Nanotextured Surfaces Impregnated with Lubricant for Enhanced Condensation
Recently, Anand et al. reported on an alternate approach to promote dropwise condensation in which the condensing surface is microscopically textured and impregnated with a lubricating liquid that will not mix with the condensed liquid. Through selection of the appropriate surface geometry, chemistry, and lubricant, the authors demonstrated a surface having enhanced condensation properties, with water droplets as small 100 µm diameter becoming mobile and continuously swept away, creating new areas for droplet nucleation. The textured surfaces studied by the authors were microfabricated posts of silicon (10µm x 10µm x 10µm) which were then solution coated with a low-energy silane to make them hydrophobic. Two kinds of lubricants were investigated and the impregnation of the surface microstructures was accomplished by dipping the substrate in a bath of the lubricant controlled by a dip-coated system. Utilizing this approach, the surface of the microstructure remains exposed, while the remainder of the surface structure is covered with lubricant.
In characterizing the condensation properties of the lubricant impregnated surface, the authors found several important parameters that could be used to determine if effective droplet formation would occur. These parameters included the contact angle, spreading coefficient, density and viscosity of the lubricant, for example. In some cases, the lubricant can promote the formation of cloaked droplets condensed on the surface, where the droplet is covered by the lubricant, and therefore has no means to move. The authors also found that while effective droplet formation occurred, the drops became pinned by the tops of the micropost structure. To resolve this issue, the tops of the microposts were roughened using an etch process. This effectively reduced the capillary forces imparted by this surface eliminating the pinning of the droplets by the textured surface. Thus a scalable method has been introduced to promote enhanced dropwise condensation on engineered surfaces for application requiring optimal heat transfer. The enhancement results from the fact that the condensed droplets stay afloat on the lubricant with minimal pinning to the surface compared with superhydrophobic surfaces, where droplets grow within textures and get strongly pinned. A range of conditions required to ensure droplet flotation and demonstrate the effect of key parameters was reported, such as texture geometry, surface energies of various phases, and lubricant cloaking on droplet growth and mobility. The extremely high mobility and the resultant sweeping effect of condensed drops observed on lubricant-impregnated surfaces makes them promising for enhanced condensation heat transfer. The longevity of these surfaces is affected by additional factors including lubricant cloaking, drainage, and miscibility, and these durability challenges can be addressed by optimizing the texture and lubricant in future studies.
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
- Anand S, Paxson AT, Dhiman R, Smith JD, Varanasi KK. 2012. Enhanced condensation on lubricant-impregnated nanotextured surfaces. ACS Nano. Artilce ASAP. http://dx.doi.org/10.1021/nn303867y