Low yield stress measurements with a microfluidic rheometer.

Abstract

Yield stress, τy, is a key rheological property of complex materials such as gels, dense suspensions, and dense emulsions. While there is a range of established techniques to measure τy in the order of tens to thousands of pascals, the measurement of low τy, specifically below 1 Pa, remains underexplored. In this article, we present the measurement of low apparent τy using a Hele-Shaw microfluidic extensional flow device (MEFD). Using the MEFD, we observe a gradient in shear stress, τ, such that τ is lower near the center or stagnation point, and higher away from the stagnation point. For a yield stress fluid, we observe that, below a certain flow rate, τ exceeds τy only in the outer region, leading to stagnation or unyielding of the fluid in the inner region. We use scaling analysis based on a Hele-Shaw linear extensional flow to deduce τy by measuring the size of the unyielded region, S. We validate this scaling relationship using Carbopol solutions with concentrations ranging between 0.015 to 0.3%, measuring τy as low as ∼10 mPa to ∼1 Pa, and comparing it with τy measured using a standard rheometer. While the experimental lower limit of our technique is 5 mPa, modifying the geometry or improving the image analysis can reduce this limit to the order of 10-4 Pa. The MEFD facilitates rapid measurement of τy, allowing for its real-time assessment. We further report τy of human blood samples between 30 to 80 mPa with their hematocrit ranging between 14 to 63%. Additionally, we determine τy for a mucus simulant (∼0.7 Pa), and lactic drink (∼7 mPa) to demonstrate the versatility of the MEFD technique.