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Research Papers

Achieving Ultra-Omniphilic Wettability on Copper Using a Facile, Scalable, Tuned Bulk Micromanufacturing Approach

[+] Author and Article Information
Nicholas Clegg, Xin He

Department of Mechanical and
Aerospace Engineering,
New Mexico State University,
1040 S. Horseshoe Street,
Las Cruces, NM 88003-8001

Krishna Kota

Department of Mechanical and
Aerospace Engineering,
New Mexico State University,
1040 S. Horseshoe Street,
Las Cruces, NM 88003-8001
e-mail: kkota@nmsu.edu

Sean Ross

Air Force Research Laboratory,
Directed Energy Directorate,
3550 Aberdeen Avenue South East,
Kirtland AFB, NM 87117

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received September 3, 2016; final manuscript received March 28, 2017; published online May 17, 2017. Assoc. Editor: Martin Jun.This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government's contributions.

J. Micro Nano-Manuf 5(3), 031003 (May 17, 2017) (7 pages) Paper No: JMNM-16-1041; doi: 10.1115/1.4036446 History: Received September 03, 2016; Revised March 28, 2017

Altering the wetting characteristics of copper will positively impact numerous practical applications. The contact angle (CA) of a water droplet on the polished copper surface is usually between 70 deg and 80 deg. This paper discusses a facile, scalable, tuned bulk micromanufacturing approach for altering the surface topology of copper concomitantly at the micro- and nano-length scales, and thus significantly influence its wetting characteristics. The resultant copper surfaces were found to be robust, nontoxic, and exhibited ultra-omniphilicity to various industrial liquids. This extreme wetting ability akin to a paper towel (CA of zero for multiple liquids) was achieved by tuning the bulk micromanufacturing process to generate connected hierarchical micro- and nano-roughness with nanocavities within the embryos of microcavities. With an adsorbed coating of ester, the same ultra-omniphilic copper surfaces were found to exhibit robust super-hydrophobicity (CA ∼ 152 deg for water).

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Figures

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Fig. 1

CA measurement using circle fitting method; the same method was employed for measuring CA for all the prepared surfaces

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Fig. 2

Homogeneous wetting of 5 mL water droplet on the ultra-omniphilic copper surface; the surface is locally over-saturated with water at the center. A finite CA could not be measured (no visible contour of the droplet shape above and separate from the surface was observed) implying that γSV = γSL + γLV, i.e., the interfacial energy between solid and vapor phases (γSV) is balanced by the solid–liquid (γSL) and liquid-vapor (γLV) interfacial tensions.

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Fig. 3

Extreme homogeneous wetting (paper towel effect) on the ultra-omniphilic copper surfaces for 5 mL droplets of (a) FC-770®, (b) water, (c) glycerol, and (d) mineral oil; the surfaces are ultra-omnihpilic (versus ultra-hydrophilic) as many liquids were observed to wet them, i.e., wetting on these surfaces was observed to be a function of only the surface roughness but not the type of the liquid

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Fig. 4

Liquid retention capability of the ultra-omniphilic copper surface compared with polished and hydrophobic copper surfaces. Each surface with 5 μL water droplets was tilted by 90 deg. It was observed that droplets leave a residue on polished copper surface, while no trace of them was found on hydrophobic surface. On the ultra-omniphilic surface, the excess water was found to drip while water absorbed by the surface remained in the entire wetting area.

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Fig. 5

Liquid retention capability test; Left—micro/nanoporous copper with excess water and mineral oil; Right—surfaces after subjecting to vigorous vibration. (a-1) A 15 × 6 cm2 ultra-omniphilic copper sample excessively saturated with water. (a-2) A 19 × 4 cm2 ultra-omniphilic copper sample wetted with mineral oil. (b-1) and (b-2) After vigorous shaking, both the liquids were retained in the omniphilic surfaces due to the strong capillary forces. The inertial forces generated by vigorous vibration were able to only drain the excess liquids in both the cases.

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Fig. 6

Mineral oil was pumped multiple times through a channel with water-wetted ultra-omniphilic walls. One percent Safranin O Stain® (or basic red 2) was added to water and the channel walls were observed under fluorescent microscope (Leica M165) (a) before and (b) after the flow experiments. Preservation of the color on the entire wall surface indicated excellent liquid retention capability of the ultra-omniphilic surfaces under the condition of another liquid flowing over it.

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Fig. 7

Ultra-omniphilic copper surface observed in a scanning electron microscope (SEM) under different resolutions. All the images are from the same copper sample wherein cavity sizes ranging from nano- to micro-scale were observed with microcavities having multiple nanocavities in their embryos. (a) SEM—37,000×, nanocavity diameter is about 600–700 nm; (b) SEM—10,000×; (c) SEM—5000×, cavity diameter is about 4.5–5.5 μm; and (d) SEM—2500×, microcavity diameter is about 24–30 μm.

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Fig. 8

Hierarchical micro/nanoroughness copper surfaces: (a) stereoscopic microscope— 10×; (b) SEM—500×; and (c) SEM—2500× (inset (b) is not the exact location shown in (a); Inset (c) is approximately the same location marked in (b))

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Fig. 9

SEM spectral element analysis (5000 counts was found to provide good results [36]). In the plot, the source of oxygen (O) is expected to be from the surface oxidation in the SEM environment while the primary source of carbon (C) is expected to be the abrasive paper (SiC). The element analysis shows that the ultra-omniphilic behavior of the resulting samples is due to the obtained roughness structure on copper but not due to the presence of a -philic element on the surface.

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Fig. 10

Observed surface oxidation on treated samples after exposing to ambient for 192 h. The formed oxide layer was found to fill the cavities at both micro- and nano-length scales affecting the -philicity property of the surface.

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Fig. 11

Hydrophobic copper surfaces obtained after carrying out an additional step to the three-step bulk micromanufacturing procedure: (a) 3 mL water droplet on a copper hydrophobic surface (sample 1); (b) side view of a copper hydrophobic surface (sample 2) with 3 mL water droplets; (c) top view of a copper hydrophobic surface (sample 2) with 3 mL water droplets exhibiting the lotus leaf effect (i.e., droplets do not stick to the surface and tend to roll); and (d) CA measurement of a 5 μL water droplet on the surface of sample 1

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