Three-dimensional convection in a two-layer system with anomalous thermocapillary effect

At temperatures somewhat above room level, the interfacial tension between a 10 cS silicone oil and ethylene glycol increases with temperature, whereas it typically decreases for other systems of immiscible viscous fluids. The convective flows produced by the combined action of this so-called anomalous thermocapillary effect and buoyancy in this particular liquid–liquid system are studied by direct three-dimensional nonlinear simulation. The liquids are situated between rigid horizontal plates that are kept at different temperatures. A pseudospectral code is used to solve the evolution equations with periodic boundary conditions in the horizontal directions. Depending on the Grashof and Marangoni numbers G and M, the motionless state can either have a stationary or oscillatory instability. The corresponding finite amplitude solutions show a variety of regular structures (stationary rolls, stationary hexagons, pulsating hexagons, alternating rolls) as well as spatio-temporal chaos. The properties of the alternating rolls are investigated in some detail. Irregular patterns arising from the transition between hexagons and alternating rolls are briefly discussed.