By Kal KaurIntroductionVan der Waals ForceBiomimetic AttachmentDry Adhesion BiomimeticsReferences
Robots that can climb glass and smooth surfaces are typically designed with a directional adhesive that prevents the robot from losing its grip. Wall-climbing behaviour of a Gecko has become the main inspiration for designing a robot that can climb smooth surfaces and maintain a strong enough force to adhere to the surface without falling to the ground.
A Canadian research team at the Simon Fraser University have designed a wall-climbing robot that takes the shape of a tank and can roll up and down walls using a unique adhesive molecule also applied by the Gecko. The Gecko is capable of maintaining a firm grip of a vertical surface through van der Waals forces. To better understand the Gecko wall-climbing ability, the video below animates how a van der Waals force works.
Van der Waals Force
During a van der Waals force, polarization forces an attraction between two neutron atoms. A negative electron clouds the positive neutrons pulling these neutrons closer to initiate an alignment of these atoms. The pulling force is based on the movement of electrons in the electron clouds that causes temporary dipoles. This polarization means that one end of an atom has slightly more electrons in the electron cloud making this side carry a more negative charge compared to the opposite end of this cloud, making this end more positively charged. This dipole means that one atom can pull closer to another atom enhancing the van der Waals force. With relation to a Gecko, it is the small hairs (setae) on the toes of the gecko, and each have a pad at the tip that initiates the van der Waals process to create an interaction between the molecules on the smooth surface. Take a look at the following video, which demonstrates the application of a Gecko-inspired robot by Simon Fraser University.
The team of researchers at Simon Fraser University responsible for this novel technology started the design process by designing a dry adhesive surface that would mimic the setae of a Gecko. They achieved this design objective by creating polymer layers that were 17 microns across with end caps like the pads to the tip of a seta (single hair). The team designed the polymer equivalent of setae to have mushroom caps on a tread to ensure that each seta can peel off the smooth surface without leaving a gel-like substance behind. Adding a cap on the end of synthetic polymer fibers can increase the adhesive force (this is a type of hierarchical adhesive mechanisms – a multi-level structure) and can be useful for testing wall-climbing robot ability against surfaces asperities. Though this multi-layer system is beneficial for tougher surfaces, it does require more energy to separate the synthetic polymer fiber layer from a vertical surface.
Research by Bharat Bushan , which focuses on smart-adhesion mechanisms for wall-climbing robots, involved the development of a model for understanding biomimetic attachment using fibrillar structures. In this model, the team designed a single-level array made up of micro/nano beams hanging from a base material. In this fibrillar structure, each micro/nano fibre is cylindrical in shape with a spherical tip. This research also highlighted, that in order to maximise the adhesion capacity, there needs to be a high density of fibres. However, if the fibres are too tightly packed together, the adhesion force between the fibers becomes too strong to give flexibility to each fibre preventing the fibrillar structure from bending on a smooth surface.
Greuter M et al , have also been inspired by the nano-fibres found on the base of a gecko’s foot to help design synthetic fibrillar pads for wall-climbing robots. The robot designed by this team of researchers uses a silicone rubber belt to help adhere to a smooth surface. The prototype created in this study was put to the test and revealed a good climbing ability (e.g., the robot was capable of climbing up a surface at a 90° angle without falling off and was able to reverse this process, walk to the side along the surface at an 87° angle at a speed of 3.3 mm/s. The researchers did highlight that due to the silicone rubber, adhesion force was not possible at a degree high enough to prevent the robot from slipping and gliding along a surface. From looking at this research, it is clear that maximum adhesion force for a wall-climbing robot is not successful if using rubber belts.
Dry Adhesion Biomimetics
Research into smart-adhesive technology for climbing robots is leaning more towards the application of dry adhesion mechanisms. Just like the Gecko, dry adhesion mechanisms use nano-fibers to allow for multiple compliant points allowing for contact between the adhesive and smooth surface. As explained earlier in this article, van der Waals forces are fundamental to a dry adhesive contact, an intermolecular force that resides at the fiber-surface points and are key to creating a very strong adhesive force.
At the NanoRobotics Lab, Carnegie Mellon University, at team of researchers have studied directional characteristics to the Gecko climbing mechanism by applying synthetic polymer fibers whilst still allowing the robot to maintain directional adhesion. The team were capable of creating an adhesive surface that could maintain strength and grip by manipulating the tip angle of the microfibers and controlling the fiber tip size. Their research also found that the level of adhesion was also easily manipulated by changing the drag distance during the loading period (i.e., measuring the adhesion strength when the synthetic polymer fibers are pressed against the smooth surface).
There are many benefits to directional dry adhesives:
- This technology can help advance the functioning of mobile robots
- In the medical industry, this technology could help anchor a surgical point inside the body
- For the clothing industry, this technology offers an alternative to Velcro and could work as gripping material for sports-based activities.
- Chakraborty, S. (2010). Microfluidics and Microfabrication. USA, New York: Springer Science and Business Media.
- Murase, K., Sekiyama, K., Kubota, N., Naniwa, T., Sitte, J. (2006). Proceedings of the 3rd International Symposium on Autonomous Minirobots for Research and Edutainment (AMiRE 2005). Germany: Springer Science.
- Krahn, J., Liu, Y., Sadeghi, A., Menon, C. A tailless timing belt climbing platform utilizing dry adhesives with mushroom caps. Smart Materials and Structures 2011 20(11): 115021.
- NanoRobotics Lab, Carnegie Mellon University. Gecko Hair Manufacturing.
- Bhusham, B. (2008). Nanotribology and Nanomechanics: An Introduction. Germany, Berlin: Springer Science.
- Russell, P.J., Hertz, P.E., McMillan, B. (2011). Biology: The Dynamic Science. USA, California: Cengage Learning.
- Murphy, M.P., Aksak, B., and Sitti, M. Gecko Inspired Directional and Controllable Adhesion. Small 2009; 5:170–175.
- Hierarchical Micro-Fibrillar Adhesives. NanoRobotics Lab, Carnegie Mellon University.
- Greuter M, et al. Toward Micro Wall-Climbing Robots Using Biomimetic Fibrillar Adhesives. Published in Proceedings of the 3rd International Symposium on Autonomous Minirobots for Research and Edutainment. (2006). Germany: Springer.