond-mat.soft↗

arXiv2024

Diffusiophoresis of ionic catalytic particles

A migration of charged particles relative to a solvent, caused by a gradient of salt concentration and termed a diffusiophoresis, is of much interest being exploited in many fields. Existing theories deal with diffusiophoresis of passive inert particles. In this paper, we extend prior models by focusing on a particle, which is both passive and catalytic, by postulating an uniform ion release over its surface. We derive an expression for a particle velocity depending on a dimensionless ion flux (Damkohler number Da) and show that a charged region is formed at distances of the order of the particle size, provided the diffusion coefficients of anions and cations are unequal. When Da becomes large enough, the contribution of this (outer) region to the particle velocity dominates. In this case the speed of catalytic passive particles augments linearly with Da and is inversely proportional to the square of electrolyte concentration. As a result, they always migrate towards a high concentration region and in dilute solutions become much faster than inert (non-catalytic) ones.

cond-mat.soft↗

arXiv2024

Surface potentials of conductors in electrolyte solutions

When we place conducting bodies in electrolyte solutions, their surface potential $Φ_s$ appears to be much smaller in magnitude than the intrinsic one $Φ_0$ and normally does not obey the classical electrostatic boundary condition of a constant surface potential expected for conductors. In this paper, we demonstrate that an explanation of these observations can be obtained by postulating that diffuse ions condense at the "wall" due to a reduced permittivity of a solvent. For small values of $Φ_0$ the surface potential responds linearly. On increasing $Φ_0$ further $Φ_s$ augments nonlinearly and then saturates to a constant value. Analytical approximations for $Φ_s$ derived for these three distinct modes show that it always adjusts to salt concentration, which is equivalent to a violation of the constant potential condition. The latter would be appropriate for highly dilute solutions, but only if $Φ_0$ is small. Surprisingly, when the plateau with high $Φ_s$ is reached, the conductor surface switches to a constant charge density condition normally expected for insulators. Our results are directly relevant for conducting electrodes, mercury drops, colloidal metallic particles and more.

cond-mat.soft↗

arXiv2023

Slippery and mobile hydrophobic electrokinetics: from single walls to nanochannels

We discuss how the wettability of solid walls impacts electrokinetic properties, from large systems to a nanoscale. We show in particular how could the hydrophobic slippage, coupled to confinement effects, be exploited to induce novel electrokinetic properties, such as a salt-dependent giant amplification of zeta potential and conductivity, and a much more efficient energy conversion. However, the impact of slippage is dramatically reduced if some surface charges migrate along the hydrophobic wall under an applied field.

cond-mat.soft↗

arXiv2023

Limiting propulsion of ionic microswimmers

Self-propulsion of catalytic Janus swimmers in electrolyte solutions is induced by inhomogeneous ion release from their surface. Here, we consider the experimentally relevant cases of particles which emit only one type of ions (type I) or equal fluxes of cations and anions (type II). In the limit of a thin electrostatic diffuse layer we derive a nonlinear outer solution for the electric field and concentrations of active (i.e. released from the surface) and passive ionic species. We show that for swimmers of type I both the maximum ion flux and propulsion velocity are constrained. This suggests that the propulsion of Janus swimmers can be optimized by tuning the concentration of active ions.

cond-mat.soft↗

arXiv2022

Transport of ions in hydrophobic nanotubes

The theory of electrokinetic ion transport in cylindrical channels of a fixed surface charge density is revisited. Attention is focused on impact of the hydrophobic slippage and mobility of adsorbed surface charges. We formulate generalised Onsager relations for a cylinder of an arbitrary radius and then derive exact expressions for the mean electro-osmotic mobility and conductivity. To employ these expressions we perform additional electrostatic calculations, with the special focus on the non-linear electrostatic effects. Our theory provides a simple explanation of a giant enhancement of the electrokinetic mobility and conductivity of hydrophobic nanotubes by highlighting the role of appropriate electrostatic and hydrodynamic length scales and their ratios. We also propose a novel interpretation of zeta potentials of cylindrical channels.

cond-mat.soft↗

arXiv2022

Theoretical modeling of catalytic self-propulsion

Self-propelling particles or microswimmers have opened a new field of investigation with both fundamental and practical perspectives. They represent very convenient model objects for experimental studies of active matter, and have implications in nano-robotics, drug delivery, and more. Here, we summarize recent advances in theoretical description of the self-propulsion of catalytic microswimmers that non-uniformly release ions, including its physical origins and the current switch in focus to non-linear effects, geometric tuning and more. In particular, we show that the ionic self-propulsion always includes both diffusiophoretic and electrophoretic contributions, and that non-linear effects are physical causes of a number of intriguing phenomena, such as the reverse in the direction of the particle motion in response to variations of the salt concentration or self-propulsion of electro-neutral particles. We finally suggest several remaining theoretical challenges in the field.

cond-mat.soft↗

arXiv2022

Self-diffusiophoresis of Janus particles that release ions

Catalytic Janus swimmers demonstrate a diffusio-phoretic motion by self-generating the gradients of concentrations and electric potential. Recent work has focused on simplified cases, such as a release of solely one type of ions or low surface fluxes of ions, with limited theoretical guidance. Here, we consider the experimentally relevant case of particles that release both types of ions, and obtain a simple expression for a particle velocity in the limit of thin electrostatic diffuse layer. Our approximate expression is very accurate even when ion fluxes and surface potentials are large, and allows one to interpret a number of intriguing phenomena, such as the reverse in the direction of the particle motion in response to variations of the salt concentration or self-diffusiophoresis of uncharged particles.

cond-mat.soft↗

arXiv2021

Instability of particle inertial migration in shear flow

In a shear flow particles migrate to their equilibrium positions in the microchannel. Here we demonstrate theoretically that if particles are inertial, this equilibrium can become unstable due to the Saffman lift force. We derive an expression for the critical Stokes number that determines the onset of instable equilibrium. We also present results of lattice Boltzmann simulations for spherical particles and prolate spheroids to validate the analysis. Our work provides a simple explanation of several unusual phenomena observed in earlier experiments and computer simulations, but never interpreted before in terms of the unstable equilibrium.

physics.flu-dyn↗

arXiv2021

Enhanced transport of ions by tuning surface properties of the nanochannel

We revisit the theory of ion transport in parallel-plate channels and also discuss how the wettability of a solid and the mobility of adsorbed surface charges impact the transport of ions. It is shown that depending on the ratio of the electrostatic disjoining pressure to the excess osmotic pressure at the walls two different regimes occur. In the thick channel regime this ratio is small and the channel effectively behaves as thick, even when the diffuse layers strongly overlap. The latter is possible for highly charged channels only. In the thin channel regime the disjoining pressure is comparable to the excess osmotic pressure at the wall, which implies relatively weakly charged walls. We derive simple expressions for the mean conductivity of the channel in these two regimes, highlighting the role of electrostatic and electro-hydrodynamic boundary conditions. Our theory provides a simple explanation of the high conductivity observed experimentally in hydrophilic channels, and allows one to obtain rigorous bounds on its attainable value and scaling with salt concentration. Our results also show that further dramatic amplification of conductivity is possible if hydrophobic slip is involved, but only in the thick channel regime provided the walls are sufficiently highly charged and the most of adsorbed charges are immobile. However, for weakly charged surfaces the massive conductivity amplification due to hydrodynamic slip is impossible in both regimes. Interestingly, in this case the moderate slip-driven contribution to conductivity can monotonously decrease with the fraction of immobile adsorbed charges. These results provide a framework for tuning the conductivity of nanochannels by adjusting their surface properties and bulk electrolyte concentrations.

cond-mat.soft↗

arXiv2021

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 cristalline packing, formation of dilute lattices of well-separated particles, and other states.

cond-mat.soft↗

arXiv2020

Inertial migration of oblate spheroids in a plane channel

We discuss an inertial migration of oblate spheroids in a plane channel, where steady laminar flow is generated by a pressure gradient. Our lattice Boltzmann simulations show that spheroids orient in the flow, so that their minor axis coincides with the vorticity direction (a log-rolling motion). Interestingly, for spheroids of moderate aspect ratios, the equilibrium positions relative to the channel walls depend only on their equatorial radius $a$. By analysing the inertial lift force we argue that this force is proportional to $a^3b$, where $b$ is the polar radius, and conclude that the dimensionless lift coefficient of the oblate spheroid does not depend on $b$, and is equal to that of the sphere of radius $a$.

physics.flu-dyn↗

arXiv2020

Achieving large zeta-potentials with charged porous surfaces

We discuss an electro-osmotic flow near charged porous coatings of a finite hydrodynamic permeability, impregnated with an outer electrolyte solution. It is shown that their electrokinetic (zeta) potential is generally augmented compared to the surface electrostatic potential, thanks to a large liquid slip at their surface emerging due to an electro-osmotic flow in the enriched by counter-ions porous films. The inner flow shows a very rich behavior controlled by the volume charge density of the coating, its Brinkman length, and concentration of added salt. Interestingly, even for relatively small Brinkman length the zeta-potential can, in some cases, become huge, providing a very fast outer flow in the bulk electrolyte. When the Brinkman length is large enough, the zeta-potential could be extremely high even at practically vanishing surface potential. To describe the slip velocity in a simple manner, we introduce a concept of an electro-osmotic slip length and demonstrate that the latter is always defined by the hydrodynamic permeability of the porous film, and also, depending on the regime, either by its volume charge density or the salt concentration. These results provide a framework for the rational design of porous coatings to enhance electrokineic phenomena, and for tuning their properties by adjusting bulk electrolyte concentrations, with direct applications in microfluidics.

physics.flu-dyn↗