Food industry, environmental monitoring, healthcare, diagnostic and micro-fluidic applications often require understanding the rheology of small volumes of simple and complex fluids. Although most of the processes affecting viscosity and density of fluids occur at microscopic length scales, traditional rheometers only allow investigating fluid properties in the bulk, leaving a gap of several order of magnitudes in length between what can be measured and what causes the observed changes in physical fluid properties. MEMS (Micro-Electro-Mechanical Systems) represent a really promising way to fill this gap, thanks to their ability of probing extremely small volumes of fluids. Traditionally, MEMS structures are excited using an external signal and their resonance frequency is used to track changes in fluid properties. However, the dynamics of such systems in a viscous fluid is not trivial and not fully controllable yet, often resulting in poor measurement signal-to-noise ratio that ultimately translates to poor accuracy and reliability. The research proposed here aims to overcome this limitation of MEMS sensors, by creating a nonlinear self-excitation mechanism capable of automatically tracking the oscillation frequency of a microcantilever immersed in a viscous fluid. Changes in fluid properties translates to oscillations frequency shifts that can be easily tracked in real time without performing the frequency sweeps that are the main cause for poor accuracy in traditional externally excited MEMS sensors. This will allow for unprecedented resolution, ease of use and reliability of viscosity and density measurements on extremely small volumes of fluid.