Mixing chambers

A chamber is a node with a volume: a sphere or a cylinder, larger than the channel that meets it. Use one as a mixing volume, a reservoir, a bubble trap, or somewhere to put a sensor.

Placing one by hand

Select a node and set Node type → Chamber in the inspector. You get two fields:

Field	What it is
Shape	`sphere` or `cylinder`
Size	Diameter, in millimetres

A sphere is the gentler shape — no internal corners to trap a bubble. A cylinder is easier to print cleanly at larger sizes because its ceiling is flat rather than curving over.

Placing them at every corner

The inspector’s Mixing chambers block adds one at every corner in the network at once. Three controls govern it:

Control	Default	What it does
Shape for new chambers	Sphere	Applied to everything the button places.
Corner threshold	150°	How sharp a bend has to be to count as a corner.
Size	3×	Chamber diameter as a multiple of the local channel width.

Then Add chambers at all corners.

The corner threshold

A node counts as a corner when the path bends by more than this much. 180° is dead straight, so a lower number means only sharper bends qualify.

At the default 150°, a right-angled turn (90° interior) is a corner and a gentle 30° deviation is not.

Each chamber stays editable afterwards. The button is a starting point, not a commitment — placing twenty and deleting the six you did not want is usually faster than placing fourteen by hand.

Size as a multiple

Sizing from the local channel width rather than as an absolute means a chip whose channels taper gets chambers that scale with them. On a 0.8 mm channel the default 3× gives a 2.4 mm chamber.

What chambers do for mixing

Less than people expect, and it is worth being clear about.

A chamber slows the flow down, which increases residence time and therefore gives diffusion longer to work. That is real, and it is often enough.

What it does not do is stir. At microfluidic Reynolds numbers there is no turbulence to develop in the larger volume — the flow stays laminar, the two streams stay side by side, and a big round room does not change that. A chamber is a delay line, not a mixer.

If you need actual mixing, the mechanisms that work are on Mixing without turbulence: more length, chaotic advection, or sustained curvature.

What chambers do to printing

Two things to watch.

A sphere is an overhang. Its upper half curves over an unsupported void. Below about 2–3 mm this is usually fine — the layers are small enough to bridge — and above that you may see sagging at the top of the cavity. A cylinder avoids this by having a flat ceiling, at the cost of two internal corners.

A chamber holds resin. It is a larger volume than the channels around it, and it drains more slowly. On a monolithic chip it is the first place to check after washing, and often the place to put a vent. See Draining uncured resin.

What the checks say about them

Chambers participate in the same checks as everything else:

Their footprint is their diameter, so a large chamber near the chip edge raises channel-near-boundary.
Two chambers close together, or a chamber close to a channel, raise channels-too-close.
A chamber at a dead end still raises dead-end-channel — being a larger volume does not give fluid anywhere to go.

Mixing without turbulence — what actually mixes.
The 3D preview — checking the chamber looks the way you meant.