Propulsion by directional adhesion


The rough integument of water-walking arthropods is well-known to be responsible for their water- repellency; however, water-repellent surfaces generally experience reduced traction at an air– water interface. A conundrum then arises as to how such creatures generate significant propulsive forces while retaining their water-repellency. We here demonstrate through a series of experiments that they do so by virtue of the detailed form of their integument; specifically, their tilted, flexible hairs interact with the free surface to generate directionally anisotropic adhesive forces that facilitate locomotion. We thus provide new rationale for the fundamental topological difference in the roughness on plants and water-walking arthropods, and suggest new directions for the design and fabrication of unidirectional superhydrophobic surfaces. Indeed, the ingenious methods employed by insects and spiders to move across a water surface rely on microphysics that is of little use to larger water walkers but of considerable interest to the microfluidics community.

See paper: Prakash & Bush (2011) .

 

Grabbing water: the elastocapillary pipette

Floating flowers inspired the elastocapillary pipette.

Grabbing water with an edible flower.

We introduce a novel technique for grabbing water with a flexible solid. This new passive pipetting mechanism was inspired by floating flowers and relies purely on the coupling of the elasticity of thin plates and the hydrodynamic forces at the liquid interface. Developing a theoretical model has enabled us to design petal-shaped objects with maximum grabbing capacity.

See: Reis, Hure, Jung, Bush & Clanet (2010)

The folding of flowers represents an example of capillary origami in nature, a few examples of which are discussed in Reis, Jung, James, Clanet and Bush (2009) .

 

The elastocapillary pipette is being adapted for use in the culinary arts, as a means of serving small volumes of liqueurs with flowers composed of edible gels (right).

PRESS:  Highlights in Chemical Technology

Can flexibility help you fly?

The influence of flexibility on the flight of autorotating winged seedpods is examined through an experimental investigation of tumbling rectangular paper strips freely falling in air. Our results suggest the existence of a critical length above which the wing bends. We develop a theoretical model that demonstrates that this buckling is prompted by inertial forces associated with the tumbling motion, and yields a buckling criterion consistent with that observed. We further develop a reduced model for the flight dynamics of flexible tumbling wings that illustrates the effect of aeroelastic coupling on flight characteristics and rationalizes experimentally observed variations in the wing’s falling speed and range.

See paper here: Tam, Bush, Robitaille and Kudrolli (2010)

Biomimetic water-walking robots

Robostrider, the first water-walking robot, confronts his natural counterpart.

We report recent efforts in the design and construction of water-walking machines inspired by insects and spiders. The fundamental physical constraints on the size, proportion and dynamics of natural water-walkers are enumerated and used as design criteria for analogous mechanical devices. We report devices capable of rowing along the surface, leaping off the surface and climbing menisci by deforming the free surface. The most critical design constraint is that the devices be lightweight and non-wetting. Microscale manufacturing techniques and new man-made materials such as hydrophobic coatings and thermally actuated wires are implemented. Using high-speed cinematography and flow visualization, we compare the functionality and dynamics of our devices with those of their natural counterparts.

See Hu, Prakash,_Chan and_Bush (2010)

PRESS:  Wikipedia ,  BBC News

Capillary feeding in shorebirds

The Phalarope seeks its prey at the water surface. PHOTO CREDIT: Rainey Shuler

The phalarope captures its prey in a droplet, then transports it mouthwards by a series of tweezer-like beak motions. PHOTO CREDIT: Rainey Schuler

A certain class of shorebirds have an extremely clever feeding mechanism. By swimming in a circle, the phalarope generates a vortex that sweeps its prey towards the surface, like tea leaves in a stirred tea cup (see video below). Then, by dipping its beak into the water and withdrawing it, the bird captures its prey inside a droplet pinned between its upper and lower bills. By moving its beak in a tweezer -like succession of openings and closings, the phalarope transports the drop and the desired prey from its beak tip to its mouth. The ability to transport fluid in this fashion was demonstrated in a series of analogue experiments (see second video below).

 

We here present the results of a combined experimental and theoretical investigation of this subtle feeding mechanism.  Our study provides a simple physical rationalization for the observation of multiple mandibular spreading cycles in feeding, and highlights the critical role of contact angle resistance. We also find a unique geometrical optima in beak opening and closing angles for the most efficient drop transport. This mechanism would seem to be a unique natural example of directed drop transport via contact angle history. This capillary ratchet mechanism may also find applications in microscale fluid transport, such as valveless pumping of fluid drops.

REFERENCE [1] Surface Tension Transport of Prey by Feeding Shorebirds: The Capillary Ratchet, M. Prakash, D. Quere and J. W. M. Bush, Science, Vol. 320 (5878), 931-934, 16 May 2008. pdf

[2] BIOPHYSICS COMMENTARY: The Intrigue of the Interface, Mark Denny, Science, Vol. 320 (5878), pp. 886, 16 May. 2008 pdf

[3] Bush, J.W.M., Peaudecerf, F., Prakash, M., and Quere, D., 2010. On a tweezer for droplets. Advances in Colloid and Interface Science, 161, 10-14. pdf

[4] POUR NOS AMIS FRANCAIS: Quere, D., Prakash, M., Bush, J.W.M., 2011. Prises de bec chez les phalaropes. Reflets de la Physique, Vol. 15, 11-14. pdf

SELECT PRESS:  MIT News , NSF News , Nature News , New York Times , Boston Globe , Pour la Science ,  Sciences et Avenir , Deutschlandfunk