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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.

When salt water displaces fresh water in sand the boundary between the two liquid phases becomes diffuse after a short period of flow. The dispersion of salt may occur at a much greater rate than can be expected by ion diffusion. The effect has been attributed to a mechanism referred to here as hydrodynamic dispersion, a general phenomenon arising from the fact that the velocity of the moving stream varies from point to point in the porous system. The mathematical treatment of the problem is based upon a statistical theory developed by Scheidegger. Equations have been derived here for the dispersion of salt water injected in the stream. The theory has been tested by displacing pure water with salt water in a column of saturated sand and determining the concentration of effluent as a function of time. The measured concentrations vary with time in approximately the manner predicted by theory.

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