Elie Raphaël (ESPCI Paris Tech, France) studies the physics on complex fluids - from interfaces to exiting capillary-gravity waves, also with granular materials. This talk is titled 'Moving at the air-water interface', and you can find the abstract below. Afterwards, there will be drinks.

If you cannot join in person, you can register and you will get the stream link: https://fd24.formdesk.com/universiteitleiden/form19599

ABSTRACT:
Gravity waves generated by an object moving at constant speed at the water surface form a specific V shape pattern, commonly known as the Kelvin wake. Kelvin found that the angle between the two arms of the V is close to 39° in deep water. In some cases, however, this angle appears to be smaller. We propose an explanation to this phenomenon based on the amplitude of the generated waves. We then consider small object (like insects) moving at the air-water interface. It is generally believed that in order to generate waves, such small perturbations must exceed the minimum wave speed (about 23 cm/s). We show that this result is only valid for a rectilinear uniform motion, an assumption often overlooked in the literature. In the case of a steady circular motion (a situation of particular importance for the study of whirligig beetles), we demonstrate that no such velocity threshold exists and that even at small velocities a finite wave drag is experienced by the object. This wave drag originates from the emission of a spiral-like wave pattern. The results presented should be important for a better understanding of the propulsion of water-walking insects. For example, it would be very interesting to know if whirligig beetles can take advantage of such spirals for echolocation purposes.