Dimension markers and lofting

A dimension marker pins a channel’s width — and optionally its height — at a point along a segment. Between two markers the channel lofts: it changes size smoothly rather than stepping.

This is how you build a flow-focusing throat, a gradual expansion, or a channel that narrows through a mixing section, without drawing any of them.

Placing one

Press D for the dimension tool and click a channel. A marker appears where you clicked, and the inspector shows its fields:

Field	Notes
Width	Millimetres. Required.
Height	Millimetres. Optional — falls back to the chip default.
Position	0–1 along the segment. Editable, so you can nudge it precisely.

Delete a marker with the bin icon beside it in the inspector.

What happens with no markers

The segment is the chip’s defaults — 0.8 × 0.5 mm out of the box — for its whole length. The inspector shows this as a greyed “default 0.8 × 0.5 mm” line under the marker list.

That is a sensible size and a large proportion of chips never need anything else.

What happens with one marker

The whole segment takes that width. One marker is a way of saying “this segment is 1.2 mm”, not a way of making it 1.2 mm at one point.

What happens with two or more

The channel interpolates between consecutive markers. Before the first and after the last, it holds that marker’s value.

So a segment with markers at t = 0.2, width 0.8 and t = 0.8, width 1.4 is:

• 0.8 mm for the first fifth,
• widening smoothly from 0.8 to 1.4 across the middle three-fifths,
• 1.4 mm for the last fifth.

Turn on the Taper preview in the toolbar (T) to see the ribbon at its true width on the canvas.

Lofting across a whole path

This is the part worth knowing, because it is not obvious.

A loft does not stop at a segment boundary. When segments form a connected chain — each joined end to end, no branching — the interpolation runs along the whole chain. A marker at the start of the first segment and another at the end of the fifth gives you a taper that runs across all five.

That is what makes it practical to dimension a long serpentine: two markers, not thirty-two.

The chain breaks at a junction. Where three or more segments meet, each branch is its own chain from there on.

Height

Optional, and usually you leave it alone.

A rectangular channel has both dimensions, and the minimum-feature check measures both — for a rect or trapezoid cross-section, height can be the limiting dimension just as easily as width. For a round channel, width is the diameter and height is ignored entirely.

Two practical reasons to set height explicitly:

• A two-piece chip whose split height you want the channel ceiling to sit at.
• A high-aspect-ratio channel — deep and narrow, or wide and shallow — where the aspect ratio is doing something for you. It changes the Poiseuille number and therefore the pressure drop; see Pressure drop.

What the checks say

Two of them are specifically about markers.

abrupt-taper fires when the width between consecutive markers changes by more than your taper ratio — 2× by default. It reports the ratio, the distance over which it happens, and the half-angle, because a 3× change over 20 mm is a gentle taper and the same change over 0.5 mm is a step.

A sharp contraction is not automatically wrong. A flow-focusing droplet generator needs one, which is why this defaults to info rather than caution.

channel-below-min-feature measures the narrowest point along the segment, not an average. A channel that tapers to 0.05 mm at one end fails there, however generous the rest of it is.

When several segments share a marker they are reported together — “3 segments are only 0.05 mm across” — because widening that one marker fixes all of them.

A worked example: a flow-focusing throat

The droplet generator template is built this way. Its throat is 0.35 mm against 0.8 mm channels either side:

1. Draw the cross — a continuous channel with two side channels meeting it.
2. Place a marker just before the junction at 0.8 mm.
3. Place a marker at the junction at 0.35 mm.
4. Place a marker just after at 0.8 mm.

The contraction and the expansion are both sharp, deliberately: the shear gradient at the throat is what pinches the droplets off.

Note that 0.35 mm is near the limit of a standard 4K printer, which is exactly what the template’s caveat says and what the checks will tell you on a coarse profile.

Next

• Chip parameters and split types — the defaults markers fall back to.
• The 3D preview — seeing the loft as a solid.