Ultrasmall Volume Single-Droplet Viscometry: Monitoring Cornering Instabilities on Omniphobic Polydimethylsiloxane...

Viscosity is an essential fluid property that is important for industrial and laboratory applications. For biological, complex, and/or precious liquid samples, the available volume of fluid is limited, yet there are few existing techniques to measure the viscosity of small volumes of liquids. We report a facile method to measure the viscosity of liquids by monitoring the sliding of single-cornered droplets on surfaces coated with an omniphobic film that minimizes the contact-angle hysteresis. The developed measurement method was capable of accurately characterizing the viscosity of various liquids and showed statistically equivalent values when compared to the literature, for fluids with viscosities ranging from 0.35 to ∼800 mPa s (acetone to castor oil). Using the developed single-droplet viscometer, the minimum volume required to measure the viscosity of hexadecane, dodecane, toluene, and ethanol was <5 μL and was <1 μL for decane and isopropyl alcohol, respectively. Further, the viscosity of hexadecane measured from 22 to 70 °C matched literature values precisely. The single-droplet, small-volume viscometer also requires minimal cleaning due to the omniphobic surface, meaning the fluid may be reused for other purposes with no liquid loss occurring due to the viscosity measurement.