Real-Time Viscosity and Mass Density Sensors Requiring Microliter Sample Volume Based on Nanomechanical Resonators

A microcantilever based method for fluid viscosity and mass d. measurements with high temporal resoln. and microliter sample consumption is presented. Nanomech. cantilever vibration is driven by photothermal excitation and detected by an optical beam deflection system using two laser beams of different wavelengths. The theor. framework relating cantilever response to the viscosity and mass d. of the surrounding fluid was extended to consider higher flexural modes vibrating at high Reynolds nos. The performance of the developed sensor and extended theory was validated over a viscosity range of 1-20 mPa·s and a corresponding mass d. range of 998-1176 kg/m3 using ref. fluids. Sepg. sample plugs from the carrier fluid by a two-phase configuration in combination with a microfluidic flow cell, allowed samples of 5 μL to be sequentially measured under continuous flow, opening the method to fast and reliable screening applications. To demonstrate the study of dynamic processes, the viscosity and mass d. changes occurring during the free radical polymn. of acrylamide were monitored and compared to published data. Shear-thinning was obsd. in the viscosity data at higher flexural modes, which vibrate at elevated frequencies. Rheokinetic models allowed the monomer-to-polymer conversion to be tracked in spite of the shear-thinning behavior, and could be applied to study the kinetics of unknown processes. [on SciFinder(R)]