4(%3)3 - Ecole nationale supérieure de chimie de Montpellier

Figure – 2.21: Evolution of healing rate with respect to perforation healing% at different pressures.Lastly, considering the perforation to be a perfectly circular in nature and healing phenomenontaking place evenly from all direction, the healing rate of the membrane could also calculated fromresistance values (Figure – 2.21). The correlation between the healing rate and healing % reveals thatrate of healing decreases rapidly with the increase in pressure. At 20% healing, a maximum healing rateof 12 µm 2 .sec -1 was observed at 0.8 bar although it decreased rapidly to 2 µm 2 /sec once 70 % healingwas achieved.To complement the self-healing calculations based on flow resistance, Scanning ElectronMicroscopy (SEM) images were obtained for unhealed and healed membranes (Figure – 2.22a, b, c andd). Due to the piercing by the needle, the perforation is caused in such a manner that the brokenfragments are aligned in the downward direction (Figure – 2.22a). From images of partially healed andfully healed membranes at 0.07 and 0.8 bar respectively, it can be seen that the fragments coming backonto the perforation plane and joining together to close the perforation (Figure – 2.22b, c). No healingwas observed for pressures higher than 1.5 bar (Figure – 2.22d) which is in good agreement with waterflow resistance recovery measurements.67