Electrokinetic properties and morphology of PDMS microfluidic chips intended for bioassays are studied. The chips are fabricated by a casting method followed by polymerization bonding. Microchannels are coated with 1% solution of bovine serum albumin ͑BSA͒ in Tris buffer. Albumin passively adsorbs on the PDMS surface. Electrokinetic characteristics ͑electro-osmotic velocity, electro-osmotic mobility, and zeta potential͒ of the coated PDMS channels are experimentally determined as functions of the electric field strength and the characteristic electrolyte concentration. Atomic force microscopy ͑AFM͒ analysis of the surface reveals a "peak and ridge" structure of the protein layer and an imperfect substrate coating. On the basis of the AFM observation, several topologies of the BSA-PDMS surface are proposed. A nonslip mathematical model of the electro-osmotic flow is then numerically analyzed. It is found that the electrokinetic characteristics computed for a channel with the homogeneous distribution of a fixed electric charge do not fit the experimental data. Heterogeneous distribution of the fixed electric charge and the surface roughness is thus taken into account. When a flat PDMS surface with electric charge heterogeneities is considered, the numerical results are in very good agreement with our experimental data. An optimization analysis finally allowed the determination of the surface concentration of the electric charge and the degree of the PDMS surface coating. The obtained findings can be important for correct prediction and possibly for robust control of behavior of electrically driven PDMS microfluidic chips. The proposed method of the electro-osmotic flow analysis at surfaces with a heterogeneous distribution of the surface electric charge can also be exploited in the interpretation of experimental studies dealing with protein-solid phase interactions or substrate coatings.

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Potential and current density distributions were modelled and measured for an electrochemical cell with a bipolar electrode. The dimension of the bipolar electrode in the direction of current flow was extended, to enable experimental determination of the electrode potential and the local current densities at various positions inside the electrolyte and in the electrode body. The experimental results showed that the most active regions of the bipolar electrode are located at the ends of the bipolar electrode facing the terminal electrodes. The equations corresponding to the mathematical model of the experimental cell were solved using the finite volume method and gave very good qualitative agreement with the experimental data. However, some discrepancies between model predictions and experimental data were evident in the active parts of the bipolar electrode and in the variation of the terminal voltage with the total current. This was explained in terms of the active electrolyte cross-section and the electrode surface area being diminished due to the presence of gas bubbles in the system.Keywords Bipolar electrode Á Mathematical modelling Á Parasitic current Á Local potential and current density distribution Nomenclature A F Cross-sectional area of the free electrolyte space beside the bipolar electrode (m 2 ) d G Inter-electrode distance (m) d E Bipolar electrode thickness (m) E Electrode potential (V) f E Current utilisation in a bipolar cell (-) I Current (A) I E Current flowing through bipolar electrode (A) I P Parasitic current (A) I T Total current (A) j Current density (A m -2 ) n Vector normal to the boundary (m) N Number of electrolytic cells (-) r Radius (m) RResistivity (X) R F Electrolyte resistivity in the fictitious electrolyser (X) R G Average resistivity of the inter-electrode gap (X) R S Short-circuit resistivity (X) S Electrode surface (m 2 ) U Voltage (V) U r Open-circuit cell voltage (V)xPosition along the cell (m)Greek letters u Galvani potential (V) r Conductivity (S m -1 ) j F Electrolyte conductivity (S m -1 ) g Overpotential (V)