Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 57

double layer at solid-liquid interfaces. This transport is caused by an ex-
ternally applied force (typically a pressure gradient) that leads to a tangential
shift of the liquid phase in a capillary cell in relation to the solid under
investigation. The convective ion transport can be detected by direct meas-
urement of the electrical current, designated as streaming current, between
two non-polarizable electrodes (e.g., Ag/AgCl electrodes) using an ampere-
meter of suciently low internal resistance (Figure 3.1). Alternatively, an
electrical potential, the streaming potential, can be measured if an elec-
trometer of suciently high input resistance is connected to the two elec-
trodes. This potential results from the steady-state balance between the
streaming current and a back conduction current that originates from the
electrical conductivity of the liquid phase in the capillary cell and from that
of the analyzed material surface, i.e., the surface conductivity. The latter
results from mobile excess charge carriers located in the electrical double
layer (excess with respect to the concentration of carriers present in bulk
solution far from the interface). 18
A typical set-up for performing streaming current and streaming potential
measurements consists of a rectangular streaming channel formed by
two identical coated sample surfaces of length L 0 and width ' (Figure 3.1).

d n 3 r 4 n g | 5

.

Figure 3.1 Schematic representation of a cell to measure the streaming current/

potential at charged soft polymer films supported by a hard carrier. The
hydrodynamic flow field, v(x), is indicated developing in the channel
under application of a lateral pressure gradient along the y coordinate.
The scheme details the nomenclature used in the theory section further.
For the sake of simplicity, charges located at the carrier surface are not
represented. Typical dimensions of the cell are: L 0 ¼ 20 mm, ' ¼ 10 mm
and H ¼ 5 mmto60mm.
Reprinted from R. Zimmermann, S. S. Dukhin, C. Werner, J. F. L. Duval.
On the use of electrokinetics for unravelling charging and structure
of soft planar polymer films, Curr. Opin. Colloid Interface Sci., 2013, 18,
83-92, Copyright 2013, with permission from Elsevier.