Surface tension, Buoyancy and Viscous instabilities in fluids

The presence of a chemical reaction can affect the physical properties of a fluid when the physical property depends on the concentration of the reactants or product. Thus a chemical reaction can induce fluid motion. Dr Trevelyan is interested in surface tension, buoyancy and viscous instabilities in fluids.

Surface tension:

Falling film reactors are used in industry to extract pollutants from a gas or as heat exchanges. When a thin film of liquid falls down a vertical wall, solitary type waves form on the interface. If the wall is hotter than the surrounding air then a perturbation to the temperature field will result in interfacial temperature gradients which can lead to solitary waves forming at the liquid/gas interface due to changes in surface tension – thermocapillary Marangoni instability. The presence of interfacial waves can enhance the efficiency of falling film reactors.

Buoyancy:

Carbon dioxide sequestration is a very active area research. When a heavy fluid overlies a lighter fluid an instability occurs – a “Rayleigh-Taylor” instability, however, several other types of buoyancy instabilities are possible. Suppose a liquid contains two species that diffuse at different rates. If a faster diffusing less dense species over lies a slower diffusing more dense species the evolving density profile becomes non-monotonic and so locally regions exist where a heavy fluid lies above a lighter fluid which induces an instability called “Diffusive-Layered-Convection”. Alternatively, if a slower diffusing less dense species over lies a faster diffusing more dense species then even when the density profile is monotonically increasing in the downwards direction an instability can occur, the so called “Double-Diffusion” instability. The situation becomes more complicated when a reaction takes place, as in the case in CO$_2$ sequestration.

Viscous instabilities:

In the oil industry water is injected into an underground reservoir to displace the oil. When a less viscous fluid is injected into a more viscous fluid in a porous media a “Saffman Taylor” instability occurs. As water is less viscous than oil, the presence of this type of instability makes the extraction process less efficient.