Transport of water droplets on metallic topographic gradients under forced vibration

Mechanical systems with moving parts and/or fluids, such as heat exchangers and turbomachinery, often experience vibration. Furthermore, these systems often benefit from effective water droplet removal to improve air-side heat transfer, avoid corrosion pitting of turbine blades, etc. Therefore, designing these systems requires a deeper understanding of a droplet's vibrational response on the surface of interest. In addition to uniform surfaces, topographic wetting gradients are also examined due to their ability to promote the spontaneous motion of water droplets on a surface for directional control. Topographic wetting gradients, produced in a one-step industrial process, have the potential to enhance heat exchanger performance, promote dropwise vs filmwise condensation, and facilitate more complete water drainage. This is enabled by net surface tension forces across the gradient, which drive droplet motion, especially when coupled with vibration. More specifically, we study the droplet behavior influenced by both vertical and horizontal vibration on polished and superhydrophobic coated aluminum surfaces as well as on uncoated aluminum surfaces containing variable-pitch micro/nanoengineered topographic wetting gradients. These experiments demonstrated a nonlinear response in the droplet behavior depending on the test surface and the combinations of forced vibration amplitude and frequency that were used to create the droplet lateral motion, which in some cases was up to 6.5 mm for droplet volumes of 5 and 10 μl. The range of examined frequencies was 20–200 Hz since these frequencies were observed on operating heat exchangers, whereas the range of displacements (i.e., amplitudes) was 50–2000 μm, which expands all previously reported data.