in situ gelation of alginate in microchannel flows: gel properties determine deposition efficiency*

Polymer flows through pores, nozzles and other small channels govern engineered and naturally occurring dynamics in many processes, from 3D printing to oil recovery in the earth's subsurface to a wide variety of biological flows. The crosslinking of polymers can change their material properties dramatically. In engineered flows, polymer crosslinking is often a situation to be avoided. For instance, in 3D printing it is greatly preferred for crosslinking to occur upon impact with a substrate rather than prior to exiting a nozzle. In blood flow, however, polymer crosslinking can either be advantageous, as in wound healing, or pathological, as in thromboembolism formation. In either of these situations, and others, it is advantageous to know a priori whether or not crosslinking polymers will lead to clogged channels or cessation of flow. In this study, we investigate the flow of a common biopolymer, alginate, while it undergoes crosslinking by the addition of a crosslinker, calcium, driven through a microfluidic channel at constant flow rate. While quantifying the limits on flowability and clogging in situ in this crosslinking polymer system, we observe a remarkable phenomenon in which the crosslinked polymer intermittently clogs the channel. We observe a pattern of deposition and removal of a crosslinked gel that is simultaneously highly reproducible, long-lasting and controllable by a variety of parameters. We map this behavior as a function of flow rate, polymer concentration, and crosslinker concentration, measuring the time-dependence of pressure that results from deposition and ablation. An analytical model of the diffusive boundary layer reasonably fits the pressure traces, suggesting that deposition is driven by a balance of diffusion and convection. Interestingly, the model suggests that deposition occurs more efficiently in regions of the phase diagram where gels are stiffer. Further, fractal analysis of the roughness of the pressure traces suggests that the route to failure, i.e. complete clogging of the channel, is signaled by a transition to non-linear dynamics, despite the fact that the flows are at low Reynolds number. This observation suggests intruguing possibilities for use of pressure traces to diagnose, and even predict, failure in crosslinking polymer flows.