(513i) Enabling Rheological Analysis of Complex Fluids at the Point-of-Need

Viscosity analysis at the point-of-need (PON) can be enabled by microfluidic systems that process small sample volumes (microliters) in portable easy-to-operate and low-cost set-ups. Self-driven capillary flow in microchannels that are large enough (~400-800 micrometers in diameter) to be monitored and recorded using a smartphone is, thus, a convenient realization for PON applications involving viscosity analysis. Such capillary filling dynamics span an important range of shear rates that can be modulated in open/closed configurations of capillary channels and, in principle, leveraged to probe the rheological behavior of the fluid. Blood rheology, for instance, has been shown to be better characterized by models that include a constant viscosity at low and high shear rates, with a Power-law like regime in between (e.g., Cross model). But an important drawback emerges due to poorly understood capillary dynamics, mainly originating from high imbibition velocities, which produce significant deviations from simple models such as the Lucas-Washburn equation and limit the ability to probe viscosity at moderate to high shear rates. Thus, failure to account for these deviations, including a dynamic contact angle, hinders the applicability of this approach for accurate rheological characterization of complex fluids. Here we introduce important advances that enable accurate analysis of non-Newtonian behavior from capillary filling phenomena over a wide range of shear rates. First, we develop an empirical correlation to account for dynamic effects and facilitate probing of a Newtonian viscosity even when high imbibition velocities are involved. We compare our model’s performance to existent dynamic contact angle models in the literature. This improved modeling is then extended to a Power-law fluid to investigate non-Newtonian behavior. To enable broader application, we formulate an innovative approach whereby the model is locally applied over small changes in shear rate to retrieve a variable Power-law exponent n as a function of shear rate, thus probing more complex rheological behavior such as the one described by the Cross model. Therefore, video recording of capillary filling using a smartphone, properly analyzed with the proposed approach, facilitates measuring the rheology of complex fluids in PON and resource-limited settings.