Hydrodynamic Flow Effects in Interfacial Interactions

The discussions so far have concentrated on equilibrium interactions between atoms, molecules, particles, and surfaces with little attention being paid to possible kinetic effects. In vacuum, one may assume that such effects will be negligible or absent until distances of separation approach molecular dimension. At that level effects may be seen that reflect any or all of the following processes  

The kinetics of ionic association or dissociation at the interface. The rate of ionization or ion pairing of surface charges will retard shifts to the equilibrium electrostatic interactions. 

The slow movement or displacement of adsorbed species such as polymers. Changes in the configuration of adsorbed species (i.e., polymers or surfactants) may be slow relative to the velocity of approach of the two surfaces, or desorption rates (especially for polymers) will be much smaller. 

The periodic packing effects discussed above come into play. The removal of the last intervening molecular layers of solvent will be more difficult due to the energy advantage gained by the periodic packing. 

Other interesting effects that may occur to the perplexed investigator looking for an explanation for unexpected results. 

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