Physics Today: Japan Edition

 

 

Our review of the hydrodynamics of water-walking creatures, published in Physics Today, was chosen as the cover article in the Japanese version of the journal.

See the related article here: Physics Today-Japan (2011).

The english version is provided here: PhysicsToday (2010) .

See also the related post on interfacial locomotion, linked to  here.

Drinking strategies in nature

 

In this special edition of Journal of Fluid Mechanics dedicated to Tim Pedley on the occasion of his 70th birthday, we examine the fluid mechanics of drinking in nature. We classify the drinking strategies of a broad range of creatures according to the principal forces involved, and present physical pictures for each style. Simple scaling arguments are developed and tested against existing data. While suction is the most common drinking strategy, various alternative styles have evolved among creatures whose morphological, physiological and environmental constraints preclude it. Particular attention is given to creatures small relative to the capillary length, whose drinking styles rely on relatively subtle interfacial effects. We also discuss attempts to rationalize various drinking strategies through consideration of constrained optimization problems. Some biomimetic applications are discussed.

See paper:  Kim & Bush, JFM (2012).

 

A number of different drinking styles arising in the natural world.

Water entry of small hydrophobic spheres

 

We considered the impact of small hydrophobic spheres on a water surface. Particular attention was given to characterizing the shape of the resulting air cavity when the cavity collapse is driven principally by surface tension rather than gravity. A parameter study revealed the dependence of the cavity structure on the governing dimensionless groups. A theoretical description based on the solution to the Rayleigh–Besant problem was developed to describe the evolution of the cavity shape and yields an analytical solution for the pinch-off time and the sphere’s depth at cavity pinch-off.

See paper Aristoff & Bush, JFM (2009) .

Science Dong-A

 

 

 

An account of our work on water strider propulsion and the resulting adventures in biomimetics was given in the Korean journal, Science Dong-A.

Read the article here .

 

Link to our post on water strider propulsion here.

A bug uprising: meniscus-climbing insects

 

“I was stunned by the perfection of the insects”.   – Pablo Neruda

1 - Meniscus at the surface of a pond

 

Wetting climbers

From our perspective, the surface of a pond appears to be flat; however, there is millimetric topography in the form of menisci that  arise where the water surface meets land, floating objects or emergent vegetation (1). To millimetric water-walking creatures, these menisci can appear as frictionless mountains that they may be unable to climb using their ordinary means of propulsion.

The waterlily leaf beetle Pyrrhalta feeds upon the plant for which it is named (2).  The larva is a poor swimmer, making  travel between lily pads difficult.  It uses a special meniscus-climbing technique to close in on emerging (left) and overhanging (right) vegetation.

2 - Waterlily leaf beetle

 

3 ) The larva of Pyrrhalta is circumscribed by a contact line with the water surface.  To climb the meniscus, the larva arches its back pulling up on the free surface with its head and tail.

3 - Meniscus-climbing larva

 

4 ) The deformation of the water surface near the head and tail of the larva is clearly visible.  In these images, it approaches an emerging wetted leaf.

4 - Deformation of water surface

Surface deformation

 

 

Non-wetting climbers

Water-walking insects are generally covered by a dense mat of hair that renders them hydrophobic.  Learning to climb the meniscus was a necessary adaptation for their terrestrial ancestors as they colonized the water surface.  Modern water walking insects ascend to land in order to escape aquatic predators and lay their eggs.

5 ) The border between land and water may appear flat to us, but to water-walking insects, there may be significant topography.  Here the water measurer Hydrometra treads carefully atop slippery rocks protruding from below the water surface.

5 - Hydrometra walking on rocks

6 ) Meniscus-climbing by the water treader Mesovelia.  The water treader approaches a meniscus, from right to left. The deformation of the free surface is evident near its front and hind tarsi.  While covered entirely with non-wetting hairs, the treader uses specialized wetting claws to pull up on the water surface.

6 - Meniscus-climbing by the water treader Mesovelia.

7 ) To climb the slippery meniscus, water-walking insects need to get a running start.  Only by running up the meniscus and using their specialized climbing mechanism as they slide back down can they generate the speed to reach land.  Mesovelia (left) and the infant water strider (right) start their sprints at the bottom of the meniscus.

7 - Mesovelia (left) and the infant water strider (right) start their sprints at the bottom of the meniscus.

8 ) Floating weeds (left) are attracted to the meniscus, so on occasion, Mesovelia can hitch a ride to draw itself closer to land. On the right Mesovelia pauses before attempting a second climb.

8 - Mesovelia climbing

9 ) Left, Two water treaders making haste to climb the meniscus. Right, the water measurer prepares to climb the mensicus by drying its non-wetting claws.

9 - Water treaders

 

10 ) Water measurers Hydrometra known for their plodding speed on the water surface.  Left, Hydrometra next to a downward sloping meniscus at the edge of a glass of water.  Surface tension both supports Hydrometra‘s weight and keeps the water from spilling out the glass.  Right, an upward sloping meniscus generated by an overhanging plant.  To asend the plant, Hydrometra must ascend the slippery meniscus.

10 - Hydrometra managing the meniscus

11 ) Hydrometra  ascending the meniscus.  By assuming a static posture in which it pushes down with its middle legs and pulls up with its front and hind legs, the creature rises to the top of the meniscus.  Once at the top (right), it uses its claws to haul itself upward.

11 - Hydrometra ascending the meniscus

12 ) Meniscus-climbing postures assumed by insects.  Shaded spots indicate the sense of the surface deflection, light being upwards and dark downwards. a, Mesovelia. b, Microvelia. c, Hydrometra. d, Pyrrhalta. e, Anurida. Figures courtesy of Brian Chan.

12

 

13 ) Meniscus-climbing by Anurida maritima.  By pulling up on the water surface with its wetting ventral tube and pushing down with its nose and tail, Anurida can deform the water surface.  Assuming this static postures allows Anurida to ascend to land and to form colonies of 50-100 individuals.

13

14 ) To travel between two colonies, Anurida combines walking with meniscus-climbing.  Meniscus-climbing is recognized by the upward deformation of the free surface, as seen around the individual on the left.

14

 

See paper here:  Hu & Bush, Nature (2005)  

 

REFERENCES

Andersen, N.M. (1976). A comparative study of locomotion on the water surface in semiaquatic bugs (Insecta, Hemiptera, Gerromorpha). Videnskabelige Meddelelser fra Dansk Naturhistorisk Forening, 139: 337-396.

Bush, J. W. M. & Hu, D. L., 2006. Walking on water: Biolocomotion at the interface. Ann. Rev. Fluid Mech. 38.

Chan, D. Y. C., Henry, J. D. J. & White, L. R. The interaction of colloidal particles collected at fluid interfaces. J. Coll. Int. Sci. 79, 410 418 (1981).

Kralchevsky, P. A. & Denkov, N. D. Capillary forces and structuring in layers of colloid particles. Curr. Opin. Coll. Interf. Sci. 6, 383 401 (2001)

Miyamoto, S. On a special mode of locomotion utilizing surface tension at the water-edge in some semiaquatic insects. Konty  23, 45 52 (1955).

Nicolson, M. The interaction between floating particles. Proc. Camb. Phil. Soc. 45, 288 295 (1949).

Whitesides, G. & Grzybowski, B. Self-assembly at all scales. Science 295, 2418 2421 (2002).

 

SELECTED PRESS:  MIT News ,  NY Times ,  National Geographic ,  PhysicsToday-Backscatter

ALBUM COVER:   Meniscus

 

Water strider locomotion: Paradox lost

 

“Like a long-legged fly upon the stream, his mind moves upon silence”.

– William Butler Yeats

Water striders Gerridae are insects of characteristic length 1 cm and weight 10 dynes that reside on the surface of ponds, rivers, and the open ocean. Their weight is supported by the surface tension force generated by curvature of the free surface5,6, and they propel themselves by driving their central pair of hydrophobic legs in a sculling motion. Previous investigators have assumed that the hydrodynamic propulsion of the water strider relies on momentum transfer by surface waves. This assumption leads to Denny’s paradox: infant water striders, whose legs are too slow to generate waves, should be incapable of propelling themselves along the surface. We here resolve this paradox through reporting the results of high-speed video and particle-tracking studies. Experiments reveal that the strider transfers momentum to the underlying fluid not primarily through capillary waves, but rather through hemispherical vortices shed by its driving legs. This insight guided us in constructing a self-contained mechanical water strider whose means of propulsion is analogous to that of its natural counterpart.

See paper: Hu, Chan and Bush, Nature (2003)    … and commentary Dickinson (2003)

SELECT PRESS:  The Economist ,  Nature , BBC News ,  SouthFlorida Sun-Sentinel , Science_Dong A , Ca m’interesse ,  Pour la Science , Biophotonics

Television:  BBC International

For an account of the project presented by David Hu, see the below video.

 

Some nice strider shots from Dr. Hu’s old webpage:

 

 

 

 

 

 

 

And some vortical flows, induced by sprinkling Thymol Blue on the water surface, and letting the striders paint…

 

 

 

 

 

Evaporative instabilities in soap films

 

A vertical soap film supported on a rectangular wire frame of height 3.5 cm and width 15 cm drains under the influence of gravity in an unsaturated environment. Evaporation at the top of the film disrupts the film shape, giving rise to a horizontal bump which grows in amplitude until becoming gravitationally unstable and generating a series of sinking plumes of relatively thick film. The plumes penetrate a finite distance into the film, giving rise to a turbulent mixed layer which slowly erodes the underlying region of stably stratified film. Note the black film adjoining the wire frame at the top of the film, and the relatively weak convective motions, associated with marginal regeneration, evident near the base of the film.

 

See article: Skotheim & Bush (2000)