Simulations reveal flow-induced compaction in membranes

Reverse osmosis membranes operate at high pressures across narrow thicknesses. In some experiments, membranes have been deformed at pressures of 150 bar, which is marked by a drop in water flux. In this work, we calculate the bulk modulus of a swollen charged polysulfone membrane in water by ``squeezing the sponge'' - we deform the membrane by applying external potentials. We record the resultant stress versus strain and measure the bulk modulus of the membrane, which is in good agreement with experiments.

We investigate how membranes deform at pressures as small as 0.1% of their bulk modulus. To simulate flow through a membrane, we push water at a constant force per water molecule through a membrane supported by a backstop. We observe that compression of membranes is localized near the support. We attribute the localized deformation to the bottom layer carrying the stress of all the upper layers during flow. Our simulations shed light on the operable limits of soft membranes at high pressures.