DAVID L. HU


            The objective of my doctoral thesis is to elucidate the propulsion mechanism of various surface swimmers through consideration of the fluid motions which they generate.  Specifically we intend to study the propulsion mechanisms of a number of semiaquatic creatures through the application of the methods of experimental fluid mechanics.

Semiaquatic insects are supported by the curvature force generated by their distorting the free surface and generate thrust by four characteristic mechanisms: walking, rowing, ascension of menisci, and ejection of surfactant.  These mechanisms rely on the manipulation of surface tension to generate thrust.  Water walkers generate horizontal components of surface tension with their feet (Bowdan 1978).  The water strider drives its central legs against the surface without breaking through (Andersen 1982).  Meniscus climbers pull on the surface with their feet to generate surface tension forces that draw themselves upward (Baudoin 1955).  The rove beetle propels itself along the surface by the ejection of surfactants that generate surface tension gradients.  We propose to elucidate these mechanisms of semiaquatic locomotion through a theoretical and experimental study of the four surface swimmers: the water strider Gerridae, the meniscus climber Mesovelia, the rove beetle Steninae, and the freshwater snail Physida.

I have to date focused on the propulsion mechanism of the water strider.  In order to drive itself forward, the water strider distorts its menisci so that the curvature force has a non-zero horizontal component.  Newton's third law requires that momentum be transferred to the fluid in an amount equal to that of the strider.  It was previously assumed that the sole means by which to transfer momentum to the fluid, and hence generate thrust, was through capillary waves.  Denny (1993) suggested that the short legs of infant water striders are too slow to generate waves; hence the strider cannot transfer the momentum necessary to move.  According to this physical picture, infant water striders cannot swim: their ability to do so has been referred to as Denny's Paradox (Suter et al 1997). 

A series of laboratory experiments were conducted in order to elucidate the propulsion mechanism of the water strider.  Particle tracking, dye studies, and high-speed videography show that the infant water strider transfers momentum to the fluid through dipolar vortices shed in the bulk by the strider's rowing legs.  Our observations indicate that 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 waves.  We were thus able to circumvent Denny's Paradox by concluding that capillary waves do not play an essential role in the propulsion of Gerridae.

In the future I intend to apply the experimental methods developed in our study of the water strider to elucidate the propulsion mechanism of other surface swimmers.  I have recently been examining the biolocomotion of the freshwater snail, that can propel itself both on hard surfaces and inverted beneath air-fluid interfaces.  On hard surfaces, snails move using undulations of their muscular foot which generate pressure in the underlying lubrication layer.  However, these pressures cannot be sustained on stress-free fluid interfaces; their propulsion mechanism beneath free surfaces remains poorly understood.

We have also designed and constructed biorobots, small self-contained swimming machines.  To date we have built a mechanical water strider, that rows on the water surface without breaking through, and a mechanical snail that propels itself on a layer of glycerol.

The goal of my graduate study is to gain proficiency in the application of mathematics to biolocomotion.  My long-range professional goal is to continue research in an academic setting.