Contact angle and droplet evaporation on the smooth and structured wall surface in a wide range of droplet diameters

Droplet evaporation in a wide range of initial volumes of 1-1000 μl on the structured and smooth surfaces was studied experimentally. It is found that the static contact angle on the structured surface for the steady equilibrium depends on the droplet shape and initial diameter; it has an extreme. With the increasing wetting diameter of water samples from 2 to 30 mm, the contact angle increases first, reaches a maximum, then decreases and tends to a constant value. To determine the contact angle, the authors have performed the comparisons by different methods in a wide range of droplet sizes. A wide range of droplet sizes is usual for spray cooling. Approximation dependences for determining droplet volume were obtained in a wide range of droplet sizes. For the droplets with the initial diameter less than 1 mm, middle and large droplets, as well as for the small initial contact angles and angles of about 85-90°, different kinetics of evaporation will be observed. Kinetics of droplet evaporation on the structured surface differs from evaporation on a smooth wall. Dimensionless Fourier number (Fo), derived by the initial droplet diameter and total time of evaporation, decreases with an increase in the wall temperature and initial droplet diameter. For large droplet diameter and a high wall superheating the derivatives d(Fo)/d(Tw) and d(Fo)/d(d0) tend to zero.