External fields colloids

Observation of the electrode under examination being exposed to an electric field may yield information about the value of E. Any charge on the electrode, which can be a wire or a colloidal particle, will result in a movement in the external field. Assuming that the movement is due to charges being present on the electrode the rate of the movement should pass through a minimum at E i.c provided that specific adsorption is absent. (Data obtained with this method are labelled ED). 

Stigter, D, Electrophoresis of Highly Charged Colloidal Cylinders in Univalent Salt Solutions. 2. Random Orientation in External Field and Application to Polyelectrolytes, Journal of Physical Chemistry 82, 1424, 1978. 

By assuming that the external field - deformed by the presence of the colloidal particle-and the field of the double layer are additive, D. C. Henry derived the following expression for mobility ... 

Because the charged particle and its ion atmosphere move in opposite directions, the center of positive charge and the center of negative charge do not coincide. If the external field is removed, this asymmetry disappears over a period of time known as the relaxation time. Therefore, in addition to the fact that the colloid and its atmosphere move countercurrent with respect to one another (which is called the retardation effect), there is a second inhibiting effect on the migration that arises from the tug exerted on the particle by its distorted atmosphere. Retardation and relaxation both originate with the double layer, then, but describe two different consequences of the ion atmosphere. The theories we have discussed until now have all correctly incorporated retardation, but relaxation effects have not been included in any of the models considered so far. 

Most data about the Ludwig-Soret effect of polymers in solution have been obtained from thermal field-flow fractionation (TFFF), developed by Giddings and coworkers [17,18]. TFFF is one member of the family of field-flow fractionation techniques, which are all characterized by a laminar flow of the polymer solution or colloidal suspension within a relatively narrow channel. An external field, which may be gravitation, cross-flow, or temperature as in TFFF, is applied... 

On behalf of all members of the Collaborative Research Centre 481 on Complex Macromolecular and Hybrid Systems in Internal and External Fields, we wish to thank the Deutsche Forschungsgemeinschaft for financial and administrative support, the voluntary reviewers of the proposals for their invaluable judgment and advice, the Bavarian State Ministry of Sciences, Research and the Arts, and the University of Bayreuth for their ongoing support to continuously develop and strengthen the interdisciplinary research focus on Macromolecular and Colloid Research at the University of Bayreuth. Undoubtedly, all of these measures helped to advance the impact and international visibility. 

The use of optical methods to study the dynamics and structure of complex polymeric and colloidal liquids subject to external fields has a long history. The choice of an optical technique is normally motivated by the microstructural information it provides, its sensitivity, and dynamic range. A successful application of an optical measurement, however, will depend on many factors. First, the type of interaction of light with matter must be correctly chosen so that the desired microstructural information of a sample can be extracted. Once selected, the arrangement of optical elements required to perform the required measurement must be designed. This involves not only the selection of the elements themselves, but also their alignment. Finally, a proper interpretation of the observables will depend on one s ability to connect the measurement to the sample s microstructure. 

A charged surface and the ions, which neutralize the surface, together create an electric double layer. The distribution of the ions can be evaluated from the Poisson-Boltzmann equation where the ions are treated as point particles and the primitive model is used. Further, all correlations between the ions are neglected, which means that the ions are interacting directly only with the colloids and through an external field given by the average distribution of the small ions. The distribution of the particles are assumed to follow Boltzmann s theorem [11]... 

In the case of crystals, one usually encounters structures that are anisotropic on a molecular scale, due to anisotropic interactions between the molecules. Clusters formed by colloidal-scale particles often do not exhibit anisotropy due to the isotropic character of the interactions on that colloidal scale. One may introduce anisotropy in that case by applying an external field, for example, a flow field or electric-magenetic field, and subsequently freezing the morphology by, for example, gelation. 

B.V. Deijaguln, (= Deryagin), S.S. Dukhln and V.N. Shilov, Kinetic Aspects of Electrochemistry of Disperse Systems, Adv. Colloid Interface Set 13 (1980) 141. (Emphasis on double layers in an external field.)... 

Colloidal interaction forces act primarily in a direction perpendicular to the particle surface forces primarily acting in a lateral direction were discussed in Chapter 10. We merely consider internal forces, which find their origin in the properties of the materials present. This excludes forces due to an external field, such as gravitational, hydrodynamic, and external electric forces these are involved in some subjects of Chapter 13. 

Normal FFF. In normal mode FFF, used primarily for colloidal materials, the particles are driven toward one of the channel walls (the accumulation wall) by the external field. As the particle concentration... 

In this section, we discuss the behavior of liquid crystal suspensions under the action of an external electric field. The behavior of colloidal suspensions in electric fields is of considerable technological interest with the so-called Electro-Rheological (ER) fluids [17, 18]. The main features of this behavior are now rather well understood. When an external field is applied, particles suspended in an isotropic fluid become polarized. Resultant dipole-dipole interactions between the particles lead to their chaining along the direction of the applied field. When suspended in a liquid crystal host, colloidal particles are also expected to be polarized upon the application of an electric field. However, new phenomena may take place because of the specific response of the liquid crystal. In this case, the external field is likely to alter the distortions of the liquid crystal alignment... 

Lapiga E. J., Sinaiski E. G., On Capture of small charged Particles by a large uncharged Particle in an external electric Field, Colloid J.,... 

The only external force on the dispersion considered so far has been the earth s gravitational field. On particles of colloidal size and especially tho.se below c. 0.5 pm diameter, and density close to the medium, the effect of gravity is completely outweighed by the thermal motion of the particles Brownian motior, leads to a structure which is close to an equilibrium state. As the particle size increases, e.g. for a suspension, then the effect of external fields has to be considered since under the influence of gravity particles tend to senle. Another importani external field occurs when the system is stirred or sheared. The easiest case to consider is the application of a simple shear gradient The flux J (velocity x... 

Stigter, D., Electrophoresis of highly charged colloidal cyhnders in univalent salt solutions. 2. Random orientation in external field and application to polyelectrolytes, J. Phys. Chem., 82, 1424, 1978. 

The list could be made longer, taking the idea of electrokinetics in a wide sense (response of the colloidal system to an external field that affects differently to particles and liquid). Thus, we could include electroviscous effects (the presence of the EDL alters the viscosity of a suspension in the Newtonian range) suspension conductivity (the effect of the solid-liquid interface on the direct current (DC) conductivity of the suspension) particle electroorientation (the torque exerted by an external field on anisotropic particles will provoke their orientation this affects the refractive index of the suspension, and its variation, if it is alternating, is related to the double-layer characteristics). 

Minor et al. [32] have analyzed the time dependence of both the electroosmotic flow and electrophoretic mobility in an electrophoresis cell. They concluded that, for most experimental conditions, the colloidal particle reaches its steady motion after the application of an external field in a much shorter time than electroosmotic flow does. Hence, if electrophoresis measurements are performed in an alternating field with a frequency much larger than the reciprocal of the characteristic time for steady electroosmosis (t 10° sec), but smaller than that of steady electrophoresis (t 10 sec), the electroosmotic flow cannot develop. In such conditions, electroosmosis is suppressed, and the velocity of the particle is independent of the position in the cell. Figure 3.6 is an example we measured the velocity of polystyrene particles in the center of a cylindrical cell using a pulsed field with the frequency indicated when the frequency is above 10 Hz, the velocity (average between the field-on and field-off values) tends to the true electrophoretic velocity measured at the stationary level. Another way to overcome the electroosmosis problem is to place both electrodes providing the external field, inside the cell, completely surrounded by electroneutral solution as no net external field acts on the charged layer close to the cell walls, the associated electroosmotic flow wiU not exist [33]. 

The dipole moment induced in a colloidal particle by an external field can be probed through its contribution to a collective behavior of the suspension, namely the dielectric constant or the... 

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