Development of a simple droplet-based microfluidic capillary viscometer for low-viscosity Newtonian fluids.

Viscosity is an easily measured macroscopic property that provides molecular information and is widely used across the sciences and engineering. Here we report a microfluidic capillary viscometer that forms droplets from aqueous samples in an immiscible carrier phase and encodes information about sample viscosity in the droplet spacing. The device shows exceptional calibration stability, with only a 0.6% drift in calibration factor from run to run, the ability to handle aqueous and nonaqueous samples, and the ability to operate with sample volumes as low as 38 nL. Operating range for aqueous sample viscosity was characterized, and was found to be 0.96-52 cP. Operating range for aqueous shear rate was found to depend on aqueous viscosity and varied from 1.9 × 10(1)-4.4 × 10(2) s(-1) for high viscosity samples to 4.1 × 10(2)-6.0 × 10(3) s(-1) for low viscosity samples. Accuracy was tested by comparing measured viscosities of several samples including crème de menthe peppermint liquor, human urine, and baby oil to viscosities of the same samples obtained with a U-tube viscometer. The device was found to be very accurate, with differences between methods as low as 0.1%. The viscometer presented requires only a basic T junction and can utilize off-chip fluorescence to measure viscosity, which could allow for easy addition of viscometric measurement capabilities to existing droplet platforms. Furthermore, the device is capable of performing measurements on Newtonian fluids without precise control over pressures or flow rates, which significantly simplifies device operation.