Colloid Journal最新文献

A theoretical analysis of the temperature change of an evaporating droplet on a superhydrophobic surface is performed taking into account heat fluxes of various types. The results show that the additional cooling effect of evaporation can lead to significant cooling and even crystallization of sessile droplets at positive temperatures. However, with a decrease in the ambient temperature, the efficiency of this additional cooling decreases. A method for continuous monitoring of the temperature of an evaporating droplet based on the measured thermodynamic parameters of sessile droplets is proposed. Experimental studies conducted at temperatures slightly above and below zero degrees Celsius demonstrated a satisfactory correlation between the results of the theoretical analysis and the experimentally measured supercooling of water droplets.

A new analytical solution has been proposed for the linearized Navier–Stokes equations and the diffusion equation. The solution makes it possible to relate the intensity of the Marangoni flow to the surface tension gradient in a droplet of a binary solvent and to study the relevant mass transfer and self-organization of solvates (nanoparticles, molecules, etc.). When deriving the equations, the smallness of the Reynolds number has been assumed, which corresponds to the smallness of the droplet size and the liquid flow velocity. The evaporation has been assumed to be slow sufficiently for ensuring the validity of the quasi-stationary approximation. The smallness of the Peclet number has also been accepted, which corresponds to low velocities of the convective flows as compared with the velocity of the diffusion transfer of an impurity. In this case, the Marangoni number may have a value from unity to several tens. The model has been tested using water–ethanol and octanol–hydrogen peroxide systems. Streamlines have been plotted for the convective flows, and the conditions for their appearance have been analyzed.

The article considers factors determining the stability of a nanobubble with a hydrate layer having a thickness of 1 nm and a dielectric permittivity of about 3. Two stability hypotheses are compared, namely, electrostatic and mechanical (ice-effect or “electrofreezing”). In the first case, the Laplace pressure is compensated by the electrostatic pressure at the bubble boundary; in the second case, it is compensated by the effect of the electrofreezing of its Δ-layer in a high electric field. It is shown that, in salt-free water, a lower nanobubble charge is required for the formation of an ice shell than in the case of the Coulomb stabilization mechanism. In seawater, the Coulomb mechanism is, on the contrary, more efficient, because icing is counteracted by dissolved salt ions. The sizes and charges of the nanobubble are determined for both stability mechanisms.

A theoretical study has been carried out for the influence of relaxation processes in water on the intensity of the electromagnetic radiation of an oscillating charged water droplet, which is assumed to be viscous and incompressible. A theoretical analytical expression of the dispersion equation has been derived for an oscillating and radiating droplet, with this expression having the form of a complex fifth-power algebraic relation. The charge relaxation in an oscillating charged water droplet affects the intensity of its electromagnetic radiation due to the conductivity of water. The highest electromagnetic radiation intensity is inherent in an ideally conducting liquid droplet. It is an order of magnitude higher than the radiation intensity of a liquid droplet with a finite conductivity. The lowest radiation intensity is inherent in a droplet of a dielectric liquid with a frozen-in charge. The surface tension relaxation affects the electromagnetic radiation of a charged oscillating droplet by disordering surface water molecules and altering the magnitude of the surface tension coefficient. The relaxation of water viscosity has no substantial effect on the damped capillary oscillations and electromagnetic radiation of cloud droplets.

Capillary forces are one of the main sources of adhesion between the elements of microtechnological devices. This phenomenon manifests itself during the fabrication or operation of a device and plays a negative or positive role. The paper describes a method that makes it possible to estimate the capillary force between hydrophilic rough surfaces as a function of the relative humidity and the nominal contact area. The method is based on counting the number of roughness asperities, which are able to form capillary bridges spontaneously. To implement the method, detailed information about the roughness of the contacting surfaces is required, which can be obtained using an atomic force microscope (AFM). The idea of the method is illustrated, using as an example, deposited gold films of different thicknesses that come into contact with a smooth silicon surface. AFM scans of a surface with an area of 20 × 20 µm² and a resolution of 4096 pixels per line are used. The developed theory reproduces the basic patterns observed experimentally. In particular, it is shown that the relative role of capillary forces decreases with an increase in the nominal contact area, and dispersion forces begin to play a major role in adhesion. The results of the work are important for the design of microsystems and for experiments measuring dispersion forces.

The isothermal molecular dynamics method and the LAMMPS software package have been employed to perform a comparative study of the stability of Pd₅₀₀₀Ni₅₀₀₀@Pt₅₀₀₀, Pt₅₀₀₀Ni₅₀₀₀@Pd_5000, and Pt₅₀₀₀Pd₅₀₀₀@Ni₅₀₀₀ core-shell nanostructures during gradual heating from 300 to 2200 K. It has been revealed that all three homotopes retain their core–shell morphology up to the onset of melting. However, the ternary Pt₅₀₀₀Ni₅₀₀₀@Pd₅₀₀₀ nanoparticles have been found to be most stable: their melting begins at a higher temperature, and they partly inherit the core–shell morphology even after the completion of melting. A conclusion has been inferred about the relation between the higher stability of PtNi@Pd nanostructures and the effect of Pd surface segregation in ternary Pt–Pd–Ni nanoparticles. In turn, the pronounced surface segregation of Pd is explained by the fact that it is this component that has the lowest specific surface energy.

The molecular dynamics method is employed to solve the problem of stationary vapor–liquid nucleation at a constant number of particles interacting via the Lennard-Jones potential for the cases of both isothermal and nonisothermal nucleation in a wide range of vapor supersaturations. A special simulation approach is used, in which clusters that have reached a certain size are removed from the system, while particles composing them are returned as monomers. The temperature distribution over cluster sizes is determined. It is found that the temperature somewhat decreases beginning from its value corresponding to monomers; however, as the cluster size approaches a critical value, it returns to its initial level and, then, rapidly increases. The temperature distribution over cluster sizes governs the distribution of their number densities and controls vapor nonideality, thus significantly affecting the nucleation rate. It is shown that the knowledge of the cluster temperature is of critical importance for analytical models, as it enables one to accurately determine the vapor supersaturation and the actual nonisothermal nucleation rate. The nucleation rates and critical cluster sizes determined for the isothermal and nonisothermal cases have shown satisfactory agreement with a theoretical model predicting a decrease in the nucleation rate under the nonisothermal conditions.

This paper presents the results of theoretical studying electroconvection emergence and development near an ion-selective region during uniform flow of an electrolyte solution through this region. The analysis of the linear stability of the stationary solution of the problem has made it possible to determine the critical electric potential difference that causes the electrokinetic instability as a function of the external flow velocity. The two-dimensional numerical simulation has revealed the peculiarities of the nonlinear electroconvection regimes. The research has shown the stabilizing effect of the external flow: the electroconvection begins at higher potential differences, whereas its regimes change each other faster with increasing potential difference. Understanding of these effects is useful for practical applications, such as the development of systems for analyte preconcentration in microlaboratories before the chemical analysis of biological liquids.

The paper presents the results of the numerical simulation performed in a unidimensional formulation for a cell containing an ion-selective region. The employed mathematical model takes into account the nonideal selectivity of the ion-exchange region and the presence of a convective electrolyte solution flow through it. The flow has been found to influence the selectivity of the ion-exchange region. The electric current through the system may be both enhanced and diminished depending on a realized current regime, namely, underlimiting or limiting. The understanding of this effect will be useful for practical applications, such as analyte preconcentration systems in microlaboratories for chemical analysis of biological liquids and electrobaromembrane separation systems.