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I'm designing a small tea strainer in the shape of a cylinder that goes into a glass bottle, which is also shaped like a cylinder. Currently, I'm still designing it in Fusion 360, but I'm facing one particular challenge.

You see, the circumference of the strainer and the inner glass wall of the bottle aren't that far apart. In other words, the strainer is supposed to fit the bottle as closely as possible. One problem that I can foresee is that whenever water enters the strainer and exits from the sides, the cohesive and adhesive forces of water will cause droplets to stick to the outer wall of the strainer and inner wall of the bottle.

What would be an appropriate solution here? Would water still build up if I added small vertical ridges to the strainer to encourage the droplets to gather and flow downwards? How far apart would the glass and stainless steel have to be in order for the problem to be negligible?

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  • $\begingroup$ Do a search on meniscus. $\endgroup$
    – Solar Mike
    Commented Aug 27, 2023 at 20:59
  • $\begingroup$ shouldn't the strainer be submerged? $\endgroup$
    – jsotola
    Commented Aug 27, 2023 at 21:15
  • $\begingroup$ Well, not quite. The design of the bottle is such that it can be flipped around, causing the strainer to become submerged in one moment. But when the tea is brewed, the bottle is flipped around again, thereby draining the water from the strainer. $\endgroup$ Commented Aug 27, 2023 at 21:19

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You are close. Use the ridges to establish a uniform gap or standoff between the strainer and the vessel walls, and then size the gap width (that is, the ridge height) so gravity forces will overcome surface tension forces in the gap. This condition will allow two-phase flow within the gap i.e., the tea can run out the bottom of the gap while air burbles its way into the gap at the same time.

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  • $\begingroup$ Thank you so much for your quick response! 😉 I'm having a little trouble understanding it though, so let me rephrase it and see if I got it: If I add the ridges and make the gap between said ridges wide enough, then the water will flow down while air flows upwards and takes its place, all within a single column. Correct? And if this is correct, would it then matter how deep the ridges are? $\endgroup$ Commented Aug 27, 2023 at 22:03
  • $\begingroup$ @AndrewJackson, no, you choose the depth of the ridges to establish the width of the gap between the strainer and the vessel walls so as to support two-phase flow within that gap. $\endgroup$ Commented Aug 27, 2023 at 23:43

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