The microscopy reveals that alginate crosslinked by calcium forms a gel which occludes the channel simultaneously with an increase in driving pressure. Fig. shows a phase contrast image of the first 1 mm of the channel downstream of the junction, obtained at t ∼ 168 s in Fig. , inverted and with the background subtracted. In the inverted image, the gelled and deposited alginate appears as bright regions within the channel. Increases in the summed intensity of the inverted image, I(t), therefore suggest an increase in gel deposition. Fig. shows I(t) of the first 500µm of the image over the five minute portion of the flow test.