Two-axis MEMS-based force sensor for measuring the interaction forces during the sliding of a droplet on a micropillar array

• A MEMS-based two-axis sensor was proposed to directly measure the interaction forces in normal and shear directions between a sliding droplet and a superhydrophobic surfaces with micropillar array.
• The sensor featured 300-nm-thick piezoresistive silicon structure as sensing element, by which a resolution of less than 10 nN was obtained for both normal and shear forces.
• A calibration method was proposed to achieve the sensitivity characteristic of the sensor.
• A direct measurement of interaction forces between a 7.5 μL water droplet and the pillar of the sensor during sliding was demonstrated.

In this paper, we proposed a 2-axis MEMS-based force sensor to directly measure the interaction forces during the sliding of a water droplet on a superhydrophobic surface with a micropillar array. In our sensor design, a 300-nm-thick silicon (Si) structure was fabricated beneath a single micrometer-sized pillar as the sensing element. Two piezoresistors were formed at the root of two opposite Si beams to detect the forces acting on the pillar surface in the normal and shear directions. The proposed sensor has a sensing resolution of less than 20 nN for both normal and shear forces. This resolution is confirmed to be sufficient to measure the forces of a droplet acting on a micropillar. The fabricated sensor was demonstrated to be able to measure the interaction forces during the sliding of a 7.5 μL water droplet on the micropillar array.