Topography Optimization for Sustainable Dropwise Condensation: The Critical Role of Correlation Length

Abstract

An effective pathway to enhance the heat transfer process is to induce the formation of highly mobile condensate droplets, employing micro‐nanoengineered superhydrophobic surfaces. However, the design of the topography of these surfaces for sustained high performance constitutes a significant scientific and technological challenge. Herein, the critical role of the correlation length of topography is demonstrated as an important factor when designing superhydrophobic surfaces for heat transfer applications. Specifically, it is shown that a) a high correlation length value corresponds to increased space between surface structures and higher lateral distances between nucleating droplets, which results in lower droplet departure diameter and significantly delayed flooding of the surface and b) correlation length has to surpass a critical value for dropwise condensation (DWC) to be sustained in hierarchical structured surfaces, when the droplets are growing in a partial Cassie state. Following this rationale, droplets are categorized in three different energy and wetting states (Wenzel droplets, Cassie droplets of low kinetic energy and high energy jumping droplets), depending on the correlation length of the topography. Heat transfer experiments demonstrate an increase of 126% in the heat transfer coefficient (HTC) of surfaces exhibiting the maximum correlation length when compared to the flat hydrophobic surface.

Durable, Ultrathin, and Antifouling Polymer Brush Coating for Efficient Condensation Heat Transfer

“…The steam was With increased subcooling, the dropwise condensation mode on PDMS brushes was maintained (Figure 3a, Videos S6−S8, Supporting Information), without a change of the circular drop shape. On our superhydrophobic reference surface (see Experimental Section for details), 66 dropwise condensation was observed at subcooling <1 K. Superhydrophobic surfaces are known for their jumping dropwise condensation mode only at low subcooling. 17,67 However, the drops show an irregular shape at subcooling of 2 K and finally turn into a liquid film at 3 K. The filmwise condensation is due to the flooding at high subcooling values because the surface remains superhydrophobic after condensation upon drying.…”

“…The reaction lasts for 4 h. Hydrophilic CuO surfaces are prepared by immersing oxide-free pristine copper in boiling water for 30 min. The superhydrophobic surfaces are fabricated as described before . Briefly, after the cleaning process, the substrates are immersed into a 9.25% V/V aqueous solution of hydrochloric acid for 10 min to fabricate microstructures.…”

“…The superhydrophobic surfaces are fabricated as described before. 66 Briefly, after the cleaning process, the substrates are immersed into a 9.25% V/V aqueous solution of hydrochloric acid for 10 min to fabricate microstructures. Then they are immersed in boiling water for 5 min to fabricate nanostructures on top of microstructures through the boehmitage process.…”

“…(c) Corresponding heat flux of the three surfaces at different subcooling. Data for superhydrophobic surface from our recent work 66.…”

Enhancing heat transfer at low temperatures by laser functionalization of the inner surface of metal pipes

“…According to 69 , surpassing a critical ratio between the distances of the surface structures (correlation length) and the distance of nucleation for the condensation of droplets in the partial Cassie state is necessary for sustainable dropwise condensation on hierarchical surfaces. It is assumed that this criterion could be met, as the enhanced heat transfer at low temperatures could be maintained over the entire measurement campaign of 4 months with a total of over 500 operating hours.…”