Contact angle and impinging process of droplets on partially grooved hydrophobic surfaces

• We investigate droplet resting and impinging on partially grooved PDMS surface.
• The contact angle parallel to grooves is larger than the perpendicular one.
• An impinging droplet would recoil faster in the direction parallel to grooves.
• More groove area or smaller droplet will make the anisotropy more significant.

Wetting states of droplets on partially micro-grooved surfaces were investigated. Grooves were fabricated using soft lithography and the width of the grooves was smaller than the droplet diameter. On the partially micro-grooved surfaces, the apparent contact angle parallel to the grooves is larger than the one on the smooth surface, while the microstructures have little effect on the contact angle perpendicular to the grooves. Increasing the fraction of the grooved area and the surface energy of the surface will result in a more anisotropic droplet. When a droplet impinged upon these partially grooved surfaces, the spreading process was similar to that on a smooth surface. However, the recoiling process was found to be quite anisotropic. The grooves enhanced the recoiling velocity in the direction parallel to the grooves while hindering the recoiling process in the perpendicular direction. The recoiling process will become more anisotropic by increasing the fraction of the grooved area. The effect of the grooves on both the contact angle and the impinging process is independent of the groove width scale and is only dependent on the fraction of the grooved area in the Cassie state.