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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.


Related in: MedlinePlus

Contact angle (θ) measurements for the fabricated silicon surfaces: (a) n-type bare silicon following the entire cleaning process; (b) silicon covered with nanowires.
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Figure 7: Contact angle (θ) measurements for the fabricated silicon surfaces: (a) n-type bare silicon following the entire cleaning process; (b) silicon covered with nanowires.

Mentions: However, if we consider the silicon nanowires synthesized on the overall surfaces in this study, they would induce extremely high surface roughness due to their nanoscale dimensions. Figure 7 displays the contact angle measurements for a bare silicon surface along with the surface having nanowires. Our measurements suggest that a nanowire-coated surface becomes highly hydrophilic at contact angles less than 10°. Once the liquid droplet falls on the surface, the contact angle comes to superhydrophilic near 0 degree by wicking. A silicon surface with dense nanowires has high water-wettability, which results in a very low contact angle at the solid and liquid interface. This is in close agreement with previous experimental results [11] and theoretical predictions [25] for nanowire-coated surfaces. In view of the roughness effect on the contact angle θ, for a hydrophilic n-type silicon surface (θsilicon = 70°) [26], nanowires having very high aspect ratio induce highly hydrophilic behavior by increasing the surface roughness [25,27]. The wicking condition in porous structure can be explained by the criteria for the surface energy based on the geometry [25]:(3)


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)

Contact angle (θ) measurements for the fabricated silicon surfaces: (a) n-type bare silicon following the entire cleaning process; (b) silicon covered with nanowires.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 7: Contact angle (θ) measurements for the fabricated silicon surfaces: (a) n-type bare silicon following the entire cleaning process; (b) silicon covered with nanowires.
Mentions: However, if we consider the silicon nanowires synthesized on the overall surfaces in this study, they would induce extremely high surface roughness due to their nanoscale dimensions. Figure 7 displays the contact angle measurements for a bare silicon surface along with the surface having nanowires. Our measurements suggest that a nanowire-coated surface becomes highly hydrophilic at contact angles less than 10°. Once the liquid droplet falls on the surface, the contact angle comes to superhydrophilic near 0 degree by wicking. A silicon surface with dense nanowires has high water-wettability, which results in a very low contact angle at the solid and liquid interface. This is in close agreement with previous experimental results [11] and theoretical predictions [25] for nanowire-coated surfaces. In view of the roughness effect on the contact angle θ, for a hydrophilic n-type silicon surface (θsilicon = 70°) [26], nanowires having very high aspect ratio induce highly hydrophilic behavior by increasing the surface roughness [25,27]. The wicking condition in porous structure can be explained by the criteria for the surface energy based on the geometry [25]:(3)

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