(488a) Portable Smartphone-Enabled Capillary-Based Viscometer and Tensiometer

Here we present a simple capillary-based approach for the measurement of viscosity and surface tension of liquids. A smartphone is used to record videos (at up to 240 frames per second) of liquid imbibition inside horizontally placed capillaries of both uniform (100-400 micrometers in diameter) and tapered circular cross-sections. Automated image analysis of the videos is then performed to detect fluid displacement and retrieve the meniscus position, L, as a function of time, t. Because liquid imbibition is measured over relatively short lengths of the capillary (typically less than 6 cm) and could possibly involve high imbibition speeds (there is no gravity slowing down the fluid as in capillary rise), the effect of a dynamic contact angle is expected in our measurements. Therefore, to enable broader application of this approach, we have developed an analytical model for the capillary-driven imbibition of a Power-Law fluid in capillaries of uniform circular cross-section, while accounting for dynamic contact angle variations in the viscous regime. For a Newtonian fluid and neglecting contact angle dynamics, the mathematical expression obtained simplifies to the well-known Lucas-Washburn equation. A systematic fitting using our analytical model is then employed to retrieve fluid properties from the L vs. t data collected with the smartphone. Results from our viscometer are in quantitative agreement with known fluids used as references. In addition, an equilibrium condition achieved by fluids inside capillaries of tapered circular cross-sections, also imaged with a smartphone, has been explored as a convenient method for measuring the surface tension of small amounts of liquids (less than 10 microliters). The materials needed (capillaries and a smartphone camera) and the analysis performed (implemented as a user-friendly software) enable viscosity and surface tension measurements to be performed rapidly and without requiring special knowledge or training. All of these advantages make this approach low cost and portable, which facilitates the study of the rheology and surface tension of various fluids in point-of-use and resource-limited settings.