An experimental and analytical study on the influence of superhydrophobic micro-textured surfaces on liquid wetting phenomena

Controlling dust, particle, gas or liquid attachment or motion over the surfaces is attracting many researchers to reduce losses or reach mission-specific goals. Therefore, researchers have been developing novel surfaces so that performance increase, such as heat transfer, or reduction of energy losses, such as pressure loss, will be achieved. One of those approaches is hydrophobic surfaces that are widely used in order to increase liquid repellence commonly by controlling the liquid-solid contact angle. To ensure desired contact angles between the surface and liquid, these micro-nano-textured surfaces are fabricated by using advanced manufacturing technologies and methods like etching, electrospinning, and chemical vapor deposition. While, these fabricated surfaces can be used for a variety of purposes, such as protecting the material e.g. from corrosion, changing the surface property to avoid deposition or preventing icing on aircraft wings; hydrophobicity is also a key parameter for heat transfer systems such as heat exchangers, refrigerators, and industrially used condensers, as the focus of current study. In this work, a set of experimental and analytical studies has been conducted to investigate the influence of micro-nano-textured surfaces on the liquid wetting phenomena. Since the liquid-wall interaction is a key parameter in terms of droplet formation, the present work represents an essential contribution to our major research focused on heat transfer performance of heat pipe condenser. Copper samples with three different surface topographies at micro-scale (unstructured-smooth, square-grooved and v-grooved) have been subjected to a chemical treatment process by applying commercially available nano-particles using dip-coating and spraycoating techniques. These types of surfaces have been chosen to understand the effect of micro-structuring/coating on wettability, droplet formation and their dynamics. Before and after the coating procedure, measurements have been performed with de-ionized water in order to determine the difference in droplet contact angle at the surfaces to be used. In the analytical part, capillary Laplace equation with available analytical correlations from the literature has been used to predict the contact angles and surface energies on the surfaces to be investigated. The experimental results have been validated with the analytical approaches. When bare surface experimental results are compared with uncoated samples, it is found that the average water contact angle (WCA) increases by 34.5% and 52.5% for the square-grooved and v-grooved surface. Analytical calculations compared to experimental results show a reasonable deviation of 2.3% and 4.1% for both square-grooved and v-grooved surface, respectively. Moreover, analytically validated results clearly show that the coated square-grooved surface has a larger average contact angle than the v-grooved surface by 4.6%.