To this point, we have considered flow in channels whose dimension was large relative to the Debye length or the size of any molecules or particles suspended in the flow. When we use channels with shallow (e.g., nanoscale) depths d, we cannot separate the EDL from the bulk fluid by using boundary-layer theory; instead, we must account for the presence of net charge density in the bulk flow field. Even if the double layers remain thin, the perturbative effects of double layers (for example, surface conductance) are more significant as the channel becomes small. Because these phenomena typically coincide with transport through nanoscale channels, the term nanofluidics is often used to refer to flows with small d * or flows with molecules or particles comparable to the size of the channel, though the scale need not be nanoscopic for these phenomena to be important, and these phenomena are unimportant in some nanoscale flows. Despite this, some authors use the term nanofluidics to refer specifically to flows in nanoscale channels with no reference to molecular size or λD. Because our interest is the interplay of electrokinetic effects with channels and molecular-scale confinement, our focus is on channels with molecular scale or of a size comparable to λD, and we pay only cursory attention to the absolute dimension of the channel.

For unidirectional flow in infinitely long, uniform-cross-section channels, thick-EDL effects are observed primarily through changes in the elements of the electrokinetic coupling matrix.