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I'm designing a funnel/bottle for a powder. My goal is to have all of the bottle's powder contents come out when inverted. However, the powder has a tendency to stick to itself (like flour or brown sugar) and so I will run into a chance that the powder will not fall into the bottom vessel by gravity alone.

To make matters more complicated, this is not an open funnel on the top. It will be more like a plastic water bottle in that when it's rotated to dumping position, the top isn't open to the air and venting doesn't currently appear to be an option.

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My question is, what funnel surface type will increase the flow to help dispel the powder contents? Some ideas that come to mind are smooth, vertical guides, or stepped stairs. Note my funnel outlet is fixed in diameter.

Smooth

This one makes the most sense as it's a "smooth" flow. However, I've been surprised by how air and water behave in relationship to fluid dynamics (ex: golf ball dimples or pickup trucks being more aerodynamic with the tailgate up). And although fluid dynamics doesn't cover powders, I would imagine there's some similarities.

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Vertical Guides

In theory, these ridges would break up the powder clumping on the way down.

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Stepped

In theory, these steps would add turbulence on the way down and help break up the powder on each "step down."

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Extra question: Regardless of the features in the funnel (or lack thereof), what's the minimum angle from vertical that would be ideal for a funnel to dispense powders by gravity alone?

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    $\begingroup$ Stiction is a function of both the material used for the funnel and the material being funneled. I doubt there's any semi-universal answer. $\endgroup$ – Carl Witthoft Oct 31 '18 at 17:46
  • $\begingroup$ Thanks for this term; I've not heard of it before. In my case, assume the funnel material is the same in the examples. I know this is something that can be tested out with trial and error, but I'm wondering if there's any knowledge out there to start with. $\endgroup$ – Davbog Oct 31 '18 at 17:55
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The problem you have is more complicated than you think. Any industry that deals with the storage and movement of granulated/powdered materials has to deal with this and most have unique solutions.

You need to look at the principles of hopper design.

Some of the things that affect any design will be:

  • The angle of repose of the material you intend to use.
  • The moisture content of the material.
  • The angle of the cone in the funnel/hopper.
  • The material used in the cone and wall of the bottle/hopper/funnel and its friction factor.
  • The ability of the powdered material to create a near vertical wall and stand up, despite its angle of repose.

A stepped cone for the funnel is a bad idea because it creates hang-ups for the material to get caught on. Also the steps reduce the overall angle of the cone. Generally, the steeper the angle of the cone, the better.

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    $\begingroup$ Some cones are vibrated to help avoid blockages and smooth delivery... $\endgroup$ – Solar Mike Nov 1 '18 at 6:34
  • $\begingroup$ Thank you! The principles of hopper design really helped me with a starting point. $\endgroup$ – Davbog Nov 2 '18 at 17:11

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