Vladimir Nikolaievich Shilov

The effect of the concentration of dispersed particles on the mechanisms of low-frequency dielectric dispersion (LFDD) in colloidal suspensions

Abstract There are two mechanisms which are currently used to explain the low-frequency (kHz range) dispersion of the dielectric permittivity of suspensions in electrolyte solutions (LFDD). The first, the surface diffusion mechanism (SDM), associates the LFDD with the diffusion of bound ions along the particle surface caused by the applied electric field. The second, the volume diffusion mechanism (VDM), follows from the generalization for alternating fields of the classical theory of the relaxation effect in electrophoresis and associates the LFDD with the diffusion of free ions in the diffuse double layer. It has been found that VDM is much more strongly dependent on particle concentration than SDM, opening new possibilities for the investigation of each of these two mechanisms separately. The reason is that when the concentration of particles in suspension increases, the characteristic length for the propagation of volume diffusion processes decreases together with the decrease of the free electrolyte volume, whereas the characteristic length for the surface diffusion remains constant. Correspondingly, when particle concentration is raised, the relaxation time of the VDM effect must decrease, whereas it must remain constant for the SDM mechanism. Thus, by varying the concentration of particles in suspension, one can separate the dispersion curves of SDM and VDM. A simple model is elaborated which can be useful to predict the volume fraction dependence of the parameters of LFDD; in particular, its amplitude and critical frequency. The results are compared with experimental data obtained with polymer latex dispersions of volume fractions ranging from 3 to 16%. It is found that the dielectric behaviour (the volume fraction dependence of both the amplitude and critical frequency of LFDD) of the dispersions is reasonably well explained with our model, thus demonstrating that VDM prevails in the systems studied. Experimental data previously found by other authors are also discussed in the light of the model presented.

Field-induced disturbance of the double layer electro-neutrality and non-linear electrophoresis

The influence of applied electric field on the field-induced variation of the electrolyte concentration (concentration polarization) disturbs the electro-neutrality of the system, represented by a dispersed particle and its electric double layer in electrolyte solution. The manifestation of this electro-neutrality disturbance in the non-linear electrophoresis was considered in the framework of a procedure of successive approximations in powers of the applied field strength. Analytic expressions describing the component of the electrophoretic velocity proportional to the cubic power of the applied field strength (cubic electrophoresis) were obtained for arbitrary values of the surface conductivity (of Dukhin number). The model restrictions are spherical non-conducting particle with homogeneous surface and thin double layer.

Fast coagulation of nearly spherical ferric-oxide (hematite) particles. 1. Formation and decomposition of aggregates - Experimental estimation of velocity constants

Abstract Using a single-particle light-scattering technique, we investigated rapid coagulation with monodisperse Fe 2 O 3 particles (haematite). It was revealed that the value of the velocity constant of coagulation, k 11 = 11.0 × 10 −12 cm 3 s −1 , was significantly closer to predictions by Smoluchowskis theory than to theories which allow for hydrodynamic and van der Waals interactions between spherical particles.

Corrected results for the influence of size, ζ potential, and state of motion of dispersed particles on the conductivity of a colloidal suspension

A correction of a recent work on the dependence of the DC conductivity of diluted colloidal suspensions on the size, zeta potential, and state of motion of dispersed particles (C. Grosse, S. Pedrosa, V.N. Shilov, J. Colloid Interface Sci. 251 (2002) 304) is presented. It is shown that the procedure used in that work to calculate the contribution of the particles to the conductivity of the suspension leads to a result that includes the variation of the conductivity of the dispersion medium. Revised analytical and numerical calculations are presented, which strongly reinforce the conclusions reached in the original work: The expression for the conductivity increment based on the value of the dipolar coefficient of the suspended particles (calculated taking into account their electrophoretic motion) appears to be valid over the whole range of particle sizes.

Low-Frequency Dielectrophoresis and the Polarization Interaction of Uncharged Spherical Particles with an Induced Debye Atmosphere of Arbitrary Thickness

The electroosmotic component of the mutual dielectrophoresis is accounted for in the theory of the polarization interaction of uncharged particles. Previously, for the case when the polarization charge layer thickness is small as compared with the particle radius, it was shown that the electro-osmosis exerts significant effect on the motion of a particle in a nonuniform field (external or induced by a neighboring particle). The electro-osmosis in the induced Debye atmosphere of arbitrary thickness is considered in this paper. A class of systems characterized by the formation of transverse linear aggregates of particles is determined. The aggregates are weakly conducting media with nonconducting particles but more polar than the medium. The possibility of the occurrence of transverse electrocoagulation structures, which was originally obtained by us using the heuristic approach, is supported by results obtained in the solution of the electrohydrodynamic problem of the long-range interaction of polarized particles. The results obtained confirm high efficiency of the heuristic method used.

Dielectrophoresis of nanosized particle

The theory of dielectrophoresis is constructed with allowance for electroosmotic perturbances. Changes in the flow regime of electrolyte in the diffuse part of electrical double layer under the action of quadratic (with respect to external field) electric forces are considered. The expression for low-frequency limit of the dielectrophoretic velocity of a spherical particle, which is valid at the arbitrary thickness of electrical double layer and fairly small value of ζ-potential (< 50 mV), is derived. It is shown that electroosmotic perturbances appeared to be rather significant for nanosized particles and their dielectrophoretic velocity under the effect of electroosmotic perturbances changes by several times.

Theory of the low frequency electrorotation of disperse particles in electrolyte solution

An interpretation is given for the cofield rotation observed in low-frequency electrorotation experiments using polystyrene particles in electrolyte solution. It is shown that the main cause of the particle rotation is the electroosmotic velocity of the electrolyte inside the thin double layer, rather than the torque exerted by the rotating electric field on the out of phase part of the induced dipole moment.

Electric characteristics of cellular structures containing colloidal silver

Electrophysical characteristics of yeast cells in which precipitates are formed by the chemical-microbiological method from colloidal solutions with different concentrations are studied. The cell ζ-potential and dc surface conductivity at frequencies of 100–600 kHz are calculated using the data from measuring the electrophoretic mobility and dispersion of cell conductivity. In our calculations, we used known electric cell model (isolating cytoplasmic membrane that separates conducting cytoplasm and external solution) supplemented by an external ion-exchange layer that models the cell wall. A comparison of changes in the dc and ac surface conductivities of cell testifies to the formation of clusters of metal silver nanoparticles in the cell wall.

Contribution of electro-osmosis to electrorotation spectra in the frequency range of the β dispersion

Abstract The theory of electrorotation of uncharged conducting spherical particles surrounded by an insulating membrane and suspended in an electrolyte solution is considered. This system constitutes the simplest model of the electrical properties of liposomes or cells. Its dielectric behaviour is characterised by two dispersions, the main one of which (β dispersion) is determined by the finite time of charging the capacity of the membrane. The charge distributions that are formed on both sides of the membrane during the polarization process have a finite thickness due to diffusion. This fact was first taken into account by Garcia et al. [J. Phys. D Appl. Phys. 18 (1985) 1891] for the calculation of the permittivity, but was never considered in electrorotation theories. Nevertheless, it is in this last case when the thickness of the field induced charge distributions is particularly important, since it not only influences the value of the dipole moment, but also leads to the appearance of an electrsmotic slip velocity of the electrolyte solution around the particle (this phenomenon has a decisive bearing on the low-frequency electrorotation of charged insulating particles [Grosse and Shilov, Colloids and Surfaces A 140 (1998) 199]. It is shown that the inclusion of all the diffusion related phenomena in the theory of electrorotation of uncharged particles with membrane, can lead to a qualitative difference with respect to the classical theories. Depending on the conductivity values, the β dispersion counterfield electrorotation peak can split into two, leading to the appearance of a co-field low frequency peak.

Equations for the non-reversible hydrodynamics of the boundary layer in an electrolyte solution

Abstract Hydrodynamic (Stokes) equations for the flow of electrolyte solution near a surface, under the action of gradients of electrochemical potentials, temperature and pressure, are deduced on the basis of a general thermodynamic approach. The derived equations are applicable independent of the nature of the surface forces acting on the ions. The results obtained are free of the restrictions of the so-called standard model of an electric double layer, which only takes into account the action of electrostatic surface forces. The general thermodynamic approach allows for corrections to the expression for the effective thermoosmotic force, and for predictions of a new effect of the action of the pressure gradient on the variations of the specific volume of an ion near the surface.