Liquid Entry Pressure (LEP or LEPW)

In membrane distillation it is of paramount importance that liquid does not fill the membrane pores, in order to keep membrane efficiency to the highest possible value. To this end hydrophobic membranes are normally used. The hydrophobicity can be quantified through the so-called liquid entry pressure (LEP) parameter, defined as the pressure difference at which liquid penetrates into the membranes pores.

The pressure required to force water through the structure is inversely proportional to the opening size and dependent on the polymeric properties of the membrane. A theoretical expression, based on Young-Laplace equation, enables the estimation of LEP (see, e.g., El Bourawi et al. 2006):

$$ \Delta P=\frac{2\gamma cos\theta}{r} $$

where γ is the liquid surface tension, θ the contact angle between liquid and membrane, and r is the pore radius.

Given the practical difficulty of obtaining the exact value of the above physical parameters, it is not uncommon to estimate LEP experimentally,...