Coalescence liquid saturation state

As the liquid content is increased, the pendular state will remain until the pendular bonds start to coalesce and liquid bridges form between non-touching points. This is known as the funicular state (Figure 11.20). The state depends on the degree of liquid saturation or voidage saturation, which is defined as the ratio of the liquid volume to the total volume of pores in the powder bed. Typically, a powder bed will remain in the pendular state until the liquid saturation reaches around 25 percent, and in the funicular state between 25 and 80 percent. 

It is readily argued that the intersection of coalescence and generation rates in Figure 11 leads to a stable steady state. If the system is perturbed away from this point, it naturally returns. Consider a small, positive perturbation in the local liquid saturation. The coalescence rate then declines, and the foam texture becomes finer. This change causes an increased flow resistance that returns the liquid saturation back to the stable operating point. The converse negative saturation perturbation is similarly argued to be stable. 

It has been established (P8, R5) that when the value of S exceeds about 0.25, the liquid bridges begin to coalesce with one another and the bonding mechanism changes over from the pendular to the funicular state. When S exceeds 0.8, the existence of discrete liquid bridges is no longer possible and now the capillary pressure state alone exists. Thus, the funicular state lies in a range of saturation bounded by the lower and upper critical limits denoted by Sp and Sc, respectively. 

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