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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.

Droplet-based transport phenomena driven by surface tension have been explored as an automated pumping source for a number of chemical and biological applications. In this paper, we present a comprehensive theoretical and experimental investigation of unconventional droplet-based motions on a superhydrophobic-patterned surface microfluidic (S(2)M) platform. The S(2)M surfaces are monolithically fabricated using a facile two-step laser micromachining technique on regular polydimethylsiloxane (PDMS) chemistry. Unlike the traditional droplet-driven pumps built on an enclosed microfluidic network, the S(2)M network pins the liquid-solid interface of droplets to the lithographically defined wetting boundary and establishes a direct linkage between the volumetric and hydraulic measures. Moreover, diverse modes of droplet motions are theoretically determined and experimentally characterized in a bi-droplet configuration, among which several unconventional droplet-driven transport phenomena are first demonstrated. These include big-to-small droplet merging, droplet balancing, as well as bidirectional transporting, in addition to the classic small-to-big droplet transition. Furthermore, multi-stage programmable bidirectional pumping has been implemented on the S(2)M platform, according to the newly established droplet manipulation principle, to illustrate its potential use for automated biomicrofluidic and point-of-care diagnostic applications.