Heavy metal particle liftoff and compound droplet formation on a hydrophobic surface

Though single droplet impact dynamics was extensively investigated, the complex hydrodynamics involved in the simultaneous interaction of droplets with a loosely held dense solid particle on a hydrophobic substrate has received less attention. In this paper, the authors report the impingement response of a water droplet colliding on a non-fixed spherical steel particle placed on a hydrophobic surface and the subsequent creation of a particle-laden compound droplet. Utilizing the kinetic energy of the water drop, the heavy metal bead is picked up from the surface during the droplet bouncing. Here, a dense metal spherical mass is located on a hydrophobic substrate having contact angle, θc=140° and drop collides with a Weber number range of 8.20 ≤We≤ 38.07. During the droplet spreading, a thin film is developed between the particle and the hydrophobic plate due to capillary action and the particle is engulfed inside the droplet while it recoils, thereby a compound droplet is created. For instance, during the rebound of a composite drop, the metal bead, having one-third the mass of the droplet, is elevated to a height of 2.5 times its diameter at We = 24.5. Phenomenological models are developed for the prediction of compound droplet rebound height and the minimum Weber number required for the composite drop creation, and it exhibited good accord with experimental observations. These results shed more light on the self-cleaning mechanism involving dense particles and provided a promising strategy for the production of a solid–liquid composite droplet.