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An analytical model of open-cavity second sound resonators is presented andvalidated against simulations and experiments in superfluid helium using a newresonator design that achieves unprecedented resolution. The model incorporatesdiffraction, geometrical misalignments, and flow through the cavity, and isvalidated using cavities with aspect ratios close to unity, operated up totheir 20th resonance in superfluid helium.An important finding of this study isthat resonators can be optimized to selectively sense either the quantum vortexdensity carried by the throughflow -- as typically done in the literature -- orthe mean velocity of the throughflow. We propose two velocity probing methods:one that takes advantage of geometrical misalignments between the tweezersplates, and another that drives the resonator non-linearly, beyond a thresholdthat results in the self-sustainment of a vortex tangle within the cavity.A newmathematical treatment of the resonant signal is proposed to adequately filterout parasitic signals, such as temperature and pressure drift, and accuratelyseparate the quantum vorticity signal. This elliptic method consists in ageometrical projection of the resonance in the inverse complex plane. Itseffectiveness is demonstrated over a wide range of operating conditions.Theresonator model and elliptic method are being utilized to characterize a newdesign of second-sound resonator with high resolution thanks to miniaturizationand design optimization. These second-sound tweezers are capable of providingtime-space resolved information similar to classical local probes inturbulence, down to sub-millimeter and sub-millisecond scales. The principle,design, and micro-fabrication of second sound tweezers are being presented indetail, along with their potential for exploring quantum turbulence.

Second sound resonators and tweezers as vorticity or velocity probes : fabrication, model and method