Drainage of foam by gravity

Notice that the film thickness is a function of time and the equation only provides an estimate. It can be used to analyse how we can influence the rate of drainage. High viscosity (rji) liquids reduce draining and provide a cushion effect, which can absorb shock energy. 

The main foam destabilization mechanism in freshly poured beer is vertical draining by gravity. The height of the fresh beer foam is measured to 5 cm and can be considered constant. It can be assumed that 25% of the cross-sectional area of the foam is used for vertical draining, in this initial phase, and that the foam structure remains constant and homogeneous. 

If the initial lamella (or film) thickness in the foam is cmdely assumed to be 100 pm (and constant), how long does it approximately take to drain the beer foam (vertically) from 10% of its hquid volume when the gas volume fraction of the foam is 75% Comment on the result. 

A gravity draining time for 10% of liquid volume in the foam must be calculated. The average vertical drainage rate is given by Equation 13.5  

The volumetric drainage rate (in s can be taken as constant because the lamella thickness is constant, leading to  

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