The inclusion of the ionic size (steric) effects into the steady and locally developed diffusioosmotic flow inside a uniformly charged slit microchannel through a theoretical analysis is the subject of this study. The results indicate essential quantitative and qualitative distinctions between the steric effects on classical electrokinetic phenomena like electroosmosis and on diffusioosmosis. For example, although the steric effect on electroosmotic flow is always unfavorable, it may have a positive influence on diffusioosmosis and even double the mean velocity under certain conditions. Moreover, finite ionic sizes can even change the flow direction with a tendency to increase the chance of flow toward higher concentrations. Another interesting finding is that, unlike electroosmosis, the steric effects on diffusioosmotic flow do not vanish even when the EDL is very thin. The main quantitative difference between the ionic size effects on diffusioosmosis and the other electrokinetic phenomena is the critical zeta potential above which these effects become important which is here found to be only about several tens of millivolts. The more sensitivity of diffusioosmosis to the steric effects and also the above-mentioned surprisingly different trends are attributed to the induced electric field which may drastically get influenced by the steric factor.
