The hydrodynamics of water-walking insects and spiders

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 142-152).We present a combined experimental and theoretical investigation of the numerous hydrodynamic propulsion mechanisms employed by water-walking arthropods (insects and spiders). In our experimental study, high speed cinematography and flow visualization techniques are used to determine the form of the flows generated by water-walkers. In our supporting theoretical study we provide a formal fluid mechanical description of their locomotion. We focus on the most common means of walking on water such as the alternating tripod gait, rowing, galloping and leaping. We also examine quasi-static modes of propulsion in which the insect's legs are kept stationary: specifically, Marangoni propulsion and meniscus-climbing. Special attention is given to rationalizing the propulsion mechanisms of water-walking insects through consideration of the transfer of forces, momentum and energy between the creature and its environment.by David Lite Hu.Ph.D

Patterns of load distribution among the legs in small water striders during standing and striding

Water striders (Gerris argentatus) move across the water surface by taking advantage of the surface tension, which supports their bodyweight without breaking. During locomotion, the midlegs are primarily responsible for generating thrust, whereas the other legs support the body. Although the aspects of standing and locomotion on the water surface are well understood, relatively fewer studies concerned the coordinated biomechanical movements of the legs. In order to maintain buoyancy of the body on the water surface, the leg positions must be adjusted to distribute the bodyweight appropriately. The present study investigates distribution of the bodyweight on the legs in relatively small water striders. We aimed to understand how loading on the legs changes during sculling that leads to sliding of the body on the water surface. The assistance of all legs at every moment enables the body to maintain its floating during standing and striding. Water striders can achieve a gentle striding through the midlegs driving phase in association with smooth load shifting among their legs, which are positioned in a specific configuration to support the insect on the water surface

Walking on water: Biolocomotion at the interface

Abstract We consider the hydrodynamics of creatures capable of sustaining themselves on the water surface by means other than flotation. Particular attention is given to classifying water walkers according to their principal means of weight support and lateral propulsion. The various propulsion mechanisms are rationalized through consideration of energetics, hydrodynamic forces applied, or momentum transferred by the driving stroke. We review previous research in this area and suggest directions for future work. Special attention is given to introductory discussions of problems not previously treated in the fluid mechanics literature, with hopes of attracting physicists, applied mathematicians, and engineers to this relatively unexplored area of fluid mechanics

Marangoni Propulsion of Active Particles

We study the surfing motion of active particles located at a flat liquid-gas interface. The particles create and maintain a surface tension gradient by asymmetrically discharging a surface tension-reducing agent. We employ theory and numerical simulation to investigate the Marangoni propulsion of these active surfers. First, we use the reciprocal theorem to establish a relationship between the propulsion speed and the release of the active chemical. This theoretical relation is utilized to examine the effect of wall confinement and geometry on the Marangoni-driven motion of active particle when the inertial effects are negligible and when the transports of the released agent is dominated by diffusion. Contrary to what might be the usual expectation, we find that the surfers may propel in the lower surface tension direction depending on their geometry and proximity to the bottom of the liquid layer. We then extend our theory beyond the Stokes regime with the aid of the perturbation theory and calculate the leading-order corrections to the propulsion speed due to the advective transport of momentum and mass when (Re, Pe) (denoted by Re and Pe, respectively) are small, but finite. Next, we develop a computational framework that enables us to study the effects of intermediate and large Re and Pe on the propulsion speed. Our numerical approach is validated against theory and available experimental data. Interestingly, our simulations reveal that the normalized propulsion speed initially increases with increasing Re and Pe from zero. It then reaches a maximum and afterward sharply declines when Re or Pe becomes large. That there exist certain intermediate (Re, Pe) at which the Marangoni propulsion reaches a peak is a new discovery that can guide engineering to design Marangoni surfers with superior performance. We also numerically analyze the translational stability of Marangoni surfers of spherical shape. An overset-grid is adopted to carry out the simulations. We demonstrate that a Marangoni surfer can retain its stability at higher Reynolds numbers relative to the same surfer moving at an interface with no Marangoni effect present. Lastly, we computationally investigate the change in the mobility of the surfers as a result of the depth of the liquid layer. We consider the motion of thin cylindrical disks and oblate spheroids for a wide range of release rates and diffusivity of the exuded chemical species, that control the effective (Re, Pe). We show that indeed the surfers can undergo a forward, a backward, or an arrested motion. We also identify the links between these modes of mobility and the forces acting on the surfers as well as the flow structure in their vicinity. Rather unexpectedly, we discover that negative pressure is the primary contributor to the fluid force experienced by the surfer and that this suction force is mainly responsible for the reverse Marangoni propulsion. Overall, our findings substantially improve the current understanding of the Marangoni-driven motion of active particles at liquid-gas interfaces and pave the way for engineering future miniature surfing robots