Fluids with light-tunable rheological properties (i.e., photorheological or PR fluids) have been studied in the recent past by many research groups. One advantage of light over other stimuli lies in its ability to be directed at a precise location from a remote source. A potential application for PR fluids that leverages the spatial selectivity of light is in the area of flow control within fluidic devices, e.g., to stop the flow at a precise location within a network of fluidic channels (valving action). However, despite a wide variety of existing PR fluid formulations, current fluids are not suitable for this application. Here, we report a PR fluid based on the biopolymer, alginate, that could be useful in this context. The fluid is composed entirely of commercially available components: alginate, a photoacid generator (PAG) as a photo-trigger, and a chelated complex of divalent strontium (Sr2+) ions (Sr-EGTA). Upon exposure to UV, the PAG dissociates to give H+ ions, which induce the release of free Sr2+ ions from the Sr-EGTA. The released Sr2+ ions then self-assemble with the alginate chains to give a nanostructured gel with a high gel modulus (~ 1000 Pa). To test the possibility of fluidic valving, we flow the above fluid through a microchannel and expose a specific point in the channel to UV light. At that point, alginate is converted into a strong gel in a few minutes, and we find that the resulting gel blocks the flow. When the UV light is removed, the gel is gradually diluted by the flow and the channel reopens. We have thus shown a new kind of light-activated fluidic valve; note that the above concept relies entirely on physical (non-covalent) bonds, i.e., on self-assembly, rather than on covalent crosslinking of dissolved monomers or polymers.

Light-activated fluidic valve by triggering the nanoscale assembly of a biopolymer

247th National Meeting (2014)

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