Capillarity-based switchable adhesion

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Review by Bryan Hassell: AP 255 Fall 11

From: Capillarity-based switchable adhesion, Michael J. Vogel and Paul H. Steen, PNAS ∣ February 23, 2010 ∣ vol. 107 ∣ no. 8 ∣ 3377–3381

Keywords: capillary or capillarity

Fig. 1. Switchable wet adhesion concept and demonstrations. (A) Concept: Top and bottom states are static equilibria, characterized by zero power consumption. Moving from one equilibria to the other is accomplished by pumping liquid into or out of the device (pump not shown). (B) Substrate-pendant experiment, with device (Upper Left) holding a Plexiglas substrate (2 cm × 2 cm, arrow points to substrate). Oscilloscope shows “attach” voltage pulse. (C) Device-pendant experiment, with device (arrow points to top surface, visible through Plexiglas substrate) holding its own weight plus a payload for over 7 min at zero voltage. (D) Substrates successfully tested in (B), left to right: plywood, sandpaper, linoleum, brick, and roof shingle. (E) Slim-profile device: 2 cm wide, 1.1mm thick, ϵ = 300 μm, and ϕ = 0.1. Electrode interconnects to either side, filling port extends down the middle, and no spacer is present here.
Fig. 2. SECAD schematic and operation. (A) Main components in cutaway (not to scale for clarity). Primary layers are labeled to the right. Letters indicate: (a) spacers; (b) holes from which droplets/bridges protrude; (c) wire interconnects to power supply; (d) electrodes; (e) epoxy seal; (f) fluid reservoir; (g) luer connector as reservoir continuation and filling port; (h) reservoir meniscus, depending on configuration; and (i) representative support post. (B) Operation of device with no substrate present. (B.i.) is just before voltage pulse, t ¼ 0 s, (B.ii.) is at t = 2.0 s. Device has ϵ = 440 μm, N ¼ 100, V ≈ 12.5 V (S ≈ 6).
Fig. 3. Force-transducer experiment. (A) Force (Upper Plot) felt by substrate over time due to voltage pulses applied (Lower Plot). Inset: experimental setup. (B) Still image from attached state in (A). Also see Movie S4. The view is through the Plexiglas substrate, as indicated in (A Inset). Device has ϵ = 500 μm, N = 448, ϕ = 0.4, α = 0.16.
Fig. 4. Adhesion strength F∕Ameas versus contact size ϵmeas, using normalized values. Legend: fabrication type is silicon wafer (SW) or machine shop (MS); control is electroosmosis (EO) or manual syringe (syr).