Design of Surface-Tension-Driven Robots Inspired by Water Striders

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Unlike the Illusionist David Blane, Gerridae or otherwise known as water striders (a family of freshwater bugs) do have the unique ability to walk on the surface of water. Take a look at the following video which demonstrates a water strider copulating. Leaving copulating aside, what you see in this video is the ability of the strider to maintain a position on top of the bed of water. 

So, just how are these insects capable of gliding on water? Well, unlike the majority of us who would avoid tension like the plague, the water strider appears to live off this instinctive behavior as a survival technique. Gerridae species have a layer of waterproof hair that resides on the base of their hind legs. During contact with water, the water molecules remain as a lattice and avoid attraction to the waterproof layer on the gerridae’s legs, creating a clean barrier and therefore allowing this insect to glide across the water. 

Another unique feature of this insect is its claws. Claw-like projections are found on the upper part of the legs of the strider, which will prevent the top layer of the water from being punctured by the insect. It is quite obvious that the ability to remain on the surface of the water is a survival technique for the gerridae by adopting a degree of surface tension. This otherwise survival instinct is exactly what has inspired researchers to design and engineer surface-tension-driven legged robots.

Inspiration

The adaption of suboptimal biological systems into robotic technology is a big challenge among design engineering. Designing a robotic water strider involves observing and paying close attention to the physical characteristics of Gerridae. Two main physical characteristics to the gerridae are particularly important:

• Its ability to stay on the water
• The sculling motions created by the middle legs that force the insect to move across the bed of water

Suhr SH et al. (2005) presented a model of a robotic water strider, which includes 1) the supporting legs, 2) the actuating legs that support the steering of the robotic body, 3) the central component to the body (made up of carbon fibers to reduce the bodyweight of the strider robot), and 4) the main body of the robot. The main body is structurally made up of a power source to drive the on-board electronics, a central control unit, a controller, and sensors.

It is important that the actuators to such a small light-weight structure are capable of steering the body of the robot with precision and speed with enough force output and little energy consumption. Though this is a difficult obstacle to overcome when working on such miniature robotic systems, Suhr SH et al did manage to further explore the problem. They found that piezoelectric actuators were ideal based on the following advantages:

• Allows a motor to move at high speed
• Lightweight structures
• Require low power consumption making them energy efficient

However, the fact that they require a high voltage also means that piezoelectric actuators adopt a small motion. This in turn will take a strider robot much longer to get from point A to point B in a controlled space. To overcome this disadvantage, Suhr SH et al. designed their strider robot to have multiple long actuator legs to allow for better complexity of motion based on creating more degrees of freedom for the robot structure.

We have already established that water striders can balance their bodies on the bed of water without puncturing the surface and plunging straight down into the deep end by adapting to the surface tension force of water. The actuator legs of a strider robot work in a similar fashion - the legs deform the water surface. The video below illustrates a buoyant force, the same concept adapted to design and engineer a strider robot that can stay above water.

The lift force is characterized as the pressure applied by the strider robot as it rests on the bed of water. This causes the weight of the water volume to push down. The effect has to be balanced out by a lifting force keeping the object resting on the water. Mathematically speaking, this phenomenon is based on the amount of water pushed down by an external force (surface tension force [fr]) that is equal to the amount of water creating an upward force (buoyancy force [fb]) which will give the effective lifting force (FL). Researchers at The NanoRobotics Laboratory, Carnegie Mellon University have engineered a microbot that uses the surface tension of water in order to maneuver (see video below).

Design and Analysis

A study by Song YS et al. (2007), focusing on the theory and applications for biologically inspired water strider robots, suggests that the length and number of actuator legs need to be proportional to the lift force. This research involved designing a strider robot with legs that were approximately 12.6 mm apart to maximize the lift force. The research team further designed the tip of the legs in an upward direction to minimize penetrating the surface of the water. The team also found that it was important for the actuator leg to not be too long as this could compromise the motion of the central actuator leg.

The research mentioned in this article opens new possibilities for studying robot locomotion on water. Moreover, it highlights what allows for better stability of a robot to maneuver on the surface of water.

Sources and Further Reading

• O. Ozcan, H. Wang, J. D. Taylor, and M. Sitti, ''Surface Tension Driven Miniature Legged Robot Inspired by Water Strider Insects,'' Journal of Research of NIST, in press.
• Y. S. Song and M. Sitti, ''Surface Tension Driven Biologically Inspired Water Strider Robots: Theory and Experiments,'' IEEE Trans. on Robotics, vol. 23, no. 3, pp. 578-589, June 2007.
• Y. S. Song, S. Suhr, and M. Sitti, ''Modeling of the Supporting Legs for Designing Biomimetic Water Strider Robots,'' Proc. of the IEEE Robotics and Automation Conference, pp. 2303-2310, Orlando, FL, May 2006.
• Suhr SH, et al. (2005). Biologically Inspired Miniature Water Strider Robot. IEEE Robotics and Automation Conference. Department of Mechanical Engineering, Carnegie Mellon University. USA.
• Brenbaum, M. (1989). Ninety-Nine Gnats, Nits, and Nibblers. USA: Board of Trustees of the University of Illinois.

This article was updated on 6^th February, 2020.