Glass Capillary Array Design Guide

Fused and Etched Structures - In an etched capillary array, chemically soluble glass acts as a temporary core or placeholder. The soluble glass is combined with a more durable matrix glass, drawn to a reduced size, assembled and often fused into a larger structure, and then removed in a mild etching solution. The resulting voids become the capillary channels.

This method is especially useful when a plate must contain a very large number of closely packed, uniform channels. The maximum useful thickness is controlled in part by how effectively the etchant can reach and remove the soluble glass through the full channel length.

Drawn-Tube Structures - A drawn capillary begins as a glass tube or tube assembly that is heated and drawn to a smaller cross-section. Because the channel is already open, it does not need to be chemically cleared after drawing. The interior is fire-polished during the draw process, and channel length is not constrained by etchant access.

Drawn tubes can be assembled into bundles or arrays, although their packing and uniformity are generally less dense than tightly fused and etched structures. They are often the better approach when long, smooth, individually routed, or nonlinear channels are required.

Common etched-array channel diameters include 5, 10, 25, and 50 µm. CHI’s capabilities extend from submicron channels to several-millimeter openings for custom designs. Individual glass tubes may be redrawn anywhere between the practical minimum and the original preform diameter, while multi-tube structures commonly contain channels from a few micrometers to approximately one millimeter.

Diameter affects flow resistance, collimation angle, particle passage, optical acceptance, wall strength, achievable tolerance, and the maximum practical channel length. A target diameter should therefore be evaluated together with thickness and open area rather than as an isolated requirement.

Circular and hexagonal holes are common in standard arrays. Square holes and other custom cross-sections may be possible when the geometry is symmetric, can be machined at preform scale, and will remain stable during drawing and fusion. Complex shapes typically require more development work and may impose limits on size, wall thickness, or uniformity.

Most of the capillary plates we have produced are in the 2–50 mm size range, but custom structures up to 200–300 mm on a side have also been possible with our fused plate production process. Plate thicknesses have ranged from roughly 75 µm to 350 mm, but not every thickness is feasible with every hole size or manufacturing method.

Etched plates are constrained by the channel aspect ratio and etch process. Drawn structures can be substantially longer; we have delivered rigid coiled tube structures up to 10 meters in total length.

Exterior formats may include round or square plates, disks, strips, blocks, tubes, and custom profiles. Final machining, edge condition, surface finish, mounting features, and flatness should be specified with the intended assembly method in mind.

Tolerance capability depends on scale, packing pattern, fusion, glass behavior, exterior machining, and the measurement method. A quotation should identify which dimensions are critical to system performance and how they will be measured. Tightening every dimension can increase development effort and cost without improving the final device.

The length-to-diameter ratio (L/D), also called aspect ratio, is the channel length divided by the channel diameter. In collimation applications, a larger L/D generally narrows the geometric line of sight angular range. In flow and filtration applications, it can increase resistance and influence pressure drop.

We can produce etched-capillary L/D ratios of up to approximately 300:1. The practical limit depends on glass chemistry, channel size, open area, etch access, straightness, and acceptable residual material. Drawn tubes have no comparable chemical-etch limit, although handling, straightness, strength, and system packaging still impose practical constraints.

Etched channels expose the interface between the structural glass and the polished soluble-core glass that occupied the channel before etching. Few remaining defects larger than one-hundredth of the channel diameter are typically present after removal. Etched holes can be highly straight and parallel, with collective curves or twists possible within process limits.

Drawn capillaries are fire-polished internally as the glass is pulled through the draw tower. The resulting surface can be smooth and nearly defect-free, making drawn tubes attractive for applications in which wall condition or long continuous channels are especially important.

Open area ratio (OAR) is the channel area divided by the total cross-sectional area of the structure. For a single circular core or channel surrounded by a circular wall:

In an array, OAR is controlled by channel size, pitch, wall thickness, packing geometry, and any non-open structural regions. Many standard capillary structures are around 40–50% open area, with custom designs extending from below 1% to 70% or higher. Higher OAR may improve throughput or transmission but can reduce wall thickness and structural margin.

Changing one design variable usually affects several others. The table below summarizes common tradeoffs CHI evaluates with customers.

Use this checklist when preparing an inquiry or RFQ.