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