Fabrication and Wetting Properties of Metallic Half-Shells with Sub-Micron Diameters
This paper presents work from Love, et al. concerning fabrication of metallic shells. Thin metallic shells are deposited on and then released from spherical molds. Treated with self-assembled monolayers, aggregates of these shells can be made superhydrophobic. In addition to forming structures that affect the wetting properties of a surface, it is postulated that the thin, metal edges of the half-shells can provide strong enhancements in optical and magnetic fields.
Fabrication of Half-Shells
As illustrated in the figure above, fabrication of the half-shells occurs in four main steps:
- - monolayers or multilayers of silica colloids are prepared by drop-casting aqueous suspensions of the particles onto glass slides
- - an adhesion layer of titanium or nickel followed by a thin film of gold, platinum, or palladium are deposited onto the colloids via electron-beam evaporation
- - spherical colloids are released from the glass slide by sonication
- - silica and adhesion metal are dissolved by etching the colloids in hydrofluoric acid (HF), leaving only the thin shell
After fabricating the half-shells, experiments were conducted to determine how much the shell geometry might modify the wetting properties of gold. For a smooth, unmodified, thin film of gold deposited by physical vapor deposition, the typical contact angle for a drop of water was ~73 degrees. The contact angle for bare, unmodified gold half-shells fabricated in the same vapor deposition was ~151 degrees. Love, et al. attribute the increased contact angle to the increased surface roughness caused by the presence of the 290nm diameter, 10nm thick gold half-shells. These contact angles were measured using a hanging 5 microliter drop of water.
A surface is considered superhydrophobic if the advancing contact angle of a water drop is greater than 150 degrees. Love, et al. have shown that by simply changing the surface roughness of chemically identical gold surfaces, the surface can be made to be superhydrophobic. However, after the hanging drop measurement, the edges of the drop remained pinned and as the drop was withdrawn from the surface, a small part of it remained behind. This indicates that the surface is not entirely superhydrophobic. By coating the gold shells with a monolayer of hexadecanethiolate, the contact angle measured by hanging drop was increased to ~163 degrees, and no parts of the drop adhered to the surface. See figure at right for details.
Naveen's comments: what is hexadecanethiolate? What would be the effect of different sizes of micro-spheres?
Love, et al. note that the current fabrication method limits the potential materials to those that can withstand etching by HF. Future work will look at creating half-shells of this scale using a larger variety of materials, with the possibility of developing multi-layer shells. A few things that might be interesting to look at:
- - Do heterogeneous mixtures of half-shells on surfaces have an impact on wetting? Is there anything to be gained or studied by mixing gold and platinum shells together?
- - Love, et al. show successful localization of half-shell clusters on a substrate. Could microfluidic channels and wells be defined on a surface using only metallic half-shells?
- - Does the concavity of the half-shells have a measurable impact on the wettability?
Naveen's comments: How could one control the orientation of the microspheres (i.e. concave up or concave down)?
 Adamson, A.W., Gast, A.P. Physical Chemisty of Surfaces, 6th edition. John Wiley & Sons. New York 1997