Rolling Droplet Dynamics over Channeled Hydrophobic Surfaces: A Comparative Study

The rolling dynamics of droplets are critically important for various applications in different sectors, including biomedical, petroleum, building construction, and self-cleaning. Droplet motion on inclined hydrophobic surfaces is governed by gravity, and the rotational speed is mainly governed by the droplet fluid properties, surface wetting state, droplet size, and angle of inclination. In some cases, such as those with a large dynamic contact angle hysteresis, rolling is accompanied by sliding over the surfaces, which lowers the droplet rotational speed. To increase the droplet rotational speed and create unidirectional rolling on surfaces, a surface topology incorporating minute-size channels may be favorable. The current study examines the comparative behavior of droplet rolling dynamics over minute-size channeled and plain hydrophobic surfaces with the same wetting states. The droplet internal flow field and pressure variation are predicted numerically, and the related droplet dynamic characteristics, including droplet puddle, wobbling, rolling speed, and droplet interfacial adhesion, are assessed from experiments incorporating high-speed camera recordings and the relevant data. It has been demonstrated that the flow developed in the droplet is driven by droplet rotation over the channeled surface, even though Marangoni and buoyancy currents are formed in the droplet because of small temperature differences between the droplet fluid and hydrophobic surface (ΔT ∼ 0.5 °C). Increasing the angle of the channeled hydrophobic surface creates a large circulation cell in the droplet fluid, unlike the case for a plain hydrophobic surface, where multiple small circulation cells are formed. The droplet contact length on the hydrophobic surface with a channel configuration becomes smaller than that on the plain surface. For the channeled hydrophobic surface, the combination of a reduced droplet contact length and low dynamic contact angle hysteresis lowers droplet adhesion and enhances the droplet rolling speed. Droplet inflection into the channel creates a unidirectional droplet rolling path on the surface. The present work provides insights into rolling droplets on channel surfaces for self-cleaning applications.