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Research Papers

Microwedge Machining for the Manufacture of Directional Dry Adhesives

[+] Author and Article Information
Paul Day

Department of Mechanical Engineering,
Stanford University,
Stanford, CA 94305;
Los Alamos National Labs,
Los Alamos, NM 87544
e-mail: pday@stanford.edu

Eric V. Eason

Department of Applied Physics,
Stanford University,
Stanford, CA 94305
e-mail: easone@stanford.edu

Noe Esparza

e-mail: noe.esparza@stanford.edu

David Christensen

e-mail: davidc10@stanford.edu

Mark Cutkosky

e-mail: cutkosky@stanford.edu
Department of Mechanical Engineering,
Stanford University, Stanford, CA 94305

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF Micro AND Nano-Manufacturing. Manuscript received April 1, 2012; final manuscript received July 18, 2012; published online March 22, 2013. Assoc. Editor: Stefan Dimov.

J. Micro Nano-Manuf 1(1), 011001 (Mar 22, 2013) (10 pages) Paper No: JMNM-12-1022; doi: 10.1115/1.4023161 History: Received April 01, 2012; Revised July 18, 2012

Directional dry adhesives are inspired by animals such as geckos and are a particularly useful technology for climbing applications. Previously, they have generally been manufactured using photolithographic processes. This paper presents a micromachining process that involves making cuts in a soft material using a sharp, lubricated tool to create closely spaced negative cavities of a desired shape. The machined material becomes a mold into which an elastomer is cast to create the directional adhesive. The trajectory of the tool can be varied to avoid plastic flow of the mold material that may adversely affect adjacent cavities. The relationship between tool trajectory and resulting cavity shape is established through modeling and process characterization experiments. This micromachining process is much less expensive than previous photolithographic processes used to create similar features and allows greater flexibility with respect to the microscale feature geometry, mold size, and mold material. The micromachining process produces controllable, directional adhesives, where the normal adhesion increases with shear loading in a preferred direction. This is verified by multi-axis force testing on a flat glass substrate. Upon application of a post-treatment to decrease the roughness of the engaging surfaces of the features after casting, the adhesives significantly outperform comparable directional adhesives made from a photolithographic mold.

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Figures

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Fig. 1

SEM micrographs of PDMS directional adhesive features: (a) unloaded microwedges from a photolithographic mold, (b) microwedges under shear loading, and (c) microwedges from a micromachined mold.

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Fig. 2

A diagram illustrating the geometry and the parameters of the micromachining process for a single cavity. “Traj.” is the tool trajectory; “S.P.” is the shear plane.

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Fig. 3

Surface comparison of PDMS microwedges cast from micromachined wax molds, using (a) no lubricant, (b) liquid soap lubricant, and (c) liquid soap lubricant and “inking” post-treatment. A broken wedge, illustrating the wedges' tapered profile, can be seen on the right.

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Fig. 4

Diagram illustrating the steps of the post-treatment process (see text for details)

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Fig. 5

Cross-section of microtome blade used in the micromachining process, showing its three different beveled sections

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Fig. 6

(a) Micrograph of a single mold cavity created in cutting force tests (θ = 50 deg, t = 100 μm) showing triangular built-up region. (b) Measured shear stress at the shear plane fs/A versus θ, with values of shear yield stress k for comparison. T, S, and C correspond to trajectories parallel to the tertiary bevel, secondary bevel, and centerline of the tool. The estimated measurement uncertainties of θ and fs/A are 0.6 deg and 0.8 MPa.

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Fig. 7

Micrographs showing the effect of trajectory angle on mold cavity shape. For some trajectories (a) and (b), a continuous chip of built-up material is formed after the final cavity. The chip disappears for θ≥56 deg (c) and (d).

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Fig. 8

Geometric data taken from characterization experiment micrographs (Fig. 7). Feature height is measured normal to the mold surface from the tip of the cavity to its upper edge.

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Fig. 9

(a) Diagram of the adhesive testing apparatus, showing the normal and shear directions. (b) Diagram illustrating the movement of the positioning stage during load-pull tests.

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Fig. 10

Comparison of the limit curves for macroscopic arrays of adhesive microwedges produced with micromachined molds and photolithographic molds. The measurement uncertainty of the data is estimated to be 2 kPa in normal and 1 kPa in shear.

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