**4. Conclusions**

In summary, this paper demonstrates the feasibility of developing TWs on micro- to millimeter-scaled piezoelectrically actuated silicon-based MEMS structures, by combining two resonant modes with the proper scheme of actuation. Values of the SWR close to the ideal TW support the efficient generation mechanism. In order to validate the experimental results, FEM analysis was performed, and a good agreemen<sup>t</sup> between the measurements and the simulations was found. Additionally, TW generation in liquid media was presented, which revealed the potential applicability of this type of structure to locomotion in liquid media, such as miniaturized propulsion systems.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-666X/10/5/283/s1: Video S1: 3D animation of the TW measured at 208.4 kHz in air; Video S2: 3D animation of the TW measured at 921.9 kHz in air; and Video S3: 3D animation of the TW measured at 583.2 kHz in isopropanol.

**Author Contributions:** Investigation, A.D.-M.; project administration, J.L.S.-R.; supervision, V.R.-D., J.H.-G., A.A., H.S., and J.L.S.-R.; writing—original draft preparation, A.D.-M.; writing—review and editing, V.R.-D., J.H.-G., A.A., H.S., and J.L.S.-R.

**Funding:** This work was supported by the European Regional Development Fund, the Spanish Ministerio de Economía y Competitividad project (TEC2015-67470-P), and the regional governmen<sup>t</sup> (JCCLM) project (SBPLY/17/180501/000139). Abdallah Ababneh and Helmut Seidel acknowledge the funding provided by the German Research Council (DFG) project "Micro resonators for applications in liquids and gases".

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
