Film 1: Adult water strider rowing

  An adult water strider rows a distance of two body lengths on the free surface.  The water strider has two gaits: it may glide or launch itself with a vertical component, as shown.  This video is slowed down 25 times.  The body length is 1 cm.

  Water striders, order Hemiptera family Gerridae, have colonized the surface of ponds, rivers, and the open ocean (Andersen 1976).  Their weight is supported by the surface tension force generated by curvature of the free surface, and they propel themselves to achieve speeds of 150 cm/s by driving their central pair of hydrophobic legs in a sculling motion. 

Film 2: Infant water strider rowing

  A one-day-old infant water strider rows along the free surface, happily unaware of Denny's Paradox.  This video is slowed down 25 times.  The body length is 1 mm; the metal tube is a hypodermic needle. 

  In order for the water strider to move, Newton's third law requires that it transfer momentum to the underlying fluid in an amount equal to that of the strider.  It has previously been assumed that capillary waves are the sole means by which to accomplish this momentum transfer (Denny 1993, Sun 2001).  Denny (1993) suggested that the short legs of infant water striders are  too slow to generate waves, and so transfer momentum to the underlying fluid.  According to this physical picture, infant water striders cannot swim: their ability to do so has been referred to as Denny's Paradox (Suter, 1997)

Film 3: The vortex wake of the water strider

  Hemispherical vortices in the wake of the water strider are visualized through application of the dye Thymol blue.  

  Our calculations show that the momentum transported by vortices in the wake of the water strider is comparable to that of the strider, and greatly in excess of that transported in the capillary wave field.  We thus conclude that capillary waves do not play an essential role in the propulsion of water striders, and thereby circumvent Denny's paradox. The strider generates its thrust by rowing, using its legs as oars and its menisci as blades.   

Film 4: Robostrider 

  A mechanical water strider, Robostrider, was designed and constructed to mimic the motion of a water strider. An important design criterion was that the force per unit length along the driving legs not exceed twice the surface tension.  The video is slowed down 25 times.  The body length is 9 cm long.

   The legs, composed of 0.2 mm gauge stainless steel wire, were naturally hydrophobic.  Its middle driving legs were powered by an elastic band (310 dynes/cm) coupled to a pulley.   High speed video indicated that the Robostrider did not break the surface despite leg tip speeds of approximately 18 cm/s.  Robostrider traveled half a body length per stroke.

Film 1: Water-walking

  Microvelia, characteristic length 1 mm, propels itself by crawling along the free surface.  Its characteristic body speed is 1 cm/s.  Microvelia transfers momentum to the underlying fluid, shown by the backwards movement of the thymol blue tracer and interspersed thymol chunks.  This video is slowed down 10x.

Film 2: Ascension of mensicus, top view

  If startled, Microvelia may propel itself by generating surface tension gradients through the ejection of surfactant (Andersen 1976).  Thus, Microvelia may generate speeds of 6 cm/s for short periods of time.  After several ejections, the insect runs out of chemical fuel, which is debated to be emitted from anal pores or through the rostrum (mouth). This video is slowed down 10x.