Levitation, oscillations, and wave propagation in a stratified fluid

TL;DR

This paper describes a simple, accessible experiment demonstrating levitation, oscillations, and wave phenomena in a stratified fluid, illustrating complex atmospheric and stellar physics concepts through hands-on student activities.

Contribution

It introduces an easy-to-implement educational experiment that models stratified fluid dynamics and related phenomena relevant to atmospheric and astrophysical physics.

Findings

Cork can be levitated in stratified water due to density gradients.

Oscillations occur when the cork is displaced from equilibrium, illustrating restoring forces.

The experiment models internal gravity waves and Brunt-Väisälä frequency in a simple setup.

Abstract

We present an engaging levitation experiment that students can perform at home or in a simple laboratory using everyday objects. A cork, modified to be slightly denser than water, is placed in a jug containing tap water and coarse kitchen salt delivered at the bottom without stirring. The salt gradually diffuses and determines a stable density stratification of water, the bottom layers being denser than the top ones. During the dissolution of salt, the cork slowly rises at an increasing height, where at any instant its density is balanced by that of the surrounding water. If the cork is gently pushed off its temporary equilibrium position, it experiences a restoring force and starts to oscillate. Students can perform many different measurements of the phenomena involved and tackle non-trivial physical issues related to the behaviour of a macroscopic body immersed in a stratified fluid. Despite its simplicity, this experiment allows to introduce various theoretical concepts of relevance for the physics of the atmosphere and stars and offers students the opportunity of getting acquainted with a simple system that can serve as a model to understand complex phenomena such as oscillations at the Brunt-V\"{a}is\"{a}l\"{a} frequency and the propagation of internal gravity waves in a stratified medium.