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Capillary choreographies at low-Friction interfaces
From Leidenfrost baths to soap films
The motion of millimetre-sized objects trapped at the liquid-air interfaces is a familiar phenomenon, seen in the clumping of cereals in a bowl or of bubbles at the surface of a sparkling drink. The deformation of the interface typically generates attractive forces between the particles, while friction with the underlying liquid strongly damp the motion.
In this talk, we will explore the dynamics of particles placed on two unusual interfaces, on which the friction is drastically reduced.
The first system is an evaporating bath of liquid nitrogen. When deposited on the cryogenic bath, ambient temperature particles are maintained in levitation by the vapour escaping from the interface. The particles spontaneously self-propel, moving in straight lines at a velocity of a few cm/s. We will explain the mechanism underlying this motion, and analyse the orbiting trajectories that appear when two particles interact.
The second system is an horizontal soap film. Here again, friction is extremely weak, while the attractive force now extends over much longer range. We study the origin of the friction, and measure the interaction force by combining two methods : analyzing particle dynamics, and using the magnetic actuation of paramagnetic beads. We show that at large distances (a few centimeters), the attraction results from film-scale deformations due to the weight the beads. At small distances (a few millimeters), the interaction force suddenly increases due to the capillary interaction between the menisci surrounding the particles.