Why doesn't my liquid material show through my transparent plastic material in cycles?

Question

I am trying to make a pipette that has a plastic transparent tip with a red transparent liquid inside. However, when I render in cycles, the material inside the tip does not show, although I can see it in the material preview tab.

This is similar to what I want (in the material preview tab):

But this is how it renders:

I have been trying different lightning but that does not seem to help very much with the transparency, even if it is quite dark now.

Does anyone know how to fix this? Thanks!

Here is my blend file: Blend file

Answer 1

The most important reason your liquid is not showing is because you have set the tip object to Shade Smooth instead of Shade Auto Smooth. Therefore the transmission gets heavily distorted. I also changed this setting on the liquid. Another thing I did is removing the Solidify modifier from the liquid and closing the top and bottom, since it makes no sense to construct the liquid as a tube.

If you then want the liquid to look more like it is filling the tip, you should probably scale it larger or "thicker" by selecting the mesh in Edit Mode and use Alt+S, because right now there is quite a gap between the liquid and the inner wall of the tip. I would also slightly decrease the Roughness of the tip material to blur the view on the liquid less and increase the Max Bounces for Glossy and Transmission (and Total of course) under Render Properties > Light Paths to lighten up the inside of the tip a little bit. But note that less roughness decreases the overall brightness of the tip since less light is scattered inside the material.

If you want it to look as if the liquid fills the tip and there is no inner wall like in the Material Preview, then you have to disable the Solidify modifier on the tip. But I would probably just make the wall of the tip thinner and the liquid larger.