Automated high-throughput viscosity and density sensor using nanomechanical resonators

•Optical excitation and read-out of microcantilevers in a microfluidic channel.•Dual-PLL to track the frequency and quality factor with millisecond time resolution.•High-throughput characterization of μL sample droplets in a two-phase configuration.•Comparison of two theoretical models to determine the liquid viscosity and mass density. Most methods used to determine the viscosity and mass density of liquids have two major drawbacks: relatively high sample consumption (∼milliliters) and long measurement time (∼minutes). Resonant nanomechanical cantilevers promise to overcome these limitations. Although sample consumption has already been significantly reduced, the time resolution was rarely addressed to date. We present a method to decrease the time and user interaction required for such measurements. It features (i) a droplet-generating automatic sampler using fluorinated oil to separate microliter sample plugs, (ii) a PDMS-based microfluidic measurement cell containing the resonant microcantilever sensors driven by photothermal excitation, (iii) dual phase-locked loop frequency tracking of a higher-mode resonance to achieve millisecond time resolution, and (iv) signal processing to extract the resonance parameters, namely the eigenfrequency and quality factor. The principle was validated by screening series of 3μL droplets of glycerol solutions separated by fluorinated oil at a rate of ∼6s per sample. An analytical hydrodynamic model (Van Eysden and Sader, 2007 [6]) and a reduced order model (Heinisch et al., 2014 [16]) were employed to calculate the viscosity and mass density of the sample liquids in a viscosity range of 1–10.5mPas and a density range of 998–1154kgm−3.