A novel microfluidic viscometer for measuring viscosity of ultrasmall volumes of Newtonian and non-Newtonian liquids

Viscosity is a critical fluid property that significantly influences fluid behavior and performance across various systems. Most commercial viscometers require relatively large sample volumes (on the order of milliliters), which restricts their utility in scenarios where only limited sample volumes are available. For instance, human tear fluid—essential for developing effective treatment strategies—is scarce (typically on microliters), especially in individuals with dry eye disease. To address this limitation, we present a novel microfluidic viscometer platform capable of measuring the viscosity of ultra-small volumes (i.e. ∼10 μl) of Newtonian and non-Newtonian fluids. The working principle is based on the Hagen–Poiseuille equation, incorporating the Weissenberg–Rabinowitsch–Mooney correction for slit-flow, and employs an optically transparent microfluidic chip integrated with supporting devices including a syringe pump, manifold, camera, and differential pressure transducer. Preliminary validation was conducted using glycerol solutions, artificial tears, and tear samples from dry and healthy eyes. This microfluidic viscometer holds promise for measuring the shear viscosity of small volumes of biofluid samples (e.g. synovial fluid, cerebrospinal fluid, tear films) or pharmaceuticals (e.g. monoclonal antibodies, ophthalmic drug delivery products) by developing surface coating materials appropriate for specific samples.