Abrasive slurry jet focusing using a shadow mask for the micromachining of helical channels

Abrasive slurry jets can be used to machine helical channels on rods for microfluidic and other applications. This paper seeks to reduce the minimum machinable feature size using a novel shadow mask setup that was attached to a slurry jet nozzle and used to fabricate narrow helical microchannels on stainless steel rods. A computational fluid dynamics (CFD) model was used to study the flow through the mask opening, and to accurately predict the machined topography. Provided they did not fully wear through their thickness, the masks were equally effective at all stages of wear. Under similar conditions, the shadow mask produced not only narrower, but unexpectedly, much deeper, channels than the unmasked case. For example, at a 7.25° helix angle, four passes of the shadow masked nozzle at top dead centre (TDC, i.e. zero lateral nozzle offset from the rod longitudinal axis) produced channels (280 μm depth, 430 μm width) that were nearly twice as deep and 67% narrower than the unmasked (145 μm depth, 720 μm width). The CFD model showed that this was due to a focusing effect and a reduction in local stagnation pressure that led to an increased centerline particle mass flux in the masked case. Offsetting the masked nozzle by 1 mm from TDC further weakened and laterally shifted the stagnation zone, increasing the centerline depth by a further ∼40%. For the tested range of helix angles (2°–41°), the cross-sectional shapes of the masked channels were nearly identical. The narrow shadow masked jet footprint allowed very tightly packed non-overlapping channels to be machined at a 2° helix angle, whereas the minimum helix angle in the unmasked case was 7.25°. Overall, the shadow mask system allowed the machining of narrower, deeper, and longer channels within a smaller axial length of the rod than the unmasked. This has important implications for inertial microfluidic devices that rely on long and narrow channels which allow inertial forces to act for a longer duration and thereby improve separation efficiency.