Interaction forces during the sliding of a water droplet on a textured surface

We have directly measured the interaction forces (in normal and lateral directions) during the sliding of a water droplet on a surface decorated with a micropillar array. The measurement was carried out using a MEMS-based two-axis force sensor array. The advantages of our sensor array were the high sensitivity and miniaturized size of the ultrathin cross-shaped piezo-resistive silicon structure fabricated under a micropillar to detect both the normal and lateral forces acting on the micropillar. A demonstrating measurement using a water droplet with volume of 18 μL sliding on the micropillar array was carried out. The measurement results showed a fluctuation in the interaction forces when the micropillar was close to the trailing edge or leading edge of the droplet. Meanwhile, in the inner region of the contact line, both the normal and lateral interaction forces were relatively stable. These results indicate that the interaction forces at the edges of the droplet are important factors controlling the sliding motion of the droplet.