Hi Deeksha Reddy , the electric field inside an electrically neutral or an evenly charged cylinder is zero indeed. If the cylinder forms a pore in a larger body which is charged and whose inside is electrically conductive then outside the pore there is an electric field with a component parallel to the axis of the cylinder. Therefore, external charge carriers of the opposite polarity are drawn to the surface of the body and to the mouth of the pore, and some will diffuse into the pore.

As soon as external charge carriers are inside the pore, the electric field at the inside is no longer zero. Since the external charge carriers repel each other they tend to form a layer at the inner wall of the cylinder. Because the charge of the body is attracted by the external charge carriers, it forms a layer at the outer wall of the cylinder.

So, one peculiarity of the double layer at the wall of a cylinder is that the external charge carriers (at the inner wall) are not attracted by the internal charge carriers (at the outer wall), provided the internal charge is evenly distributed (which is not necessarily valid at the molecular resp. atomic level). However, the charge at the outer wall is attracted by the charge at the inner wall, of course.

During discharging, for the external charge carriers the balance between being drawn into the cylinder by the field in front of the mouth of the pore and being expelled by the charge carriers which are deeper inside the cylinder tips toward ejection, due to the decreasing field of the body. As a consequence, the outer layer formed by the inner charge is also reduced.

Thank you Joerg Fricke for the elaborate answer. It cleared many doubts I had. Could you please give me one clarification? When you said, ions diffuse into the pore in the last line of your first paragraph, did you mean movement due to both diffusion and migration?

Hi Deeksha Reddy , you immediately spotted an inconsistency in my explanation, namely the "diffusion" in the first paragraph, and "the balance between [forces exerted by electric fields]" in the last paragraph. Excellent!

"Diffusion" was meant to answer the question "Why does a charged particle move into a volume of space where the electric field is zero?". But this question doesn't apply here because your statement regarding the electric field and my affirmation of it is not true:

The axial component of the electric field inside an infinitely long charged cylinder is zero, due to symmetry. In this case, from Gauss' law follows that the radial component has to be zero, too. But consider a charged circle as an extreme case of a cylinder of finite length: At the disc inside the circle we'll have an electric field containing exclusively a radial component, and Gauss' law is satisfied if we take the axial components originating on both sides of the disc into account, too. If we extrude this circle into a short cylinder, the electric field inside the cylinder will be very weak but not exactly zero. (The field would vanish completely if the cylinder were closed by conductive discs at both ends.) However, the field has to satisfy the condition that it is perpendicular to the surface of the cylinder everywhere on the surface.

BTW, "evenly charged" in my first sentence was my second blunder: "Evenly" is true regarding the phi dimension (circumference) but in axial direction only in the case of infinitely long cylinders, again; in the absence of external fields, the charge density on a cylinder of finite length is highest at its ends.

By using the convenient properties of infinitely long cylinders in order to explain a phenomenon involving cylinders of finite length I disregarded Einstein's advice to "Make things as simple as possible but not simpler!" Sorry! ;-)

So, on second thought I would say that the formation of a double layer at the wall of pores can be explained exclusively by migration but under usual conditions (far from 0° K) diffusion might accelerate the formation.

Thank you Joerg Fricke for the explanation. :)