Experiment 4 - Surface Treatment of PDMS

Cured PDMS has a very low surface energy. That is, it is difficult for chemical species to interact with the surface, and adhesion is poor. Changing the surface characteristics of the polymer to give it a higher surface energy will enhance the adhesion characteristics of the polymer.

Materials

• Cured Sylgard Elastomer 184 polydimethylsiloxane, obtained from Ellsworth Adhesive System, with a smooth surface (such as the cured slab fabricated in Experiment 2; the non - diffracting surface should be sufficiently smooth)
• Marker, such as a Sharpie
• Hand - held Tesla coil used to test for leaks

Procedures and Explorations

• Place the cured PDMS slab on a level surface with the smooth side up. Place a drop of water on the surface. The drop will "bead," i.e., the water in the drop will be attracted more to itself than to the surface of the PDMS and it will attempt to form a spherical shape. Because water is not attracted to the PDMS, the surface is referred to as being hydrophobic ("water-fearing"). Examples of items containing hydrophobic surfaces include waxed paper and waxed cars.

Figure 1. Water beading on a hydrophobic PDMS surface. (Dye has been added to the water to provide contrast; it is not necessary for this experiment.)

• Dry the slab and attempt to write on it with a marker. Most marker inks are also not attracted to the PDMS and will bead up on the surface.

Figure 2. Marker ink beading on a hydrophopic PDMS surface. (Titanium oxide has been mixed into the PDMS to provide contrast; it is not necessary for this experiment)

• The PDMS surface may be oxidized with the discharge from a Tesla coil. This corona discharge electrically breaks down chemicals comprising the air. Some of the chemical species produced by this process can react with the PDMS surface to convert it to a less hydrophobic surface. High energy electrical discharges like this are used to modify many other surfaces. Warning: Tesla coils are electrical devices with very high voltages; please follow recommended usage instructions. Moving the tip of the coil back and forth ~3 cm above the surface for ~30 seconds appears to provide sufficient surface oxidation time. Shut off the Tesla coil when it is not being used.

Figure 3. Oxidizing the surface of PDMS with a Tesla coil.

• Place a drop of water on the oxidized PDMS surface. The drop will "wet" the surface, i.e., the water in the drop will be attracted more to the surface of the PDMS than to itself and attempt to flatten. Because water is attracted to the PDMS, the surface is referred to as being hydrophilic ("water-loving").

Figure 4. Water wetting a hydrophilic PDMS surface. (Dye has been added to the water to provide contrast; it is not necessary for this experiment.)

• Dry the slab (you will probably find it harder to dry) and attempt to write on it with a marker. Most marker inks are now attracted to the surface (as they are to paper, which typically is hydrophilic) and will form higher-quality marker lines.

Figure 5. Marker ink on a hydrophilic PDMS surface. (Titanium oxide has been mixed into the PDMS to provide contrast; it is not necessary for this experiment)

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Experiment 1 - The "Intial" Stamp

Experiment 2 - A Flexible Diffraction Grating

Experiment 3 - Bouncing PDMS Balls