arXiv subjects

Yuki Uematsu

Publications and source records attributed to Yuki Uematsu.

17 recordsLinked to original sources

arXiv2026

Negative Differential Capacitance from Composition-Dependent Stern Capacitance in a Binary Mixture

We develop a thermodynamic theory of electric double layers in binary liquid mixtures by allowing the Stern-layer capacitance to depend on the local solvent composition. This coupling produces an additional negative contribution to the inverse differential capacitance. As a result, the surface potential can become a nonmonotonic function of the surface charge density, leading to negative differential capacitance and a voltage-induced first-order transition between two electric-double-layer states. We determine the coexistence condition using a common-tangent construction for the surface-charge-controlled grand potential and obtain phase diagrams in terms of the Stern-capacitance contrast, surface charge density, bulk composition, and surface potential. We also compare the theory with capacitance data for tetrabutylammonium chloride in water/1-propanol mixtures, finding semi-quantitative agreement in the continuous-response regime. These results suggest that solvent exchange inside the Stern layer can strongly control the capacitance and interfacial phase behavior of binary mixtures.

physics.chem-ph↗

arXiv2025

Statistical-Mechanical Theory on the Probability Distribution Function for the Net Charge of an Electrolyte Droplet

Droplets of electrolyte solutions in an insulating medium are ubiquitous in nature. The net charges of these droplets are normally nonzero, and they fluctuate. However, a theory on the probability distribution function for the net charge of droplets is lacking, so far. Thus, a statistical-mechanical theory of a charged droplet is developed with including the effect of the electrostatic energy of charging as well as the random distribution of ions. Then, the probability distribution function for the net charge of an electrolyte droplet is calculated assuming that it is generated from a macroscopic solution with the different cation and anion concentrations. Using the Gaussian approximation and Stirling's formula, the analytic results for the average and variance of the net charge of a droplet are obtained.

physics.chem-ph↗

arXiv2025

Ion adsorption and zeta potential of hydrophobic interfaces

Hydrophobic interfaces have unique physicochemical properties and are used in various chemical products such as food, cosmetics, soap, and medicine and technologies such as pan coating and ski wax. In this chapter, we describe the fundamental concept of hydrophobic interfaces and explain their ion adsorption and zeta potential by using experimental data from the literature. Thus far, these electrical properties are considered universal for solid/water, liquid/water, and gas/water interfaces; however, a careful comparison in this chapter will reveal significant differences among them. To confirm that the affinity of H$^+$ ions for all hydrophobic interfaces is stronger than that of OH$^-$ ions, more experimental data on hydrophobic liquid/water and solid/water interfaces are required.

physics.chem-ph↗

arXiv2025

Diffuse-Layer Capacitance at the Potential of Zero Charge in Binary Mixtures

The capacitance of the electric double layer has potential applications in supercapacitors, and theoretical investigations of the double-layer capacitance in binary mixtures are important. In this work, we develop the theory of the electric double layer in binary mixtures, and the diffuse-layer capacitance at the potential of zero charge is obtained analytically. Furthermore, we observe a divergence of the capacitance in the phase diagram, suggesting a surface instability. The obtained capacitance is different from that derived using single-liquid approximation unless the preferential solvation energies of cations and anions are the same. When the system is close to the surface instability line, the capacitance strongly deviates from the results of single-liquid approximation.

physics.chem-ph↗

arXiv2025

Ostwald Ripening of Buoyancy-Driven Microbubbles

Microbubble solutions have a wide range of industrial applications, including heat transfer, agriculture, and water treatment. Therefore, understanding and controlling the size variation of bubbles is critical. In this study, we develop a theoretical framework for Ostwald ripening in buoyancy-driven microbubbles by introducing a height-dependent size distribution function. For the first time, we show that the population balance equation in steady state can be interpreted within the Lifshitz-Slezov-Wagner theory when the distribution function is redefined as the density distribution of the buoyancy-induced flux. Notably, in this form of Ostwald ripening, the distribution function approaches a scaled universal distribution, not over time, but as a function of height. We analytically derive the scaled universal distribution and show that the fifth power of the mean radius of the bubbles grows linearly with height.

physics.chem-ph↗

arXiv2024

Decomposition of Friction Coefficients to Analyze Hydration Effects on a C$_{60}$(OH)$_{\rm n}$

To analyze hydration effects on macromolecular diffusion, the friction coefficients of macromolecules were examined using molecular dynamics simulations with an all-atom model. In the present study, a method was introduced to decompose the molecular friction coefficient into the contributions for each site on the macromolecule. The method was applied to several fullerenols in ambient water. The friction coefficients for the hydrophilic part, such as the OH group, were larger than those for the hydrophobic part, such as the C. The hydration effect did not depend only on the kind of functional group but also on surface roughness. This approach would be useful in explaining the experimentally observed large changes in diffusion coefficients of proteins that were accompanied by conformation changes.

physics.bio-ph↗

arXiv2023

Ubiquitous preferential water adsorption to electrodes in water/1-propanol mixtures detected by electrochemical impedance spectroscopy

The electric double layer is an important structure that appears at charged liquid interfaces, and it determines the performance of various electrochemical devices such as supercapacitors and electrokinetic energy converters. Here the double-layer capacitance of the interface between aluminum electrodes and water/1-propanol electrolyte solutions is investigated using electrochemical impedance spectroscopy. The double-layer capacitances of mixture solvents are almost the same as those of water-only electrolyte solutions, and the double-layer capacitance of 1-propanol-only solutions are significantly smaller than those of other volume fractions of water. The qualitative variation of the double-layer capacitances with the water volume fraction is independent of the electrolyte types and their concentrations. Therefore, these results can be explained by ubiquitous preferential water adsorption caused by the hydrophilicity of the electrode surface.

cond-mat.soft↗

arXiv2023

Electrophoretic mobility of a water-in-oil droplet separately affected by the net charge and surface charge density

Water-in-oil emulsions and droplets exhibit completely different physico-chemical properties compared to oil-in-water emulsions and droplets. Thus, directly applying a standard theoretical model to water-in-oil systems cannot describe these anomalous properties. Here, the electrophoretic mobility of a water-in-oil droplet is analytically investigated using Debye-Hückel linearization and neglecting Marangoni effect. The resulting electrophoretic mobility is shown to be separately dependent on the net charge of the droplet and the surface charge density at the droplet interface. Furthermore, when the net charge is negligible, electrophoretic mobility is proportional to the surface charge density with a negative coefficient. This indicates that the internal electric double layer inversely contributes the electrophoresis. This theory is applied to experimental data of water-in-oil emulsions and droplets in the literature, and qualitative and quantitative verification of the theory is discussed.

cond-mat.soft↗

arXiv2023

Ostwald ripening of aqueous microbubble solutions

Bubble solutions are of growing interest because of various technological applications in surface cleaning, water treatment, and agriculture. However, their physicochemical properties such as the stability and interfacial charge of bubbles are not fully understood yet. In this study, the kinetics of radii in aqueous microbubble solutions are experimentally investigated, and the results are discussed in the context of Ostwald ripening. The obtained distributions of bubble radii scaled by mean radius and total number were found to be time-independent during the observation period. Image analysis of radii kinetics revealed that the average growth and shrinkage speed of each bubble is governed by diffusion-limited Ostwald ripening, and the kinetic coefficient calculated using the available physicochemical constants in literature quantitatively agrees with the experimental data. Furthermore, the cube of mean radius and mean volume exhibit a linear time evolution in agreement with the Lifshitz-Slezov-Wagner (LSW) theory. The coefficients are slightly larger than those predicted using the LSW theory, which can be qualitatively explained by the effect of finite volume fraction. Finally, the slow down and pinning of radius in the shrinkage dynamics of small microbubbles are discussed in detail.

cond-mat.soft↗

arXiv2023

Analytic theory of nonlinearly coupled electrokinetics in nanochannels

The nonlinear electrokinetic response of ionic solutions is important in nanofluidics. However, quantitatively understanding the mechanisms is still a challenging problem because of a lack of analytic approaches. Here, a general framework for calculating the nonlinear electrokinetic coefficients of strongly confined electrolytes is constructed using a perturbation scheme of the pressure and voltage differences across a nanochannel. The theory is applied to an electrically neutral nanochannel filled with electrolytes, and analytic expressions for the first- and third-order electrokinetic coefficients are obtained. We demonstrate that the combination of high hydrodynamic permeability and ion-wall friction plays an essential role in nonlinear electrokinetics. Furthermore, we analytically demonstrate that the external flow induces uniform excess charge inside the nanochannel.

physics.flu-dyn↗

arXiv2021

Electrification of water interface

The surface charge of a water interface determines many fundamental processes in physical chemistry and interface science, and it has been intensively studied for over a hundred years. We summarize experimental methods to characterize the surface charge densities developed so far: electrokinetics, double-layer force measurements, potentiometric titration, surface-sensitive nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we elucidate physical ion adsorption and chemical electrification as examples of electrification mechanisms. In the end, novel effects on surface electrification are discussed in detail. We believe that this clear overview of state of the art in a charged water interface will surely help the fundamental progress of physics and chemistry at interfaces in the future.

physics.chem-ph↗

arXiv2018

Intentionally added ionic surfactants induce Jones-Ray effect at air-water interface

The Jones-Ray effect is an anomalous minimum in the surface tension of aqueous electrolytes at millimolar salt concentrations. We experimentally demonstrated that intentionally added ionic surfactants induce the Jones-Ray effect. The one-dimensional Poisson-Boltzmann theory, including the effect of surfactant adsorption and salt depletion, excellently agrees with the obtained experimental data. All the parameters of the ion-specific surface affinities used in the theory are consistent with previous experiments. These results strongly suggest that the Jones-Ray effect observed so far has been induced by the inevitable contamination of the air-water interfaces.

physics.chem-ph↗

arXiv2018

Linear response functions of an electrolyte solution in a uniform flow

We study the steady state response of a dilute monovalent electrolyte solution to an external source with a constant relative velocity with respect to the fluid. The source is taken as a combination of three perturbations: an external force acting on the fluid, an externally imposed ionic chemical potential, and an external charge density. The linear response functions are obtained analytically and can be decoupled into three independent terms, corresponding to (i) fluid flow and pressure, (ii) total ionic number density and current, and (iii) charge density, electrostatic potential and electric current. It is shown how the uniform flow breaks the equilibrium radial symmetry of the response functions, leading to a distortion of the ionic cloud and electrostatic potential, which deviate from the standard Debye-Hückel result. The potential of a moving charge is under-screened in its direction of motion and over-screened in the opposite direction and normal plane. As a result, an unscreened dipolar electric field and electric currents are induced far from the charged source. We relate our general formalism to several experimental setups, such as colloidal sedimentation.

physics.flu-dyn↗

arXiv2015

Nonlinear electro-osmosis in dilute non-adsorbing polymer solutions with low ionic strength

Nonlinear behavior of electro-osmosis in dilute non-adsorbing polymer solutions with low salinity is investigated with Brownian dynamics simulations and a kinetic theory. In the Brownian simulations, the hydrodynamic interaction between the polymers and a no-slip wall is considered with Rotne-Prager approximation of Blake tensor. In a plug flow under a sufficiently strong applied electric field, the polymer migrates toward the bulk, forming a depletion layer thicker than the equilibrium one. Consequently, the electro-osmotic mobility increases nonlinearly with the electric field and gets saturated. This nonlinear mobility qualitatively does not depend on the details of rheological properties of the polymer solution. Analytical calculation of the kinetic theory for the same system reproduces quantitatively well the results of the Brownian dynamics simulation.

cond-mat.soft↗

arXiv2014

Electrophoresis of electrically neutral porous spheres induced by selective affinity of ions

We investigate the possibility that electrically neutral porous spheres electrophorese in electrolyte solutions with asymmetric affinity of ions to spheres on the basis of electrohydrodynamics and the Poisson-Boltzmann and Debye-Bueche-Brinkman theories. Assuming a weak electric field and ignoring the double-layer polarization, we obtain analytical expressions for electrostatic potential, electrophoretic mobility, and flow field. In the equilibrium state, the Galvani potential forms across the interface of the spheres. Under a weak electric field, the spheres show finite mobility with the same sign as the Galvani potential. When the radius of the spheres is significantly larger than the Debye and hydrodynamic screening length, the mobility monotonically increases with increasing salinity.

cond-mat.soft↗

arXiv2013

Electro-Osmotic Flow of Semidilute Polyelectrolyte Solutions

We investigate electro-osmosis in aqueous solutions of polyelectrolytes using mean-field equations. A solution of positively charged polyelectrolytes is confined between two negatively charged planar surfaces, and an electric field is applied parallel to the surfaces. When electrostatic attraction between the polymer and the surface is strong, the polymers adhere to the surface, forming a highly viscous adsorption layer that greatly suppresses the electro-osmosis. Conversely, electro-osmosis is enhanced by depleting the polymers from the surfaces. We also found that the electro-osmotic flow is invertible when the electrostatic potential decays to its bulk value with the opposite sign. These behaviors are well explained by a simple mathematical form of the electro-osmotic coefficient.

cond-mat.soft↗

arXiv2012

Effects of strongly selective additives on volume phase transition in gels

We investigate volume phase transition in gels immersed in mixture solvents, on the basis of a three-component Flory-Rehner theory. When the selectivity of the minority solvent component to the polymer network is strong, the gel tends to shrink with an increasing concentration of the additive, regardless of whether it is good or poor. This behavior originates from the difference of the additive concentration between inside and outside the gel. We also found the gap of the gel volume at the transition point can be controlled by adding the strongly selective solutes. By dissolving a strongly poor additive, for instance, the discontinuous volume phase transition can be extinguished. Furthermore, we observed that another volume phase trasition occurs far from the original transition point. These behaviors can be well explained by a simplified theory neglecting the non-linearity of the additive concentration.

cond-mat.soft↗