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(Color online) (a) Motor construction (not to scale); the entire device is around 32 mm × 12 mm × 1 mm and the top of the Si chamber is shown open for clarity. (b) Scanning electron microscope (SEM) image of the Si chamber used to house the rotors; this is viewed from the side that is bonded to the piezoelectric substrate; note the 40 μm diameter Si central pin used to mount the rotors. (c) Miniaturized (∼55 μm) thick steel rotor in an 80 μm deep, 1 mm diameter Si chamber. (d) Disc shaped rotor with 85°, 160 μm deep notches (top), and a 60° angled “bow tie” rotor (bottom) (enhanced online). [URL: http://dx.doi.org/10.1063/1.3676660.1]10.1063/1.3676660.1
(Color online) (a) Rotation speed for various rotor designs without applied preload. Due to its design asymmetry, we observed the rotor with 3 arms was displaced out-of-plane against the chamber roof, therefore impeding its rotation after an initial spin-up transient of around 5 ms. First-order exponential response least-squares fit to the data are shown for each rotor. (b) The start-up rotor torque reveals a linear relationship with respect to the surface area in contact with the SAW; the use of rotors with different geometries facilitated variations in the surface area. The linear fit shows a gradient of c = 5.7 nNm/mm2 for the steel rotors. (c) Comparison of the unloaded rotor speed with the preloaded case (approximately 220 μN ± 50 μN) shows a significant increase in steady-state velocities, at the cost of rotary speed stability.
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