In Situ Visualization of Concentration Polarization during Membrane Ultrafiltration Using Microscopic Laser-Induced Fluorescence

A novel noninvasive techniquemicroscopic laser-induced fluorescence (micro-LIF)has been applied to achieve in situ visualization of concentration polarization (CP) of nanoparticles during cross-flow ultrafiltration at high resolutions. The reversible, highly dynamic nature of CP and its sensitive response to the filtration conditions were investigated and validated by direct visualization of the CP layer and the well depicted concentration profile near the membrane surface. Using micro-LIF, the formation of a CP layer during filtration and its back-diffusion after the filtration ceased can be directly observed. The dynamic variation of the CP layer with the cross-flow velocity and transmembrane pressure (TMP) change has also been demonstrated. The results showed that CP reached the steady state approximately 1 min after the filtration condition change. A higher cross-flow velocity and/or a lower TMP decrease the CP concentration and thickness. Further quantitative analysis of the filtration test results using the film theory model helps to obtain the particle concentration at the membrane surface and the thickness of the CP layer (30–50 μm). Accordingly, the nature of CP dynamics was characterized and the deficiency of the traditional CP model was explored.