Glossary

Terms are defined as this wiki uses them. Where a word has a broader meaning in the literature, the narrower sense here is the one that matters when you are reading a warning or a readout.

Flow and fluids

Chaotic advection — Mixing by repeatedly stretching and folding the interface between two streams, rather than by waiting for diffusion. The mechanism behind the herringbone mixer. Not turbulence: the flow stays laminar throughout.

Dean flow — A pair of counter-rotating secondary vortices set up in the cross-section of a curved channel. Strengthens with flow rate, which is why the spiral mixer improves as you push it harder.

Diffusion — Net transport by random molecular motion. The only mixing mechanism available in a straight microchannel. Mixing time scales with the square of the distance to be crossed, which is why halving channel width cuts mixing length by four.

Dynamic viscosity (μ) — Resistance to shear, in Pa·s. Water is about 0.001 Pa·s (1 mPa·s). The single most important fluid property for pressure drop.

Hagen–Poiseuille — The relation between pressure drop, flow rate and channel geometry for laminar flow. See Pressure drop for the form OpenChip uses and the assumptions it carries.

Hydraulic diameter (Dₕ) — 4A/P: four times cross-sectional area over wetted perimeter. Lets non-circular channels be compared with round ones. For a circular channel it is simply the diameter.

Hydraulic resistance — The pressure a unit of flow costs, in Pa·s/m³. A laminar channel network is a resistor network, and OpenChip solves it as one.

Laminar flow — Flow in smooth layers that do not mix across. Essentially universal in microfluidics. See Laminar flow and the Reynolds number.

Newtonian fluid — One whose viscosity does not depend on how hard you shear it. Water, ethanol, glycerol blends. Not whole blood, polymer solutions, or most cell suspensions.

Poiseuille number (Po) — f·Re, a pure function of cross-section shape. 64 for a circle, about 57 for a square, 96 for infinite parallel plates. Using the circular value for a rectangular channel under-predicts pressure drop by up to 50 %.

Prandtl number (Pr) — Ratio of momentum diffusivity to thermal diffusivity. Used in the heat-exchange estimate.

Residence time (τ) — How long fluid spends in a given length of channel. Internal volume divided by volumetric flow rate.

Reynolds number (Re) — ρvDₕ/μ. The ratio of inertial to viscous forces. Below about 2000 flow is laminar; microfluidic values are typically well under 1.

Shear-thinning — Viscosity falls as shear rate rises. Whole blood does this. Not modelled.

Wall shear stress (τ_w) — Force per unit area the fluid exerts on the channel wall. The number to watch if you are flowing cells.

Geometry and CAD

Bezier — A curve defined by its endpoints and control handles. One of the two segment types in OpenChip; the other is straight.

Chamber — A widened volume at a node — a sphere or a cylinder. Used as a mixing volume or a reservoir.

Channel graph — The design: nodes joined by edges. OpenChip’s central data structure, and everything else is derived from it.

Cross-section — The shape of a channel where you cut across it: round, rectangular, or trapezoid. Set per segment.

CSG — Constructive solid geometry. Building a solid by adding and subtracting primitives. The channels are subtracted from the block.

Dimension marker — A width (and optionally height) pinned at a position along a segment. The solid lofts between consecutive markers. See Dimension markers and lofting.

Edge — One channel segment, between two nodes.

Loft — A smooth transition between cross-sections of different size along a path. What OpenChip does between dimension markers.

Manifold — A mesh that encloses a solid volume with no holes or self-intersections. What a slicer needs. manifold-3d is the geometry kernel OpenChip uses, and it is the same one modern OpenSCAD uses internally.

Node — A point in the channel graph: a waypoint, a port, or a chamber.

Port — An opening to the outside, with a bore and a fitting geometry. See Ports and fittings.

Waypoint — A node that is just a bend or a junction — no port, no chamber.

Printing

Bonding — Joining the two halves of a two-piece chip. See Bonding and sealing a two-piece chip.

Build volume — The largest box the printer can produce, in X, Y and Z.

Cure-through — Light penetrating past the layer being exposed and curing resin beyond it. The reason thin walls between channels merge, and the reason channels-too-close exists.

FEP — The transparent film in the bottom of a resin vat.

Layer height — The Z step per exposure. Typically 30–50 µm for this work.

Minimum feature size — The smallest feature your machine and resin reliably resolve. The most important number in your printer profile.

Monolithic — Printed as one sealed block with the channels as internal voids. Compare two-piece.

mSLA — Masked stereolithography. A UV LED array shining through an LCD that masks each layer. What “desktop resin printer” means in practice.

Overhang — A downward-facing surface unsupported from below. Past roughly 45° from vertical it sags or needs supports.

Post-curing — UV exposure after washing, to finish polymerising the part.

Resin — The photopolymer. Its properties — cure depth, viscosity, shrinkage — vary enormously between brands and matter more than most people expect.

Two-piece — Split at a cut plane into a channel plate and a lid, bonded after printing. Far easier to drain. Compare monolithic.

Vent — A deliberate hole added so uncured resin can escape a sealed channel. See Draining uncured resin.

Washing — Removing uncured resin with isopropanol or a resin-specific solvent, before post-curing.