Instability in sugar–salt double-diffusive systems

Abstract The International Young Physicists’ Tournament (IYPT) is a worldwide, annual competition for high school students. This paper is adapted from the author’s investigation into problem 8, Sugar and Salt, of the 28th IYPT. When a container with a layer of sugar water placed above a layer of salt water is illuminated, a distinctive fingering pattern may be seen in the projected shadow. This phenomenon is known as double-diffusive convection. Although the theoretical foundations for double-diffusive systems have been established, explicit experimental verifications of the instability condition are limited. This paper aims to qualitatively investigate the dependence of the growth of the fingers on different sugar and salt concentrations, verify the downward flux of sugar, and quantitatively verify the instability condition for the sugar–salt double-diffusive system. This experiment gives students the experience of tackling complicated problems in physical chemistry and fluid dynamics in a laboratory setting, in the spirit of the IYPT.