Passive flow control in microdevices using thermally responsive polymer solutions

Thermally responsive polymer solutions have previously been used in microchannels with integrated heaters for active valving. Here we present a detailed study of the rheological behavior of these complex fluids, suggesting the broad range of conditions under which active valving could be used, along with an investigation of their use for passive microflow control in which viscous heating leads to channel blockage. We discuss the scaling of viscous heating in the channels under constant flow rate and constant pressure drop conditions. In the case of pressure-driven flow through a microchannel, we demonstrate that viscous heating can cause periodic gel formation leading to flow instabilities. Velocity fields above and below the gel temperature were mapped using digital particle image velocimetry; above the gel temperature, flow fluctuations may be caused by the competing mechanisms of thermothickening and shear-thinning.