# Water does not flow through 4 mm hole when there is a small amount, how to let water flow through small hole even when there is low pressure?

I have a 3D print where there are 5 holes with a diameter of 4 mm in a cup, and I would like liquid to flow through all 5 holes at once while slowly draining the cup (by slowly I mean: just take a few seconds and not drain instantly). Therefore the holes can not be very large.

When I fill water in the cup it drains fine until there is a small amount of water left, and then it just stops with a small layer of water flowing over the holes.

I'm guessing it is due to surface tension and not enough pressure from water above to push the water through...

Is there a hole design that fixes this problem? I don't know what to Google or if this is the right place to ask the question. It just takes too long to guess my way through and print every attempt at the right size or shape of hole (which I have done so far and still got nothing).

• This is almost an engineering question, specifically regarding hydrodynamics and as you suggest, surface tension. How thick is the wall (floor?) surrounding the holes? Oct 19, 2020 at 20:35
• less about water resistancy of prints, more about the design and the post processing to solve this problem. While generally an engineering problem, there are solutions for the 3D printed variant. Oct 19, 2020 at 20:48
• Thank you for correcting that Trish. Fred, the floor is 1 mm in thickness :) Oct 19, 2020 at 21:01
• I think a related question can and should be asked in Physics SE or Engineering SE. If you ask a related but different question (so as to avoid cross-posting), like the relationship between hole diameter and back pressure where dripping stops and factors related to the hole that might affect it, and then mention this question for context, it would have some different and useful answers.
– uhoh
Oct 20, 2020 at 11:06
• My hunch is that water being so darn sticky there isn't much you can do that will make more than a small difference, but I don't know for sure. Water really, really likes itself!
– uhoh
Oct 20, 2020 at 11:07

What you encounter there is a combination of Adhesion, Cohesion, and Capillary Force.

Cohesion is what holds the water together. Adhesion is the force to retain water against a wall or hanging from a pen's end, it is proportional to the surface wetted. Capillary Force is the resulting effect where water moves up through a thin tube, it is anti-proportional to the diameter and in the opposite direction of the weight (force). Their relation can be shown in this picture, where a droplet hangs on the end of a glass rod, which has a capillary in it:

How to reduce the water sticking in the cup then?

• Make the straight part of the bore as short as possible. This can be done by having a thin cup. The shorter the hole, the less surface there is the water can adhere to vertically, and you might overcome capillary force.
• Smooth the hole. Maybe print it 3.5 mm and drill it up to your 4 mm diameter. This reduces adhesion.
• Smooth the inside surface. Reducing the adhesion to the inside by having less steps.
• Chamfer the inside of the holes. This alters the whole geometry and flow setup in the very low water level case, especially when the surface separates into several areas, above each hole. Then the larger volume belonging to each hole on the inner side means there is a little more pressure and you can get out some more water - and it also shortens the distance the hole has to bridge.
• make sure there is some slope everywhere inside so that the water will collect in one of the holes.

An example for a (non measured) design which relies heavily on chamfering to guide the water to the already chamfered holes and then keeps the straight section as short as possible could look like this: the central hole has a very wide chamfer, the whole plate directs water to the center and each of the other holes has a chamfer to guide out water.

However, there is a lower limit to where just tweaking the design will workd, which is based on cohesion. Cohesion is what results in surface tension and viscosity. You can only shift those limiting factors by altering the properties of the liquid, for example by adding an agent that lowers the surface tension and viscosity (soap).

• Thank you! I will try those suggestions, the floor is already thin, at 1mm, and at that scale I dont think champfers would be very noticeable. Should I go for thinner floor? or thicker floor and add Chamfer? or just add Chamfer to the 1 mm? And just to be sure, it is the top edge of the hole I should chamfer, right? Oct 19, 2020 at 21:05
• @JacobWelin both? and yes, the top. If you can, chamfering the lower would alter the flow pattern but might reduce the kept water even more. Oct 19, 2020 at 21:16
• I tried printing 3 test prints with just one hole. 1) First print was with 1mm floor and a champher. 2) Secound print was with 0.3mm floor and no Champher (No room for a champher anyways) 3) And third print was with a 3.2mm floor to allow for a bigger Champher - This third one is the one that worked the best, almost all of the water runns out and only the small champher is left with water in it, but that is just a few drops. So thank you, that worked fine! Oct 19, 2020 at 22:48
• @JacobWelin the chamfer does aide in sloping the bottom floor and at the same time does reduce the straight part... let me rephrase the first point a little XD Oct 19, 2020 at 23:04
• Another quality Trish answer. @JacobWelin, you can also use a hydrophobic coating on the inside of the cup to reduce the adhesion. Oct 20, 2020 at 2:50

If it is possible to add a downward pointing thin needle (hair, AWG-40 wire etc.) to the edge of (each) hole, do it. The droplets will smoothly glide along it to its end, where the needle circumference becomes so tiny that surface tension cannot hold the droplet anymore. Surface tension won't have a chance!

With a very thin needle and funnel-like hydrophobic surface of the vessel, you may get rid of virtually any (microliter) amount of water. It can be hard to make it by 3D-printing, though.

• You can easily make "hairs" with a 3d printer, but a slicer won't do it for you in places you want them, only places you don't. :-) Oct 20, 2020 at 14:28