Viscous fingering enables equipment-free rheometry of complex fluids in resource-constrained settings*

Rheological characterization of complex fluids often holds the key for a wide gamut of analytical applications. However, installation and deployment of sophisticated instruments like rheometer is unpragmatic in case of resource-limited settings. As a simple alternative, we have developed an equipment-free methodology called 'Rheometry-on-paper', in which fluid rheological properties can be accurately predicted by analyzing the viscous fingering patterns of a drop of fluid on a wet paper strip. The fingering instability, also known as the Saffman-Taylor instability, arises as a less viscous fluid tries to displace a more viscous one, such as glycerol, through a porous domain. Through box counting method, the fractal dimension value of the topologically complex interface is determined and mapped with the corresponding rheological indices, thereby enabling rapid rheometrical characterization at ultra-low cost without the help of any sophisticated instrumentation. Using this method, we have been able to accurately measure the rheological parameters of complex Non-Newtonian fluids such as particle suspensions in aqueous polymeric solutions, as well as determine the effect of particle volume fraction and polymer concentration on the overall rheological signature of the fluid.