Re: Helmholtz Layer electrode

[Frederick Sparber]

This is a Word Document that gave me a rough time getting the URL.

 

www.ilo.org/encyclopedia/?doc&nd=857100211&nh=0

----- Original Message -----

From: Frederick Sparber

To: vortex-l

Sent: 6/5/2006 4:32:56 AM

Subject: Re: Helmholtz Layer electrode

"The polarity (relative positivity and negativity) of two insulators in contact with each other depends on each material’s electron affinity. Insulators can be ranked by their electron affinities, and some illustrative values are listed in table 40.3. The electron affinity of an insulator is an important consideration for prevention programmes, which are discussed later in this article.

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Table 40.3      Electron affinities of selected polymers*

 

Charge	Material	Electron affinity (EV)
-	PVC (polyvinyl chloride)	4.85
	Polyamide	4.36
	Polycarbonate	4.26
	PTFE (polytetrafluoroethylene)	4.26
	PETP (polyethylene terephthalate)	4.25
	Polystyrene	4.22
+	Polyamide	4.08

    

* A material acquires a positive charge when it comes into contact with a material listed above it, and a negative charge when it comes into contact with a material listed below it. The electron affinity of an insulator is multifactorial, however.

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Although there have been attempts to establish a triboelectric series which would rank materials so that those which acquire a positive charge upon contact with materials would appear higher in the series than those that acquire a negative charge upon contact, no universally recognized series has been established.

 

When a solid and a liquid meet (to form a solid-liquid interface), charge transfer occurs due to the migration of ions that are present in the liquid. These ions arise from the dissociation of impurities which may be present or by electrochemical oxidation-reduction reactions. Since, in practice, perfectly pure liquids do not exist, there will always be at least some positive and negative ions in the liquid available to bind to the liquid-solid interface. There are many types of mechanisms by which this binding may occur (e.g., electrostatic adherence to metal surfaces, chemical absorption, electrolytic injection, dissociation of polar groups and, if the vessel wall is insulating, liquid-solid reactions.)

 

Since substances which dissolve (dissociate) are electrically neutral to begin with, they will generate equal numbers of positive and negative charges. Electrification occurs only if either the positive or the negative charges preferentially adhere to the solid’s surface. If this occurs, a very compact layer, known as the Helmholtz layer is formed. Because the Helmholtz layer is charged, it will attract ions of the opposite polarity to it. These ions will cluster into a more diffuse layer, known as the Gouy layer, which rests on top of the surface of the compact Helmholtz layer. The thickness of the Gouy layer increases with the resistivity of the liquid. Conducting liquids form very thin Gouy layers.

 

This double layer will separate if the liquid flows, with the Helmholtz layer remaining bound to the interface and the Gouy layer becoming entrained by the flowing liquid. The movement of these charged layers produces a difference in potential (the zeta potential), and the current induced by the moving charges is known as the streaming current. The amount of charge that accumulates in the liquid depends on the rate at which the ions diffuse towards the interface and on the liquid’s resistivity (r). The streaming current is, however, constant over time.

 

Neither highly insulating nor conducting liquids will become charged-the first because very few ions are present, and the second because in liquids which conduct electricity very well, the ions will recombine very rapidly. In practice, electrification occurs only in liquids with resistivity greater than   or less than , with the highest values observe! d for  .

 

Flowing liquids will induce charge accumulation in insulating surfaces over which they flow. The extent to which the surface charge density will build up is limited by (1) how quickly the ions in the liquid recombine at the liquid-solid interface, (2) how quickly the ions in the liquid are conducted through the insulator, or (3) whether surface or bulk arcing through the insulator occurs and the charge is thus discharged. Turbulent flow and flow over rough surfaces favour electrification."

 

 

"A person wearing insulating shoes is a common example of an insulated conductor. The human body is an electrostatic conductor, with a typical capacitance relative to ground of approximately 150 pF and a potential of up to 30 kV. Because people can be insulating conductors, they can experience electrostatic discharges, such as the more or less painful sensation sometimes produced when a hand approaches a door handle or other metal object. When the potential reaches approximately 2 kV, the equivalent to an energy of 0.3 mJ will be experienced, although this threshold varies from person to person. Stronger discharges may cause uncontrollable movements resulting in falls. In the case of workers using tools, the involuntary reflex motions may lead to injuries to the victim and others who may be working nearby. Equations 6 to ! ! 8 in table 40.2 describe the time course of the potential.

 

Actual arcing will occur when the strength of the induced electrical field exceeds the dielectric strength of air. Because of the rapid migration of charges in conductors, essentially all the charges flow to the discharge point, releasing all the stored energy into a spark. This can have serious implications when working with flammable or explosive substances or in flammable conditions.

 

The approach of a grounded electrode to a charged insulating surface modifies the electric field and induces a charge in the electrode. As the surfaces approach each other, the field strength increases, eventually leading to a partial discharge from the charged insulated surface. Because charges on insulating surfaces are not very mobile, only a small proportion of the surface participates in the discharge, and the energy released by this type of discharge is therefore much lower than in arcs.

 

The charge and transferred energy appear to be directly proportional to the diameter of the metal electrode, up to approximately 20 mm. The initial polarity of the insulator also influences charge and transferred energy. Partial discharges from positively charged surfaces are less energetic than those from negatively charged ones. It is impossible to determine, a priori, the energy transferred by a discharge from an insulating surface, in contrast to the situation involving conducting surfaces. In fact, because the insulating surface is not equipotential, it is not even possible to define the capacitances involved."