Micrometer-sized droplet impingement dynamics on flat and micro-structured surfaces

An 85 μm diameter droplet impinging on a flat surface and on micro-structured surfaces with groove/stud characteristic sizes of 25-50 μm was simulated using the VOF method with dynamic contact angles calculated by Blake's model to study the impingement dynamics and wetting behavior for spray cooling system in nuclear plants and other high heat flux applications. The simulations of the droplet impinging on the flat surface agreed well with experimental results (van Dam and Le Clerc, 2004). The results show that the droplet was successively dominated by the inertia force, viscous force and surface tension as it impinged on the surface. The total wetted area on the grooved surfaces at steady state was slightly larger than that on the studded surfaces with the same characteristic structure size, followed by that on the flat surface. The larger wetted area led to decreased droplet oscillations up to steady state for the micro-structures which increased the contact area between the droplet and the surface and the viscous dissipation. The increased viscous dissipation restricted the free movement and caused the droplet to reach steady state earlier. The studded surfaces were more conducive to enhancing the bottom surface wetting than the grooved surfaces.