The microfluidic device design used in these experiments consists of two inlets meeting at a Y-shaped intersection (Fig. ). Inlet channels, 40 µm wide and 6 mm long meet at a 60°angle. The outlet channel is also 40 µm wide, but is longer, at 11 mm. All channels are 25 µm high. Soft lithography is used to manufacture these devices according to standard methods with one variation. Rather than core an opening in the PDMS to insert outlet tubing in an outlet well, the final millimeter of the outlet channel is cut to allow the effluent to discharge directly from the channel onto the glass slide. This is done to prevent the build-up of alginate gel in the outlet well.