Evaporation of sessile water droplets on horizontal and vertical bi-philic patterned surfaces

This paper presents an experimental study on thermal transport to single water droplets evaporating on heated bi-philic surfaces consisting of a superhydrophobic matrix with circular hydrophobic patterns. A single water droplet of 8 ul volume is placed on a preheated surface and allowed to evaporate completely at initial surface temperatures between 45 and 74 C. We investigate the influence of the substrate orientation (horizontal and vertical) on evaporation dynamics. Using optical and infrared imaging, we report droplet fluid dynamics and heat transfer characteristics of the evaporating droplet. We identify three major findings: (i) While the highest local evaporation flux occurs at the three-phase contact line region for both orientations, many evaporation characteristics, such as contact angle evolution, temperature and heat flux distribution, the time-varying shape of the droplet, and convective currents within the droplet are different on the vertical surface compared to those on the horizontal surface. (ii) For the vertical surface, the temperature and heat flux distributions are non-uniform along the perimeter of the droplet until the contact angles at the upper and lower contact lines are equal. We attribute this special behavior to the altered inner convective currents compared to a droplet evaporating on a horizontal sample. (iii) Overall, evaporation is more efficient on the vertical surface, exhibiting higher total heat transfer rates and up to 10% shorter evaporation times.