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Micro-nano hybrid structures with manipulated wettability using a two-step silicon etching on a large area.

Kim BS, Shin S, Shin SJ, Kim KM, Cho HH - Nanoscale Res Lett (2011)

Bottom Line: The fabrication process is readily capable of producing MNHS covering a wafer-scale area.By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability.MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, Yonsei University, 262, Seongsanno, Seodaemun-gu, Seoul 120-749, Korea. hhcho@yonsei.ac.kr.

ABSTRACT
Nanoscale surface manipulation technique to control the surface roughness and the wettability is a challenging field for performance enhancement in boiling heat transfer. In this study, micro-nano hybrid structures (MNHS) with hierarchical geometries that lead to maximizing of surface area, roughness, and wettability are developed for the boiling applications. MNHS structures consist of micropillars or microcavities along with nanowires having the length to diameter ratio of about 100:1. MNHS is fabricated by a two-step silicon etching process, which are dry etching for micropattern and electroless silicon wet etching for nanowire synthesis. The fabrication process is readily capable of producing MNHS covering a wafer-scale area. By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability. MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

No MeSH data available.


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Schematics for behavior of water cooling agent on the surfaces with MNHS: (a) MNHS with microcavities and nanowires fabricated by acetone-mediated process; (b) MNHS with microcavities and nanowires fabricated by asher-mediated process to remove the hydrophobic barrier structure.
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Figure 8: Schematics for behavior of water cooling agent on the surfaces with MNHS: (a) MNHS with microcavities and nanowires fabricated by acetone-mediated process; (b) MNHS with microcavities and nanowires fabricated by asher-mediated process to remove the hydrophobic barrier structure.

Mentions: The heat transfer area is significantly increased by the additional microscale pattern structures in hierarchical MNHS than the surface with just nanoscale structures. Moreover, microscale patterns designed in accordance with vapor size (tens or hundreds of microns in a boiling process) and porous vacancies between nanowires can also play a role as artificial bubble nucleation sites in boiling application [12,28]. In addition, by controlling the sidewall structure formation during the fabrication of MNHS and then removing the polymeric passivation layer on it, it is possible to control the surface wettability, and thereby the boiling performance. Using the asher process to remove the passivation layer deposited during DRIE process, nanowires are densely synthesized over an entire area, resulting in very high surface wettability. High surface wettability efficiently pumps and supplies water to the boiling surface, which extends the surface burn-out limit (critical heat flux, CHF limit) [11,12]. On the other hand, MNHS fabricated with polymer-passivated sidewalls may decrease the boiling performance by their hydrophobic behavior. Figure 8 explains the behavior of water cooling agent on those surfaces with MNHS. Surface wettability affects the bubble generating and detaching schemes from heat-emitting surfaces [29]. The hydrophobic sidewalls fabricated by the acetone process prohibit water from coming through the porous structures towards the artificial micropatterns as nucleation sites. Then the cooling efficiency would be decreased by merged vapors that would not be detached from the surface by insufficient refreshing water and suppress heat emission, and this finally leads to the burn-out of the local areas. However, hierarchical MNHS having highly hydrophilic characteristics can supply and refresh water to the initiative bubble generating regions more collaboratively than one having hydrophobic barrier structures. Thus, the burn-out of the surface and film boiling can be retarded according to the increasing of a CHF limit [30]. Based on these wicking mechanisms as shown in Figure 8, boiling performance could be improved by MNHS that are accompanied by the geometrical combination of micro- and nanoscale structures and the superhydrophilic surface wettability.


Micro-nano hybrid structures with manipulated wettability using a two-step silicon etching on a large area.

Kim BS, Shin S, Shin SJ, Kim KM, Cho HH - Nanoscale Res Lett (2011)

Schematics for behavior of water cooling agent on the surfaces with MNHS: (a) MNHS with microcavities and nanowires fabricated by acetone-mediated process; (b) MNHS with microcavities and nanowires fabricated by asher-mediated process to remove the hydrophobic barrier structure.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3211421&req=5

Figure 8: Schematics for behavior of water cooling agent on the surfaces with MNHS: (a) MNHS with microcavities and nanowires fabricated by acetone-mediated process; (b) MNHS with microcavities and nanowires fabricated by asher-mediated process to remove the hydrophobic barrier structure.
Mentions: The heat transfer area is significantly increased by the additional microscale pattern structures in hierarchical MNHS than the surface with just nanoscale structures. Moreover, microscale patterns designed in accordance with vapor size (tens or hundreds of microns in a boiling process) and porous vacancies between nanowires can also play a role as artificial bubble nucleation sites in boiling application [12,28]. In addition, by controlling the sidewall structure formation during the fabrication of MNHS and then removing the polymeric passivation layer on it, it is possible to control the surface wettability, and thereby the boiling performance. Using the asher process to remove the passivation layer deposited during DRIE process, nanowires are densely synthesized over an entire area, resulting in very high surface wettability. High surface wettability efficiently pumps and supplies water to the boiling surface, which extends the surface burn-out limit (critical heat flux, CHF limit) [11,12]. On the other hand, MNHS fabricated with polymer-passivated sidewalls may decrease the boiling performance by their hydrophobic behavior. Figure 8 explains the behavior of water cooling agent on those surfaces with MNHS. Surface wettability affects the bubble generating and detaching schemes from heat-emitting surfaces [29]. The hydrophobic sidewalls fabricated by the acetone process prohibit water from coming through the porous structures towards the artificial micropatterns as nucleation sites. Then the cooling efficiency would be decreased by merged vapors that would not be detached from the surface by insufficient refreshing water and suppress heat emission, and this finally leads to the burn-out of the local areas. However, hierarchical MNHS having highly hydrophilic characteristics can supply and refresh water to the initiative bubble generating regions more collaboratively than one having hydrophobic barrier structures. Thus, the burn-out of the surface and film boiling can be retarded according to the increasing of a CHF limit [30]. Based on these wicking mechanisms as shown in Figure 8, boiling performance could be improved by MNHS that are accompanied by the geometrical combination of micro- and nanoscale structures and the superhydrophilic surface wettability.

Bottom Line: The fabrication process is readily capable of producing MNHS covering a wafer-scale area.By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability.MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, Yonsei University, 262, Seongsanno, Seodaemun-gu, Seoul 120-749, Korea. hhcho@yonsei.ac.kr.

ABSTRACT
Nanoscale surface manipulation technique to control the surface roughness and the wettability is a challenging field for performance enhancement in boiling heat transfer. In this study, micro-nano hybrid structures (MNHS) with hierarchical geometries that lead to maximizing of surface area, roughness, and wettability are developed for the boiling applications. MNHS structures consist of micropillars or microcavities along with nanowires having the length to diameter ratio of about 100:1. MNHS is fabricated by a two-step silicon etching process, which are dry etching for micropattern and electroless silicon wet etching for nanowire synthesis. The fabrication process is readily capable of producing MNHS covering a wafer-scale area. By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability. MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

No MeSH data available.


Related in: MedlinePlus