The hydrodynamics of water strider locomotion
By Sung Hoon Kang
Title: The hydrodynamics of water strider locomotion
Reference: David L. Hu, Brian Chan and John W. M. Bush, Nature 424, 663 (2003).
Soft matter keywords
surface tension, hydrophobic, capillary
Abstract from the original paper
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 surface, 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.
Soft matter example
Hydronamics of the surface locomotion of semiaquatic insects is an interesting subject which is not well understood. In general, there are two ways of walking on water depending on the relative magnitudes of the body weight (Mg) and the maximum curvature force (σP), where M is the body mass, g is the gravitational acceleration,σ is the surface tension and the P is the contract perimeter of the water-walker .
Water-walkers with Mc = Mg/σP > 1, such as the basilisk lizard, uses the force generated by their feet slapping the surface and propelling water downward, whereas creatures with Mc = Mg/σP < 1, such as the wter strider rely on the curvature force by distortion of the free surface as shown in Fig. 1. They have non-wetting body and legs covered by thousands of hairs [2-3].
The force balance on a stationary water strider can be written as Mg = Fb + Fc, where Fb is the buoyancy force and Fc is the curvature force. Fb is obtained by integrating the hydrostatic pressure over the body area in contact with the water, while Fc is deduced by integrating the curvature pressure over this area, or equivalently the vetical component of the surface tension, σ sin θ, along the contact perimeter (Fig. 1b). For a long thin water-strider leg, this ratio is Fb/Fc ~ w/Lc <<1, where the leg radius, w ~ 40 um, the capillary length, Lc = (σ/ρg)1/2 ~ 2 mm, and ρ, the density of water. The strider's weight is supported almost by surface tension.
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 de Gennes, P.-G., Brochard-Wyart, F. & Quere, D. Gouttes, Boules, Perles et Ondes (Belin, Collection Echelles, Paris, 2002).