Abstract
A body in motion tends to stay in motion but is often slowed by friction. Here we investigate the friction experienced by centimeter-sized bodies sliding on water. We show that their motion is dominated by skin friction due to the boundary layer that forms in the fluid beneath the body. We develop a simple model that considers the boundary layer as quasi-steady, and is able to capture the experimental behaviour for a range of body sizes, masses, shapes and fluid viscosities. Furthermore, we demonstrate that friction can be reduced by modification of the body’s shape or bottom topography. Our results are significant for understanding natural and artificial bodies moving at the air-water interface, and can inform the design of aerial-aquatic microrobots for environmental exploration and monitoring.
Highlights
Friction is the force that opposes the relative motion of solid bodies and fluids and can present itself in both dry and fluid forms
We found that ridges parallel to the direction of motion significantly reduce the skin friction, with the friction being lower as the spacing between ridges is increased
Our experiments demonstrate that centimetric bodies sliding at an air-water interface are decelerated by skin friction, which is dominant with respect to other forms of hydrodynamic drag
Summary
Friction is the force that opposes the relative motion of solid bodies and fluids and can present itself in both dry and fluid forms. Stokes drag tends to dominate for small Re and is proportional to U, while form drag is significant for larger Re and is proportional to U2. Another component of hydrodynamic resistance is the skin friction due to the viscous boundary layer that forms in the vicinity of the body surface. In the case of unseparated laminar flow on a flat plate (Re 106), skin friction is proportional to U3/2 1 Quantifying these types of friction is of critical importance for the design of boats, robots, and projectiles as well as for understanding the motion of living organisms in water and air. Our findings are relevant for the design of robots[3,17,18,19,20,21,22] and microrobots[23,24] at the water-air interface for environmental monitoring and exploration
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