Open sessile droplet viscometer with low sample consumption†
This paper reports a portable viscometer that requires less than 10 μL of sample for a measurement. Using a two-droplet Laplace-induced pumping system on an open microfluidic substrate, the device measures the viscosity of a liquid by determining the time required for one droplet to completely pump into a second droplet. The pumping behaviour follows the Hagen–Poiseuille and Laplace relations where the flow rate, Q, is proportional to the liquid's kinematic viscosity, μ. The progress of pumping is measured by tracking the change in curvature of one of the droplets using a laser that is positioned perpendicular to the microfluidic chip and directed at the “tail” of the shrinking droplet. The angle of incidence and degree of refraction changes depending on the size of the droplet, which is tracked by a linear diode array placed beneath the microfluidic chip. Droplet reservoirs and connecting channels were defined by precise patterning of a glass substrate coated with a commercially available omniphobic coating (Ultra Ever Dry®) using laser micromachining. A 500 μm wide and 20 mm long channel with circular reservoirs (d = 1.5 mm) enabled the measurement of dynamic viscosities in the range of η = 1.0–2.87 mPa s. The materials cost for the entire viscometer (fluidics and electronics, etc.) is <15 USD.