Motion of a Droplet on an Anisotropic Micro-grooved Surface

Kumar, Manish and Bhardwaj, Rajneesh and Sahu, Kirti Chandra (2019) Motion of a Droplet on an Anisotropic Micro-grooved Surface. Langmuir. ISSN 0743-7463

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Abstract

We experimentally characterise the sliding angle of water droplets (volume 3.1 $\mu$L - 22.2 $\mu$L) migrating on inclined micro-grooved surfaces along the longitudinal and transverse directions of the grooves. The rectangular micro-grooves are manufactured on silicon wafers using standard photolithography techniques. We tilt the surface gradually using a rotating stage mechanism until the incipience of the sliding. The droplet migration in the longitudinal and transverse directions to the grooves is recorded using a high-speed camera. For the droplets migrating downward in the transverse direction, the contact line exhibits a ``stick-slip'' type motion, i.e. the advancing contact line is attached with the surface, while receding contact line is detached from the surface. However, no significant change in the relative position of the advancing and receding contact lines is observed in case of the longitudinal migration of the droplets. The sliding behaviour of the droplet the longitudinal direction is similar to that observed in case of a smooth surface. The sliding angle in the longitudinal direction of motion is found to be smaller as compared to that in the transverse motion of the droplet. In both the longitudinal and transverse migrations, increasing the pitch of the grooves increases the contact angle, which in turn decreases the sliding angle. As the droplet volume is increased, the component of the gravitational force in the direction of the inclination increases, which acts to decrease the sliding angle. A theoretical analysis is also conducted to predict the sliding angle of a droplet on micro-grooved surfaces. The model predictions agree with the trends observed in our experiments, and thus validates the proposed sliding mechanisms in the longitudinal and transverse migrations of the droplet.

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