Nanoliter Droplet Viscometer with Additive-Free Operation

Measurement of the viscosity of a sample solution is an important analytic technique for medical diagnosis, pharmaceutical development, and industrial processing, among other applications. The use of droplet-based "digital" microfluidic operation allows nanoliter-scale sample volumes, much smaller than those in continuous-phase microfluidics. By observing the flow rate of a sample plug through a drastic constriction, we achieve accurate and precise measurement of the plug viscosity without addition of labels or tracers. Sample plugs in our device geometry had a volume of ~30 nL, and measurements had an average error of 7.2% ± 6.2%, with an average relative standard deviation of 2.65% ± 4.08%. We tested glycerol-based samples with viscosities as high as 101 mPa*s, with the only limitation on samples being that their viscosity should be higher than that of the continuous oil phase.