Dropwise

Dropwise condensation is a mode of heat transfer during vapor condensation in which the condensate forms as discrete droplets that grow, coalesce, and depart from the surface rather than spreading as a continuous liquid film. On appropriately engineered surfaces, this regime can yield substantially higher heat transfer coefficients than filmwise condensation, where a persistent liquid film blankets the surface.

The improved performance arises because the liquid-vapor contact resistance is reduced while droplets leave the surface, allowing fresh vapor to contact the surface more readily. Droplets nucleate at active sites, grow by condensation, and shed when their weight or surface forces overcome adhesion, producing a cyclical sequence of droplet formation and removal that minimizes surface coverage between events.

Realizing stable dropwise condensation requires surface engineering to create high-contrast between solid and liquid such that droplets exhibit low contact angle hysteresis and readily shed. Hydrophobic or fluorinated coatings, micro- and nano-texturing, and slippery liquid-infused surfaces have been explored to promote dropwise behavior, with varying degrees of durability under high temperature, vapor exposure, and cleaning.

Materials and approaches include hydrophobic coatings (such as fluoropolymers or silanes) applied to metals used in condensers, and textured or composite surfaces designed to resist film formation. Some experiments use slippery liquid-infused porous surfaces to reduce pinning; however, long-term stability remains a key challenge.

Applications of dropwise condensation aim to improve energy efficiency in condensers for power plants, desalination, and heat exchangers. In practice, maintaining dropwise behavior over time is difficult, and performance gains can diminish as coatings degrade or contamination causes film formation. Research continues to address durability, scalability, and cost.