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Self-Cleaning Surfaces

Fabrication Strategies of Synthetic Self-Cleaning Surfaces

To fabricate synthetic self-cleaning surfaces, there are a variety of methods used to obtain the desired nanotopography and then characterize surface nanostructure and wettability.

Templating

Templating utilizes a mold to add nanostructure to a polymer (Li et al., 2007). Molds can come from a variety of sources including natural sources, such as the lotus leaf. For example, polydimethylsiloxane (PDMS) was cast over the lotus leaf and used to make a negative PDMS template. PDMS was then used to make another template from the first. As the natural lotus leaf structure enables pronounced self-cleaning ability, this templating technique was able to replicate the nanostructure, resulting in a similar surface wettability (Sun et al., 2005).

Imprint nanolithography also utilizes templates, pressing a hard mold into a polymer above the glass transition temperature. Thus, the driving forces for this type of fabrication are heat and high pressure (Li et al., 2007). Porous templates consisting of aluminum with anodized aluminum oxide (a hard mold) were used to imprint polystyrene. To achieve this, the polystyrene was heated to 130 degrees Celsius and pressed against the template. (lee et al 2004).

Capillary nanolithography

patterned elastomeric mold (e.g. polyurethane with acrylate group) placed on spin coated polymer (e.g. PEG), polymer raised above Tg (REFERENCE Suh and Jon from li et al)

Often dissolve away mold

Photolithography

use x=ray or electron beam to etch substrates, often silicon, typically require added hydrophobic treatment (e.g. sputtering with gold then organic coat such as hexadecanethiol) (REFERENCE Furstner from li)

Plasma Treatment

Plasma treatment (plasma etching link) of surfaces is essentially a dry etching of the surface. This is achieved by filling a chamber with gas, such as oxygen, fluorine, or chlorine, and accelerating ions species from an ion source through plasma. The ion acceleration towards the surface forms deep groups within the surface. In addition to the topography, plasma treatment also offers surface functionalization by using different gases to deposit different elements on surfaces (li et al 2007). (fresnais et al 2006)

Chemical Deposition

Characterization of Synthetic Self-Cleaning Surfaces

Synthetic sample Characterization intro (Neinhuis and Barthlott, 1997)

Scanning Electron Microscopy (SEM) link to page

Scanning electron microscopy is used to exam morphology of fabricated surfaces. It enables the comparison of natural surfaces with synthetic surfaces. The size of nanotopography can be measured. To prepare samples for scanning electron microscopy, surfaces are often sputter coated using platinum, gold/palladium, or silver (reference). Example

Contact Angle link to page

As described above, contact angle is used to characterize surface wettability. A droplet of solvent, typically water for hydrophobic surfaces, is placed perpendicular to the surfaces. The droplet is imaged and the angle between the solid/liquid and liquid/vapor interfaces is measured. Example

Atomic Force Microscopy (AFM) link to page

AFM is used to study the local roughness and mechanical properties of a surface. AFM is also used to characterize adhesion and friction properties for micro- and nano-patterned superhydrophobic surfaces. (Yong Chae Jung and Bharat Bhushan) Example

FTIR

Synthetic surfaces (Blossey, 2003)
 * Synthetic surface preparation methodology (Li et al., 2007; Wen and Guo, 2016; Yan et al., 2011)
 * Sample methods: laser ablation and photolithography-based microfabrication, solidication of melted alkyltene dimmer, microwave plasma enhanced CVD of trimethoylmethoxysilane, phase separation, domain-selective oxygen plasma treatment, solgel method (page 647 Sun et al)
 * Add micro-/nanostructures to polymer surfaces (sun et al)
 * Rolling press technique uses porous aluminum as a template along with high pressure and temp above tg to get to fill
 * Significantly increases contact angle when make ~28 nm nanopillars compared to flat surfaces; however, difficult to make smaller pillars