Switchable adhesion: Ability to walk on walls

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2010

Researchers develop a device that can create reversible adhesion using electronic control. 

KEY CONCEPTS
• A switchable, electronically controlled capillary adhesion device has been developed that mimics the muscular action of a leaf beetle.
• Liquid bridgelets are formed by using an electric field to pump water droplets through an orifice in the SECAD to contact a solid substrate.
• On reversal of the electric field, the bridgelets break contact with the substrate while the liquid is pumped back through the orifice.

Surface adhesion is a necessary element in lubrication as there is need for a specific component, whether a fluid or grease, to adsorb onto a surface in order to minimize friction and wear. One goal of researchers has been to develop a reversible adhesive that can adhere to a surface for a finite time frame and then release. This process would be followed by establishing the adhesion on a different surface at a later time.

Inspiration for reversible adhesion comes from our imagination and from Mother Nature. Many of us know about the comic book hero, Spiderman, who can literally walk on walls and climb up buildings.

Animals have been found that produce adhesives, which develop high levels of adhesion with surfaces. In a previous TLT article, an adhesive gel developed by the slug, Arion subfuscus is described (1). This slug secretes an adhesive, which is a combination of carbohydrates and proteins that exhibits a shear strength of 100 kilopascals. The adhesive is also highly effective in water.

In seeking to develop a switchable adhesive device, Paul Steen, professor of chemical and biomolecular engineering at Cornell University in Ithaca, N.Y., took inspiration from a leaf beetle native to the state of Florida studied by his Cornell colleague, Tom Eisner. Steen says, “The beetle develops adhesive forces that can resist loads that exceed 100 times its body weight. But the beetle also has the unique ability to switch off this bond, move in a relatively quick fashion (less than one second) and adhere at another spot on the leaf.” 

The key to the beetle’s switchable adhesion behavior is that it can make contact with the leaf surface through the use of approximately 10⁵ feet. Steen says, “The beetle has the ability to use the force generated by surface tension of liquid situated between its feet and the leaf. This force is amplified by the large number of contacts between the beetle and the leaf.” 

The surface tension generated is analogous to the force produced when a liquid droplet holds two glass slides together. Steen realizes that preparing a switchable adhesive device, which utilizes muscular action to mimic the beetle, is quite challenging.

But development of a device that can create reversible adhesion using electronic control is doable. Such an approach has now been realized.

SECAD
Steen and his research group have developed a device that combines the surface tension force exhibited by the beetle with electronic control that enables the adhesion to be reversible. This device is known as the switchable electronically controlled capillary adhesion device or SECAD and contains two plates that sandwich a middle layer.

The surface tension is established by the droplet between the SECAD and the substrate. One contact by itself does not produce much adhesion, but Steen has built a SECAD made from silicon that contains a top plate with nearly 5,000 holes each with a diameter of 150 microns. The SECAD has a bottom plate below the holes that contains a water reservoir. In-between the plates a porous, glass frit middle layer acts as an electroosmotic (EO) pump.

The SECAD is approximately 1 millimeter thick and can be made from common materials such as glass, silicon, plastic and epoxy. It contains no moving solid parts.

Steen says, “We combined the utilization of surface tension force with electronic flow through the use of an EO pump to develop the SECAD. The pump moves water droplets at sizes down to 10 microns up through an orifice to contact a solid substrate, thereby forming liquid bridgelets. Pumping is done by the electric field. On reversal of the electric field, the bridgelets break contact with the substrates as liquid is pumped back through the orifice array, eliminating the adhesive effect.” 

Experiments were conducted to show the ability of the SECAD to attach to substrates and also to place substrates on the SECAD. An example shown in Figure 2 is the placement of 73 paper clips weighing just over 32 grams on the SECAD for just over seven minutes. Adhesion is maintained until a release voltage is applied.


Figure 2. Substrates can be reversibly attached to the SECAD, as shown with the placement of 73 paper clips weighing just over 32 grams in just over seven minutes. Adhesion is maintained until a release voltage is applied. (Courtesy of Cornell University)

One of the challenges faced by the researchers is making sure that the individual water droplets do not coalesce. Steen says, “This system would like to minimize free energy by making water droplets coalesce. We built the SECAD so that the droplets do not overlap, and the size of the pores in the glass frit middle layer is an order of magnitude smaller than the holes to resist back flow and thereby to minimize liquid rearrangement and coalescence.” 

Two elements were designed into the SECAD to reduce the possibility of coalescence. Steen says, “We fabricated the geometry so that a lip is present around the hole to retard adjacent drops from combining. The second step is the laying down of a superhydrophobic, non-wetting coating to waterproof the environment outside of the contact line.” 

Steen believes that coalescence will eventually occur, but it will take many days for the water droplets to combine. Evaporation can also be an issue in using water. The leaf beetle has overcome this issue by utilizing a long-chain hydrocarbon as the liquid instead of water.

Using smaller holes also makes for faster switching of the SECAD from the attached to the detached states. Steen says, “In the paper clip example (see Figure 2), switching between the states was achieved in a fraction of a second.” 

A nine-volt battery can be used to generate the electric field necessary for the attach and detach steps. Steen estimates that this battery can power approximately one million, half-second cycles.

Future work will be devoted to improving the adhesive force that can be generated by the SECAD. Steen indicates that the magnitude of the adhesion can be increased through the reduction of the droplet size, as this parameter is inversely proportional to pressure. He says, “Our objective is to develop adhesion that is comparable to atmospheric pressure. We believe that hole sizes less than 0.1 micron should enable us to develop pressures greater than 1 atmosphere, which could lead to the development of switchable adhesives with strengths comparable to epoxies.” 

Steen envisions that the SECAD could be used as a controllable friction device when two surfaces are moving next to each other. Additional information can be found in a recent publication (2) or by contacting Steen at phs7@cornell.edu. 

REFERENCES
1. Canter, N. (2009), “Molluscan Adhesive Gels,” TLT, 65 (12), pp. 14–15.
2. Vogel, M. and Steen, P (2009), “Capillarity-Based Switchable Adhesion,” Proceedings National Academy of Science, 107 (8), pp. 3377–3381.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.