Many wave phenomena seem to show up across very different types of waves. Here, we consider a water wave analogue to so-called optical tweezers used to trap microparticles in focused laser light.
The photograph above captures converging circular waves emitted by a vibrating arc-shaped wavemaker (frequency 20 Hz). A submerged disk-shaped body (diameter 3 cm) tends to drift to the focal point and thereafter stay put, a demonstration of hydrodynamic trapping or tweezing.
Some insight comes from the simpler case of a triangular body in plane traveling waves. As shown below, the body acts as a prism that bends and refracts the waves passing over it. By virtue of the momentum flux in water waves and Newton’s laws, the redirection of waves induces forces that cause the body to move with its apex leading. Refraction and the associated forces are also responsible for the trapping effect, both in our system and in optical tweezers.
The soft, blue-white tones and moonlit quality of these images is due to lighting with a Xenon lamp stroboscope that is synchronized to the waves. The lamp is shone on a screen above the wave tank, and long-exposure photographs capture light reflected from the surface over hundreds of flashes.