All entries for nenad milijkovic

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Scalable Superhydrophobic Copper Oxide Surfaces for Enhanced Heat Transfer

• Authors: N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, E. N. Wang
• Sponsorship: MIT S3TEC Center

Vapor condensation is observed in everyday life and routinely utilized in industry as an effective means of transferring heat. In industrial systems, the condensed vapor typically forms a thin liquid film due to the high surface energy associated with the majority of industrial heat exchanger materials (i.e., clean metals and metal oxides). This filmwise condensation mode is not desired due to the large thermal barrier for heat transfer.  If the condensing surface is coated with a functional hydrophobic coating, for example a long chain fatty acid or polymer coating, the vapor forms discrete liquid droplets. These droplets shed at sizes approaching the capillary length (≈2 mm for water) and refresh the surface for re-nucleation. Furthermore, when micro- or nanostructures are combined with the coating, a superhydrophobic surface can be created with smaller droplet shedding radii (≈10-100 µm) than dropwise condensation and potential droplet jumping during coalescence due to the conversion of surface energy to kinetic energy (Figure 1a). In this work^[1], we experimentally demonstrated that a 25% higher overall heat flux (Figure 2a) and 30% higher condensation heat transfer coefficient (Figure 2b) can be achieved using silanized CuO superhydrophobic surfaces (Figure 1b and c) compared to conventional dropwise condensing Cu surfaces. We show that these CuO surfaces offer ideal condensation behavior in terms of emergent droplet morphology and coalescence dynamics, and a significant enhancement in heat transfer performance. Furthermore, the chemical-oxidation-based CuO fabrication process provides a simple and readily scalable method to create superhydrophobic condensation surfaces that can sustain droplet jumping behavior. Accordingly, these surfaces are attractive for applications such as atmospheric water harvesting and dehumidification.



Figure 1: (a) Schematic of the droplet jumping mechanism showing two droplets with radii R, prior to (state 1) and immediately after (state 2) coalescence. (b) Side view field emission scanning electron microscopy FESEM image of a nanostructured CuO surface. The sharp, knife-like CuO structures have characteristic heights, h ≈ 1 μm, solid fraction, φ ≈ 0.023, and roughness factor, r ≈ 10. (c) Jumping-droplet superhydrophobic condensation on a nanostructured CuO tube (Inset: magnified view of the jumping phenomena). The tube sample has outer diameter DOD = 6.35 mm, inner diameter, DID = 3.56 mm, and length L = 131 mm.



Figure 2: (a) Experimental steady state log mean water to vapor temperature difference (ΔTLMTD) as a function of overall surface heat flux (q”) for tube surfaces undergoing filmwise, dropwise, flooded, and jumping condensation with chamber vapor pressure Pv = 2700 ± 68 Pa. Rapid droplet removal due to coalescence induced droplet jumping results in the highest heat fluxes for the jumping sample (S < 1.12). (b) Experimental and theoretical steady state condensation coefficient (hc) as a function of saturated vapor pressure (Pv) for tube surfaces undergoing filmwise, dropwise, flooded, and jumping condensation. Jumping condensation shows the highest condensation heat transfer coefficient for low supersaturations (S < 1.12).

1. N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, and E. N. Wang, “Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces,” Nano Letters, vol. 13, no. 1, 2013, pp. 179-187. [↩]

Condensation on Micro/Nanostructured Superhydrophobic Surfaces

• Authors: N. Miljkovic, R. Enright, E. N Wang
• Sponsorship: MIT S3TEC Center

Water condensation on surfaces is a ubiquitous phase-change process that plays a crucial role in nature and across a range of industrial applications including energy production, desalination, and environmental control. Nanotechnology has created opportunities to manipulate this process through the precise control of surface structure and chemistry, thus enabling the biomimicry of natural surfaces, such as the leaves of certain plant species, to realize superhydrophobic condensation. However, this “bottom-up” wetting process is inadequately described using typical global thermodynamic analyses and remains poorly understood. In this work, we study condensation on well-defined structured surfaces spanning a wide range of length scales from 100 nm to 10 μm and functionalized using several hydrophobic thin films (Figure 1). We found that the emergent morphology of isolated droplets interacting with the surface structures during growth is primarily defined by the pinning behavior of the local contact line within the structures^[1]. Depending on the relationship between the structure length-scale and the droplet nucleation density, the dominant condensed droplet morphology can switch to one that is thermodynamically unfavorable in a global sense due to local contact line de-pinning. We show how these isolated condensed droplet morphologies arise by quantitatively describing growth in terms of characteristic local energy barriers and extend this view to explain the role of droplet-droplet interactions in determining emergent droplet morphology (Figure 2). This result contrasts the common macroscopic view of wetting behavior for individual droplets. The understanding of droplet growth behavior developed provides a rational basis to develop optimized condensing surfaces. This mechanistic framework also has implications for understanding condensation behavior on nature’s superhydrophobic surfaces and ice formation on structured surfaces that initiates with the condensation of sub-cooled water.



Figure 1: Scanning electron micrographs of fabricated pillar geometries. (a) Electrodeposited Au nanopillars defined by an anodic alumina template. (b) Si nanopillars fabricated using interference lithography and metal-assisted wet etching. (c) Si nanopillars fabricated using e-beam written mask and DRIE. (d) Si micropillars fabricated using optical lithography and DRIE.



Figure 2: Regime map characterizing the dominant wetting behavior observed during condensation with coordinates of 〈L〉/l and E*, where 〈L〉 is the average spacing between droplet nucleation sites, l is the pillar-to-pillar pacing, E^*=-1/(r∙cosθ_a), r=1+πdh/l^2, d is the pillar diameter, h is the pillar height, and θa is the smooth surface advancing contact angle. Cassie morphologies emerge at large 〈L〉/l and E^*≤1 (shaded region). Wenzel morphologies emerge at low 〈L〉/l and/or E^*≳1.

1. R. Enright, N. Miljkovic, A, Al-Obeidi, C. V. Thompson, and E.  N. Wang, “Superhydrophobic Condensation: The role of energy barriers and size scale,” Langmuir, 2012, vol. 28, no. 40, p. 14424–14432. [↩]