Forced drainageFoam drainage plays an important part in the formation and evolution of liquid foams. A freshly formed foam is not in equilibrium under gravity, and liquid drains out of it until such an equilibrium is attained. This is free drainage. Forced drainage, on the other hand, is the steady flow through an otherwise static foam, which can be produced by continuous addition of liquid at the top. We have observed forced drainage in simple experiments in which liquid is added steadily to the top of a column of soap froth, which has first been allowed to drain freely so that it is close to the dry limit over most of its height. These experiments revealed a striking effect: immediately after liquid is introduced at the top, or the flow rate is increased, a front travels down the tube at a constant velocity, like the bore on certain rivers. Theoretical investigation has shown that this is indeed an example of a solitary wave. The experimental arrangement, with a steady flow rate Q, is illustrated below. The diagram on the right shows the solitary wave, with higher liquid fraction fl behind the wave front. (If the diagram is static, reload the page with the diagram in centre screen to see the motion of the wave front). |
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| Rheology |
