Charged particles dispersions

Electrophoresis Movement of charged particles dispersed in a stationary liquid phase An externally applied electrical gradient... 

EPD is achieved through migration toward an electrode of charged particles, dispersed in a suitable liquid, in an applied electric field. 

Two different electrodeposition techniques are currently in use electrolytic and electrophoretic. Electrolytic deposition occurs at the surface of the cathode when water is reduced to produce hydrogen gas and hydroxyl ions, which results in an increased pH at the electrode surface [115, 118]. Electrophoretic deposition occurs when charged particles, dispersed or suspended in liquid medium, are attracted to and deposited onto a conductive substrate of opposite charge in the presence of an electric field [112, 113, 119]. 

Electrophoretic deposition (EPD) is a wet electrolytic deposition technology for thin films. EPD employs the electrophoresis mechanism, as illustrated schematically in Fig. 1.11. The electric field is applied between two electrodes and charged particles dispersed or suspended in a liquid medium move toward the oppositely charged... 

Until recently the self-organization of charged particles, like lipid A-diphosphate, its analogues and their effects on complex fluids received little attention. In addition, charged particle dispersions in nano-size the regime such as lipid A-diphosphate can influence the stability of the system. This is a result of a segregation of domains near... 

Keep in mind that Eqs. (8-14) are only valid for small Kr, when the electrophoresis retardation (electric-field-induced movement of ions in the electric double layer, which is opposite to the direction of particle movement) is unimportant [41J. This limitation is inherent to the Hiickel equation. Practically, a colloidal suspension always contains charged particles dispersed in a medium with surfactants (or electrolytes) of both polarities. In this case the Poisson s equation must be used for deriving the surface charge density and Zeta potential relationship. Under the Dcbyc-Hiickel approximation, i.c., the small value of potential, zey/ kgT, where v is the potential and z is the valency of ion, a simple relationship between the surface charge density and Zeta potential can be easily obtained [7], The Poisson s equation simply says that the potential flux per unit volume of a potential field is equal to the charge density in that area divided by the dielectric constant of the medium. It can be mathematically expressed as ... 

For originally non-charged particles dispersed in polar media, the surface preferential adsorption of ions, surface molecular group dissociation, isomorphic substitution, adsorption of polyelectrolytes, and even surface polarization will make them behave similarly to charged particles. The Stem layer and diffuse layer comprise what is commonly known as the electrical double layer, the thickness of which depends on the type and concentration of the ions in the suspension as well as on the particle surface. A parameter, called the Debye-Huckel parameter k, is used to characterize electrical double layer thickness. K has the dimension of reciprocal length. For smooth surfaces in simple electrolytes. 

Electrophoretic Deposition (EPD) EPD is a colloidal process wherein ceramic bodies are shaped directly from a stable colloid suspension by a DC electric field. EPD process is applicable to produce ceramic bodies of films, plates, cylindrical and complex shapes using substrates. In EPD process, charged particles dispersed in a suspension migrate towards oppositely charged substrate (electrode) on application of DC electric field (this phenomenon is called electrophoresis ), and the particles are deposited on the substrate as shown in Figure 3. EPD process has advantages of simple apparatus, short deposition time, easy to control process parameters, no restriction of the shape of deposition substrate, low production cost, suitability for deposition of laminates and mass production. Furthermore, in EPD process, the preparation of the suspension is important (for example, the concentration and pH of the suspension, homogeneous dispersion of particles in the suspension). 

Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

Dielectric constant (Section 8 12) A measure of the ability of a matenal to disperse the force of attraction between oppo sitely charged particles The symbol for dielectnc constant IS e... 

Response to Electric and Acoustic Fields. If the stabilization of a suspension is primarily due to electrostatic repulsion, measurement of the zeta potential, can detect whether there is adequate electrostatic repulsion to overcome polarizabiUty attraction. A common guideline is that the dispersion should be stable if > 30 mV. In electrophoresis the appHed electric field is held constant and particle velocity is monitored using a microscope and video camera. In the electrosonic ampHtude technique the electric field is pulsed, and the sudden motion of the charged particles relative to their counterion atmospheres generates an acoustic pulse which can be related to the charge on the particles and the concentration of ions in solution (18). 

An example of a practical dielec trofilter which uses both of the features described, namely, sharp electrodes and dielectric field-warping filler materials, is that described in Fig. 22-34 [H. I. Hall and R. F. Brown, Lubric. Eng., 22, 488 (1966)]) It is intended for use with hydrauhc fluids, fuel oils, lubricating oils, transformer oils, lubricants, and various refineiy streams. Performance data are cited in Fig. 22-35. It must be remarked that in the opinion of Hall and Brown the action of the dielec trofilter was electrostatic and due to free charge on the particles dispersed in the hquids. It is the present authors opinion, however, that both elec trophoresis and dielectrophoresis are operative here but that the dominant mechanism is that of DEP, in wdiich neutral particles are polarized and attracted to the regions of highest field intensity. 

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