Fabricating Hierarchical, Directional Adhesives Suspension Stalks Microwedges Assembly Variations and Optimization Performance A challenge in creating hierarchical adhesives is that the manufacturing processes used for creating features at the micrometer scale are different from those used at millimeter scales and larger. To create large (e.g. 10cm x 10cm) hierararchical patches, we use two different processes to create molds and we assemble the molded products. The text and graphics below describe the general process as of Summer 2009. We are exploring different materials, bonding methods, and suspension stalk diameters and angles, to improve performance. For details on performance, etc. see the adhesion publications page.

Hierarchical, directional adhesive (version 1) with 0.3mm dia. posts and 20um base-width wedges

Suspension Stalks

The process begins by creating a variation on the original DPS stalks, as used by Stickybot for climbing on smooth surfaces. In this case the stalks are suspension elements to help overcome surface variations and errors in alignment. The mold for the stalks is created in Delrin, using a custom-ground cutter to make grooves and a small (380um) end mill to make angled holes that intersect the grooves. A flexible top cap seats against the grooves, producing angled stalks with oblique cut tips. The stalks are typically molded from Vytaflex 60 or Tap Blue silicone.

Stalks are created in a Delrin mold. An elastomeric top cap (gold) is pressed in place to create sharp tips.

Microwedges

In parallel, we create an array of microwedges, triangular shapes with 20 or 50 micrometer base width. The mold in this case requires a dual-exposure lithographic process using SU-8 as a negative photoresist. This work is done at the Stanford Nanofabrication Center. The first, angled, exposure is done from below, using an aluminum mask directly deposited on a UV transparent quartz wafer. The second explosure is from above, through a contact mask.

(1-2) Aluminum mask is created on a UV transparent wafer. (3-4) Angled exposure from below. (5) Second, vertical exposure from above. (7) Unexposed material is dissolved away. (8) Elastomer (e.g. PDMS) is cast in place and spun to control backing layer thickness.

Overmasking with top mask to reduce overexposure.

Overmasking in the top exposure helps to ensure that triangular cavities will have sharp tips and crisply defined surfaces. The microwedges are molded from Sylgard 170 PDMS (Dow Corning). The backing layer thickness is controlled by spinning, to between 100um and 400 um thick.

Assembly

The suspension stalks are dipped in a thin (200-300um) film of Silpoxy or RTV 118 to wet the tips of the stalks, and assembled to the microwedge backing layer with a very light pressure (to avoid deflecting the stalks).

Suspension stalks are dipped in a thin layer of liquid adhesive for bonding to the backing layer of the microwedges

Variations and Optimization

Different combinations of materials and geometries will affect the performance of the hierarchy. Making the suspension stalks slightly longer and more angled than the original DPS stalks on Stickybot results in a softer suspension with a more uniform pressure:

Large-strain ANYSYS model of the suspension, with wedges approximated by a contact model having similar behavior, to assess effects on pressure of varying stalk geometries, stalk-web connections, etc.

If the stalks get too long, they will stick to each other, or clump; however, there is less danger than with the original DPS stalks because the tips are attached to the backing layer of the microwedges.

Due to limitations on the double lithography process (underexposure, overexposure, alignment) there are fewer variations possible at the micrometer scale. Presently, best results are obtained using 200um long "squeegee wedges" with triangular cross sections having a 20um base width and 80um height. Performance The attached video (Quicktime movie) gives an idea of the typical performance of the Hierarchy III patches on a variety of surfaces. For more details see the Climbing Adhesion Publications page.

"Squeegee" wedges for greater fill ratio

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-- MarkCutkosky - started 26 Oct 2009