Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 61

where K B is the conductivity of the bulk electrolyte. In eqn (3.5), the ratio (I str /
DP)/(U str /DP) represents the channel conductance that can be alternatively
determined by applying a lateral electric field across the cell in the absence
of flow and measuring the corresponding current. 29 If the electrokinetic or
impedance measurements are performed at varying channel heights, K s
corresponds to the intercept in the plot of the channel conductance versus
the channel height. 28
The surface conductivity, K s , consists of a migration component, K s m ,re-
sulting from the ion transport in the tangential field, U str /L 0 , along the
interface:

d n 3 r 4 n g | 5

1 = 2

Z

X

N

K m ¼ HF 2
RT

dX

j z i j D i c i b i (X)e z i y(X) 1

(3 : 6)

i ¼ 1

0

and a convective contribution, K eo , due to the electro-osmotic charge
transport originating from the action of the field U str /L 0 on mobile ions lo-
cated within and outside the soft layer: 31

1 = 2

Z

V eo (X) X K eo ¼ He o e r RT(kH) 2

N

z i c i e z i y(X) dX

(3 : 7)

Z

i ¼ 1

0

where D i is the diffusion coecient of ion i in the bulk electrolyte solution,
b i (X) is the ratio between the mobilities of ion i at position X and in the bulk
electrolyte solution, and e 0 e r is the dielectric permittivity of the medium. For
soft films with suciently high water content, it is justified to assume
b i (X)

.

1. 29 The electro-osmotic flow field, V eo (X), in eqn (3.7) is determined
by the segment distribution at the interface and can be calculated applying
the Brinkman equation [eqn (3.1)] after replacing the right hand side with a
charge source term and making the substitution V(X)

B

V eo (X). 31 The overall
surface conductivity, K s , is then given by the sum K s m þ K eo . Simulations of
electro-hydrodynamics

-

at diffuse soft interfaces are explained in

Appendix A.2.

3.3 Electrokinetic Analysis to Unravel Electrosurface
Phenomena at Biointerfaces

3.3.1 Segment Distribution of Stimuli-responsive Soft
Thin Films
Stimuli-responsive coatings on the basis of thermo-, light- or pH-sensitive
polymers have gained widespread interest in numerous biomedical appli-
cations, because the properties of these 'smart' materials can be tuned by an
external trigger. 38-41 Here we present an example that demonstrates how
streaming current, surface conductivity, and swelling measurements over a
broad range of pH and salt concentrations (pH 2.5-10, 0.1-10 mM KCl,