Partial wetting is a vital tool to improve the quality factor of dynamic micromechanical sensors operated in liquids owing to the reduced viscous damping. This technique employs meniscus formation which so far could only be stabilized for a hydrophobic sensory surface excluding biosensing applications. Here, we report on the geometrically tuned wettability of particular hybrid bridge resonators (HBR) with an integrated overhang structure. This allows low-loss operation irrespective of the sensory surface material. The impact of the overhang structure on wetting is explained in a simplified model. Experimental evidence is adduced operating the HBR coated with hydrophilic thin films in water. With an in-liquid quality factor Q of 91 and a small mass m≈5 ng of the HBR, the m/Q-ratio, which is proportional to the limit of detection for mass sensing, was significantly improved in comparison to immersed resonators presented so far.

We present a semi-analytical model, describing the mechanical behavior of a partially wetted low-loss microresonator, designed for mass sensing applications in both gaseous and liquid environment. In the course of this detailed modeling, realistic boundary conditions have been implemented, and the impact of the support structure on the spectral behavior of the resonator has been investigated. We proof the existence of localized eigenmodes, in spite of the diminishing clamping of the resonator, considering higher modes of the resonator/support system. The quality factor of the partially wetted resonator was evaluated including various types of damping mechanisms. The quantification of the detection limit is the key to optimize the geometry of this sensor type.

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Weighing individual micro- or nanoscale particles in solution using dynamic micromechanical sensors is quite challenging: viscous losses dramatically degrade the sensor's performance by both broadening the resonance peak and increasing the effective total mass of the resonator by the dragged liquid. While the virtual mass of the resonator was discussed frequently, little attention has been paid to the virtual mass of particles attached to the resonator's surface and its impact on the accuracy of mass sensing. By means of the in situ detection of a polystyrene microbead in water using a bridge-based microresonator, we demonstrate that the virtual mass of the bead significantly affects the observed frequency shift. In fact, 55% of the frequency shift was caused by the virtual mass of the adsorbed bead, predicted by Stoke's theory. Based on the observed shift in the resonator's quality factor during particle adsorption, we confirm this significant effect of the virtual mass. Thus, a quantitative analysis of the mass of a single adsorbed particle is strongly diminished if dynamic micromechanical sensors are operated in a liquid environment.