Stabilisation of adverse density gradients in binary mixtures via transverse confinement: comparison of theory, numerics and experiment

We investigate the effect of the transverse aspect ratio (

) of a thermogravitational column (TGC) on the stability of a binary fluid mixture with a negative separation ratio (

psi ψ $\psi$

), which generates an adverse vertical density gradient. In extended systems, this configuration is unstable and limits separation. Here we show that confinement fundamentally alters this behaviour and can fully suppress the instability. Combining linear stability analysis of the three-dimensional basic state, three-dimensional simulations and experiments, we identify

upper B B $B$

as the key control parameter. Linear stability analysis yields a correlation for the critical aspect ratio

upper B Subscript italic c Baseline left parenthesis psi right parenthesis B c ( ψ ) $B_{\textit{c}}(\psi )$

separating unstable and stable regimes. For

upper B less than upper B Subscript italic c B < B c $B \lt B_{\textit{c}}$

, the system remains in a quiescent state despite the presence of the adverse density stratification. Energy budget analysis shows that confinement enhances transverse diffusive transport, preventing the growth of long-wave instabilities that dominate in extended systems. Direct numerical simulations confirm the predicted transition and reveal the nonlinear evolution towards convection when

upper B greater than upper B Subscript italic c B > B c $B \gt B_{\textit{c}}$

. Experiments performed with different mixtures, Tol

vertical bar | $|$

Ch 0.50

vertical bar | $|$

0.50, Tol

vertical bar | $|$

EtOH 0.80

vertical bar | $|$

0.20 and Tol

vertical bar | $|$

MeOH 0.8420

vertical bar | $|$

0.1520, and geometries using two independent

mu μ $\mu$

TGCs (

upper B B $B$

= 5.88 and

upper B B $B$

= 1.69) validate the theoretical predictions. These results demonstrate that transverse confinement provides a robust and tunable mechanism for controlling thermogravitational stability.