A bulk acoustic mode micro-electro-mechanical dual resonator platform is utilised to study the evaporation of sub-microliter water droplets from the surface of the resonator. An analytical formulation for the observed frequency shift and the measure dependence of resonant frequency on the modes of evaporation which is consistent with the optically derived data. The resonators access only a thin layer of the liquid through shear contact and, hence, the response is not affected by the bulk mass of the droplet to first order. A relationship between the droplet contact area and the elapsed time was established for the evaporation process and is used to derive a value of the diffusion coefficient of water in air that is found to be in reasonable agreement with literature values. This work introduces a new tool for the electro-mechanical monitoring of droplet evaporation with relevance to applications such as biosensing in liquid samples of sub-microliter volumes.

Current portable particle detection instruments typically rely on optical methods which are limited to 100 nm diameter particles. Microfabricated bulk acoustic resonators, when used as mass balances, could take particle detection below this limit. This study examines the collection of particles onto piezoelectric bulk acoustic mode resonators from gaseous flow using classical impaction. Collection of both polystyrene latex-pinene secondary organic aerosol particles was examined in terms of frequency shift and collection efficiency. A new experimental setup was introduced which allows for adjusting major impactor, resonator, and aerosol properties. Preliminary results show the setup works for both particles while the saturation limit was not reached within an hour despite highly elevated particle concentrations.

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Knowledge of the spatio-temporal changes in the characteristics and distribution of subsurface fluids is key to properly addressing important societal issues, including: sustainable management of energy resources (e.g., hydrocarbons and geothermal energy), management of water resources, and assessment of hazard (e.g., volcanic eruptions). Gravimetry is highly attractive because it can detect changes in subsurface mass, thus providing a window into processes that involve deep fluids. However, high cost and operating features associated with current instrumentation seriously limits the practical field use of this geophysical method. The NEWTON-g project proposes a radical change of paradigm for gravimetry through the development of a fieldcompatible measuring system (the gravity imager), able to real-time monitor the evolution of the subsurface mass changes. This system includes an array of lowcosts microelectromechanical systems-based relative gravimeters, anchored on an absolute quantum gravimeter. It will provide imaging of gravity changes, associated with variations in subsurface fluid properties, with unparalleled spatio-temporal resolution. During the final ∼2 years of NEWTON-g, the gravity imager will be field tested in the summit of Mt. Etna volcano (Italy), where frequent gravity fluctuations, easy access to the active structures and the presence of a multiparameter monitoring system (including traditional gravimeters) ensure an excellent natural laboratory for testing the new tools. Insights from the gravity imager will be used to i) improve our knowledge of the causeeffect relationships between volcanic processes and gravity changes observable at the surface and ii) develop strategies to best incorporate the gravity data into hazards assessments and mitigation plans. A successful implementation of NEWTON-g will open new doors for geophysical exploration.

The spatial sensitivity of bulk acoustic mode resonators can influence calibrations when they are implemented as accurate mass sensors of surface-bound particles. A new spatial sensitivity model based on images of the resonator surface is introduced from early principles. The adsorption of particles was studied empirically by repeatedly drying particle laden droplets on the surface of two 3.14 MHz bulk acoustic mode resonators. Theoretical and experimental results were compared to identify three scenarios over the course of consecutive droplet evaporation with varying spatial sensitivity influences. Examining different surface treatments for the resonators revealed the hydrophilic surface to have a higher rate of particle stacking and conglomeration.