micromachines Article A Pot-Like Vibrational Microfluidic Rotational Motor Suzana Uran, Matjaž Malok, Božidar Bratina * and Riko Šafariˇ c   Citation: Uran, S.; Malok, M.; Bratina, B.; Šafariˇ c, R. A Pot-Like Vibrational Microfluidic Rotational Motor. Micromachines 2021, 12, 177. https://doi.org/10.3390/mi12020177 Academic Editor: Sung-Yong Park Received: 30 December 2020 Accepted: 9 February 2021 Published: 11 February 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Faculty of Electrical Engineering and Computer Science, University of Maribor, Koroška c. 46, 2000 Maribor, Slovenia; suzana.uran@um.si (S.U.); matjaz.malok@student.um.si (M.M.); riko.safaric@um.si (R.Š.) * Correspondence: bozidar.bratina@um.si; Tel.: +386-2220-7170 Abstract: Constructing a micro-sized microfluidic motor always involves the problem of how to transfer the mechanical energy out of the motor. The paper presents several experiments with pot-like microfluidic rotational motor structures driven by two perpendicular sine and cosine vibrations with amplitudes around 10 μm in the frequency region from 200 Hz to 500 Hz. The extensive theoretical research based on the mathematical model of the liquid streaming in a pot-like structure was the base for the successful real-life laboratory application of a microfluidic rotational motor. The final microfluidic motor structure allowed transferring the rotational mechanical energy out of the motor with a central axis. The main practical challenge of the research was to find the proper balance between the torque, due to friction in the bearings and the motor’s maximal torque. The presented motor, with sizes 1 mm by 0.6 mm, reached the maximal rotational speed in both directions between −15 rad/s to +14 rad/s, with the estimated maximal torque of 0.1 pNm. The measured frequency characteristics of vibration amplitudes and phase angle between the directions of both vibrational amplitudes and rotational speed of the motor rotor against frequency of vibrations, allowed us to understand how to build the pot-like microfluidic rotational motor. Keywords: microfluidics; micro-sized streaming of a liquid in the pot-like structure; micro-sized rotational motor; piezoelectric driven vibrations 1. Introduction The miniaturized microfluidic rotational motors have been at the forefront of the research efforts of scientists and engineers who have been developing micro rotational motors as a part of microelectromechanical systems (MEMS) for more than three decades. The first microfluidic rotational motors, published in the 1990s [1], had rotary gear trains made on the silicon rotor (diameter 60 μm to 1600 μm). They were built into micro- sized channels, where fluidic linear stream of liquids was used as the driving forces for the gears. The maximal rotational speed was 390 rad/s with a high max torque of 8.7 pNm. Their rotational speed was controlled in only one direction. The rotary gear trains with the rotor were closed in a micro-sized channel, and the rotational mechanical power could not be transferred outside the micro-sized channel. The mm-sized rotor of a motor, controlled in both directions by electro wetting, was reported in [2]. The electro-wetting microfluidic motor was made by a 3.0 μL liquid droplet between two electric plates. The maximal rotational speed 19 rad/s was performed on a rotor with a diameter of 2 mm. There was no report of max torque. The rotational mechanical power could not be transferred outside the motor. Amjadi et al. [3] published rotating the thin film of liquid between the electric plates using an electric field. The experiment had not reported the actuation of the disc on the surface of the liquid, probably because the layer of liquid was too thin. Shilton et al. and Yeo et al. [4–6] reported succeeding rotating the disc on the thin layer of liquid. Rotational microfluidic motors with micrometer to millimeter-sized diameters of rotors, used the vibration energy of surface acoustic waves (SAW) at the frequency 20 MHz to drive a rotor floating in the thin layer of liquid. This method used two oppositely Micromachines 2021, 12, 177. https://doi.org/10.3390/mi12020177 https://www.mdpi.com/journal/micromachines