Rotary Liquid Droplet Microbearing

A rotational stage with a 10-mm-diameter single-crystal silicon rotor supported by liquid droplet ball bearings is described. The 100-300-μm-thickness droplet bearings are retained on the rotor surface with a micropatterned amorphous-flouropolymer-based superhydrophobic (SHP) surface coating that yields a 156° contact angle. The droplets slide on a SHP bearing raceway that is formed from laser-roughened poly-dimethylsiloxane (PDMS) on the surface of the stator, achieving a 10°-contact-angle hysteresis that results in very low sliding friction. The stage is driven by a rotating external magnetic held that provides up to 3 μN · m torque through a permanent magnet mounted on the rotor. The liquid bearing provides a passive wear-free interface between rotor and stator with a measured drag coefficient of 0.94 · 10^-3 μN · m/r/min, rotating up to a speed of 2400 r/min, and a mean minimum operating torque of 0.3 μN · m. The bearing design is stable in position and tip/tilt, with a tip mode stiffness of 5.4 μN · m/deg and measured nonrespectable rotor wobble of 0.3 mrad. The experimentally measured bearing stiffness, drag coefficient, and startup torque are shown to compare well with values predicted from analytical models based on surface tension forces on the droplet bearings.