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I recently purchased a little e-cig type device that operates on these "pods," they look like this: Pod

Now, as you puff on the device, the liquid is consumed and starts to go lower in the pod, at which point it forms noticeable menisci on both walls of the pod. Here's my attempt to capture that

meniscus

As you can see, the meniscus is quite pronounced and "sharp." Why is this so? What factors contribute to the formation/shape of the meniscus? Is this due to the viscosity of the vape-liquid?

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It is due to the interaction between the fluid and the wall, in the following way:

First, the molecules that make up the fluid have a certain affinity for their neighbor molecules. At the surface of a sample of that liquid which is in contact with air, those molecules at that interface have fewer near neighbors to attract and so they share a greater force of attraction with those neighbors they do have. This enhanced attractive force is called surface tension and it causes the surface of a sample of fluid to behave as if it were a stretched elastic membrane; in response, a small body of fluid with a high value of surface tension will naturally try to draw itself up into a spherical shape.

Second, the molecules of a solid substance are similarly capable of exhibiting an affinity for molecules of a given liquid, or indifference (if you will) towards them, or a repulsion for them.

Then, when a small amount of a liquid is then placed in contact with a surface in the presence of air, an equilibrium of sorts gets established at the triple junction where liquid, air, and solid meet: if the surface has a greater affinity for the liquid than the liquid does, the liquid will spread out onto that surface and cover it with a thin film. If the surface has about the same affinity for the liquid as the liquid does for itself, then the liquid will form a hemispherical shape on it. If the surface has less affinity for the liquid than the liquid does for itself, then the edges of the body of liquid will tuck under and may actually form a ball that touches the surface in only a small area- like raindrops on the hood of a freshly-waxed car.

Finally, when the surface involved is a tube of small diameter with the liquid inside it, if the surface attracts the liquid and the liquid has a high surface tension, then that attraction will cause the liquid to try and climb up the wall of the tube and drag enough fluid up with it to form an upwardly-curved surface called a meniscus. This is why the surface of the fluid in your vaping rig is upwardly-curved inside the reservoir.

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  • $\begingroup$ "If the surface has about the same affinity for the liquid as the liquid does for itself..." How can a liquid have affinity for itself? Inside a liquid, surface tension is zero. $\endgroup$
    – Deep
    May 10, 2018 at 10:11
  • $\begingroup$ taking water as an example, there are hydrogen bonds present between adjacent molecules which cause them to attract one another in the bulk. it is those forces which are magnified at the free surface, thereby giving rise to surface tension. $\endgroup$ May 10, 2018 at 16:39
  • $\begingroup$ Agreed. I just wanted to say that surface tension is a property of the surface separating two different media, and not of a surface lying inside the same medium. $\endgroup$
    – Deep
    May 11, 2018 at 4:15

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