The initial idea for blocked pillars was inspired by a discussion of colloids- dispersed particles within a medium. These particles tend to be controlled mostly by van der waals forces, and a lot of colloid science has focused on ways to defeat this van der waals forces, to keep the particles apart and then colloid stable. We've got a similar problem- if the pillars of the sticky feet are too close together and too soft/sticky then they'll clump. I've been investigating some of the techniques used to keep particles apart. Of them, two sound possible- electrostatic repulsion and polymer interference. The polymer idea seems superior to me- the idea is that you coat the particle with polymer, then the polymer chains push away other particles before they get close enough for van der waals forces to matter. Talking with Mark, something similar had been attempted, but was problematic- the lubricating materials worked their way into the sticky tips. As an alternate approach, I would like to chemically modify the surface, covalently bond materials that will reduce the stickiness of the pillars. Conveniently, the polyurethane cures by making urethane bonds, as described in this page: [] So any chemical with alcohol or cyanate groups can bond covalently to the polyurethane as it cures. This is, for example, why curing the material in a moist environment causes it to soften- the hydrogen groups in the water go in and bind to the cyanate groups in the polyurethane. This prevents crosslinking- softening the material. A complication is that I'm not sure by which mechanism modification will stop the van der waals forces from acting- my research so far hasn't explained why the polymer doesn't experience van der waals forces.

I'm working on a protocol to test this idea, it looks like this:

  • 1. Make a set of shapes(Is there a superior shape for testing?) in a mold.
  • 2. Spray a set of chemicals sequentially into the rectangles.
  • 3. Blow out excess with canned air (This should probably be done in a fume hood, and will this necessarily guarantee a thin film? The thickness will be determined by the properties of the liquid- may not be desirable).
  • 4. Pour the polyurethane, let it cure.
  • 5. Wash the shapes with soap and water to remove excess blocking agent, and test them on the force plate for adhesive properties.

My ideal would involve a sticky pad production process that would use the same polyurethane, but nonsticky the stalk and sticky the tip.

Some candidate chemicals:

  • Polyethylene Glycol (used in harder grades of polyurethane) (What molecular weights would we want? Higher? Lower?)
  • Ethanol, Isopropyl alcohol (Polyethylene is really slippery, so these hydrocarbon alcohols have the potential to be really slippery)
  • Mold release compound (Mold release compounds mostly lack the chemistry needed to covalently bond. I think we may see some effect anyway since many use rubbing alcohol as a solvent)
  • Soap
  • Glycerol

What are mechanisms through which polymer blocking could prevent interaction? lower surface energy Roughen surface stiffen surface thermodynamic exclusion (sort of like roughen surface- i'm not sure if there's a difference) electrosteric exclusion (like thermodynamic exclusion, plus electrostatics)

I don't want to work only by stiffening the surface- because we maintain the tradeoff between performance and anticlump. Rather, it would be preferable to operate via the rest of the possibilities.

From my reading, it sounds like the preferable solution would involve very rigid, projecting polymers that covalently bind to the substrate. I'm not sure what polymers fulfill this criterion.

(How much of each chemical would we need, and would it be applied with a simple spray bottle? I need to find out what sorts of research the Fearing group was doing.)

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