Denny's paradox

In biology, Denny's paradox refers to the apparent impossibility of surface-dwelling animals such as the

If

${\displaystyle c_{m}}$

${\displaystyle c_{m}=\left({\frac {4g\sigma }{\rho }}\right)^{1/4},}$

where ${\displaystyle g}$ is the acceleration due to gravity, ${\displaystyle \sigma }$ is the strength of surface tension, and ${\displaystyle \rho }$ the density of water. For standard conditions, this works out to be about 0.23 m/s. In fact, water striders' legs move at speeds much less than this and, according to this physical picture, cannot move.^[1]

Writing in the Journal of Fluid Mechanics, David Hu and John Bush state that Denny's paradox "rested on two flawed assumptions. First, water striders' motion was assumed to rely on the generation of capillary waves, since the propulsive force was thought to be that associated with wave drag on the driving leg. Second, in order to generate capillary waves, it was assumed that the strider leg speed must exceed the minimum wave speed, ${\displaystyle c_{m}=\left(4g\sigma /\rho \right)^{1/4}\simeq 0.23\mathrm {m/s} }$. We note that this second assumption is strictly true only for steady motions".^[2]

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