Laboratoire Matière et Systèmes Complexes

May 3rd 2021. Exceptionally the seminar will start at 11h00.

There will be a live diffusion with Zoom. Contact Michael Berhanu to attend the video seminar.

Vibrations defy gravity and time

Emmanuel Fort, Institut Langevin, ESPCI, PSL University

In this seminar, I will discuss how shaking a liquid can lead to counterintuitive phenomena like making boats float under levitating liquids and creating time mirrors for waves. In a first part, I will focus on how it is possible to perform a time control of wave propagation using vertical vibrations. I will introduce the concept of instantaneous time mirror which make waves relive their previous “life” using time discontinuities and show their experimental implementation. This concept is general and can apply to any type of waves. In a second part, I will discuss how vertical shaking can stabilize upside down liquid interfaces and how vibrations can induce other paradoxical phenomena on immersed or floating objects. Air bubbles can for instance sink in vibrated fluids and an air layer can be created to sustain large liquid layers. Vertical shaking also creates stable buoyancy positions on the lower interface of the liquid, as though the gravitational force were inverted. Bodies can thus float upside down on the lower interface of levitating liquid layers. I will discuss how stabilization is the result of the dynamical averaging effect of the oscillating effective gravity and present experimental observations of inverted buoyancy.

Boats floating on both sides of a levitating liquid layer

Snapshots of waves refocusing on a source in the shape of a smiley using an instantaneous time mirror