To test the dependence of alginate deposition on surface chemistry of the devices, we prepare several devices with varying surface chemistry. To verify that the same alginate deposition occurs on PDMS as well as glass, some devices are prepared with PDMS-coated glass slides. The same 8:1 PDMS:curing agent mixture is degassed and poured onto clean glass slides to a depth of 4 mm. These are then used in the same manner as uncoated glass slides. The difference in experimental results between PDMS coated and uncoated glass slides is within the variance seen from device-to-device (SI Fig. ). In other tests, PDMS devices are prepared, treated with plasma, and bonded to glass slides in the usual way before surface modification. Before beginning flow tests, devices are flushed with chemical treatments to adjust the degree of hydrophobicity of the PDMS and glass surfaces. We use polyvinyl alcohol (PVA) to make the surfaces hydrophilic . Commercial glass treatment Aquapel makes the surfaces hydrophobic .