Liquid droplet micro-bearings on directional circular surface ratchets

This paper presents de-ionized water droplets used as torque-generating micro-bearings between a glass plate and a micromachined Si substrate. The pattern on the Si substrate includes circular tracks, which allow droplet motion in a single direction. When vertical vibration is applied to the system, a rotation in the transverse plane is triggered. The system can be tailored to respond to a specific vibration frequency, from 36.5 to 83 Hz for droplet volumes from 13 to 1 μL. The system is tested for its frequency response at different droplet sizes and droplet counts. A figure of merit is determined to quantify the responsiveness of the system. The largest angular speed is recorded as 0.302 rad/s for a vibration at 61 Hz.