Non-Equilibrium Surface Forces

A theory of the non-equilibrium double layer and of non- equilibrium surface forces is presented. Departures from the equilibrium state in the double layer are observed whenever there . is flow in this structure. In the theory only external field influences are treated. Dipolophoresis and diffusiophoresis are defined, and conditions for a steady-state are described. Application of the theory in interpreting phenomena of electrocoagulation, electrofiltration, bubble flotation, and electrodeposition of polymers is briefly discussed

Non-equilibrium Surface Forces and Hydrodynamics of Thin Films

The notion of »non-equilibrium surface forces« is advanced for the application in a number of cases of colloid stability and transport phenomena. The idealized concept of full equilibrium in the double layer, as used in the DLVO theory is shown to be too simple to describe the stability of a colloid particle not in equilibrium with the surrounding medium. Cases are described where surface forces are stipulated by adsorption layer effects, controlled by diffusion, such as flotation, crystal growth, hydrodynamic transport, and in describing the stability of living cells in suspension

A Classical Density-Functional Theory for Describing Water Interfaces

We develop a classical density functional for water which combines the White Bear fundamental-measure theory (FMT) functional for the hard sphere fluid with attractive interactions based on the Statistical Associating Fluid Theory (SAFT-VR). This functional reproduces the properties of water at both long and short length scales over a wide range of temperatures, and is computationally efficient, comparable to the cost of FMT itself. We demonstrate our functional by applying it to systems composed of two hard rods, four hard rods arranged in a square and hard spheres in water

Electrophoretic mobility of a charged colloidal particle: A computer simulation study

We study the mobility of a charged colloidal particle in a constant homogeneous electric field by means of computer simulations. The simulation method combines a lattice Boltzmann scheme for the fluid with standard Langevin dynamics for the colloidal particle, which is built up from a net of bonded particles forming the surface of the colloid. The coupling between the two subsystems is introduced via friction forces. In addition explicit counterions, also coupled to the fluid, are present. We observe a non-monotonous dependence of the electrophoretic mobility on the bare colloidal charge. At low surface charge density we observe a linear increase of the mobility with bare charge, whereas at higher charges, where more than half of the ions are co-moving with the colloid, the mobility decreases with increasing bare charge.Comment: 15 pages, 8 figure

Diffuse-Charge Dynamics in Electrochemical Systems

The response of a model micro-electrochemical system to a time-dependent applied voltage is analyzed. The article begins with a fresh historical review including electrochemistry, colloidal science, and microfluidics. The model problem consists of a symmetric binary electrolyte between parallel-plate, blocking electrodes which suddenly apply a voltage. Compact Stern layers on the electrodes are also taken into account. The Nernst-Planck-Poisson equations are first linearized and solved by Laplace transforms for small voltages, and numerical solutions are obtained for large voltages. The ``weakly nonlinear'' limit of thin double layers is then analyzed by matched asymptotic expansions in the small parameter $\epsilon = \lambda_D/L$, where $\lambda_D$ is the screening length and $L$ the electrode separation. At leading order, the system initially behaves like an RC circuit with a response time of $\lambda_D L / D$ (not $\lambda_D^2/D$), where $D$ is the ionic diffusivity, but nonlinearity violates this common picture and introduce multiple time scales. The charging process slows down, and neutral-salt adsorption by the diffuse part of the double layer couples to bulk diffusion at the time scale, $L^2/D$. In the ``strongly nonlinear'' regime (controlled by a dimensionless parameter resembling the Dukhin number), this effect produces bulk concentration gradients, and, at very large voltages, transient space charge. The article concludes with an overview of more general situations involving surface conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference

A simulational and theoretical study of the spherical electrical double layer for a size-asymmetric electrolyte: the case of big coions

Monte Carlo simulations of a spherical macroion, surrounded by a size-asymmetric electrolyte in the primitive model, were performed. We considered 1:1 and 2:2 salts with a size ratio of 2 (i.e., with coions twice the size of counterions), for several surface charge densities of the macrosphere. The radial distribution functions, electrostatic potential at the Helmholtz surfaces, and integrated charge are reported. We compare these simulational data with original results obtained from the Ornstein-Zernike integral equation, supplemented by the hypernetted chain/hypernetted chain (HNC/HNC) and hypernetted chain/mean spherical approximation (HNC/MSA) closures, and with the corresponding calculations using the modified Gouy-Chapman and unequal-radius modified Gouy-Chapman theories. The HNC/HNC and HNC/MSA integral equations formalisms show good concordance with Monte Carlo "experiments", whereas the notable limitations of point-ion approaches are evidenced. Most importantly, the simulations confirm our previous theoretical predictions of the non-dominance of the counterions in the size-asymmetric spherical electrical double layer [J. Chem. Phys. 123, 034703 (2005)], the appearance of anomalous curvatures at the outer Helmholtz plane and the enhancement of charge reversal and screening at high colloidal surface charge densities due to the ionic size asymmetry.Comment: 11 pages, 7 figure

Study of the effectiveness of different methods of mulching potatoes (Solanum tuberosum) with cereal straw in eastern Ukraine

Received: March 19th, 2023 ; Accepted: June 16th, 2023 ; Published: July 2nd, 2023 ; Correspondence: [email protected] the conditions of global climate change, there has recently been a tendency to worsen weather conditions during the growing season in the East of Ukraine. The significant increase in average daily air temperatures and the uneven nature of rainfall do not meet the biological requirements of potato plants, which leads to a decrease in their productivity. The purpose of the research. Improve the microclimate on potato plantations by mulching them with different types of materials. Results of research. Using wheat straw or basalt mineral wool as mulch reduces overheating of the upper layers of the soil by 6.6–7.0° and contributes to moisture conservation. At the same time, there is a significant decrease in temperature and relative humidity fluctuations under the mulch layer compared to the soil surface. The use of spent mineral basalt wool for this purpose, which was previously used as a substrate in greenhouses, solves the disposal problem. Deterioration of soil chemical parameters and product quality was not noted. The use of nitrogen fertilizers (urea) or microbial preparations Azotofit-r and Ecostern before mulching with wheat straw has a positive effect on plant nutrition, which contributes to the improvement of biometric indicators and increases the yield of potatoes by 4.2–4.7 t ha-1 . Potato mulching with zero tillage involves planting tubers on its surface. At the same time, their contact with the soil is significantly reduced, decreasing the supply of nutrients from it after the transition of potato plants to autotrophic nutrition. Under these conditions, minimal tillage of the soil during its pre-planting preparation improves the development of the root system and promotes the formation of stolons and tubers at a certain depth. Depending on the number of technological operations, the degree of loosening of the soil significantly affects the yield of potatoes. Its growth with minimal tillage is from 1.3 to 10.7 t ha-1 compared to zero tillage. This allows you to compensate for additional costs related to mulching and collecting additional products. The improvement of the main economic indicators confirms the high efficiency of mulching potatoes with wheat straw with minimal tillage. Results were expressed as averages. Chemical composition was analyzed using analysis of variance with P ≤ 0.05 for yield and plant height using the program of statistical analysis (SAS) v. 9.1.3

Can airborne ultrasound monitor bubble size in chocolate?

Aerated chocolate products consist of solid chocolate with the inclusion of bubbles and are a popular consumer product in many countries. The volume fraction and size distribution of the bubbles has an effect on their sensory properties and manufacturing cost. For these reasons it is important to have an online real time process monitoring system capable of measuring their bubble size distribution. As these products are eaten by consumers it is desirable that the monitoring system is non contact to avoid food contaminations. In this work we assess the feasibility of using an airborne ultrasound system to monitor the bubble size distribution in aerated chocolate bars. The experimental results from the airborne acoustic experiments were compared with theoretical results for known bubble size distributions using COMSOL Multiphysics. This combined experimental and theoretical approach is used to develop a greater understanding of how ultrasound propagates through aerated chocolate and to assess the feasibility of using airborne ultrasound to monitor bubble size distribution in these systems. The results indicated that a smaller bubble size distribution would result in an increase in attenuation through the product

The electrical double layer for a fully asymmetric electrolyte around a spherical colloid: an integral equation study

The hypernetted chain/mean spherical approximation (HNC/MSA) integral equation is obtained and solved numerically for a totally asymmetric primitive model electrolyte around a spherical macroparticle. The ensuing radial distribution functions show a very good agreement when compared to our Monte Carlo and molecular dynamics simulations for spherical geometry and with respect to previous anisotropic reference HNC calculations in the planar limit. We report an analysis of the potential vs charge relationship, radial distribution functions, mean electrostatic potential and cumulative reduced charge for representative cases of 1:1 and 2:2 salts with a size asymmetry ratio of 2. Our results are collated with those of the Modified Gouy-Chapman (MGC) and unequal radius Modified Gouy-Chapman (URMGC) theories and with those of HNC/MSA in the restricted primitive model (RPM) to assess the importance of size asymmetry effects. One of the most striking characteristics found is that,\textit{contrary to the general belief}, away from the point of zero charge the properties of an asymmetric electrical double layer (EDL) are not those corresponding to a symmetric electrolyte with the size and charge of the counterion, i.e. \textit{counterions do not always dominate}. This behavior suggests the existence of a new phenomenology in the EDL that genuinely belongs to a more realistic size-asymmetric model where steric correlations are taken into account consistently. Such novel features can not be described by traditional mean field theories like MGC, URMGC or even by enhanced formalisms, like HNC/MSA, if they are based on the RPM.Comment: 29 pages, 13 figure