1. Example: Electrokinetic valve

2. Electrokinetic valve
Introduction
This model shows the modeling of electrokinetic flow in micro channels in MEMS, bio chips, and micro laboratories.
The model shows the combination of ready-to-use applications in COMSOL Multiphysics and the possibility of using free equation-based expressions in the edit fields.
The model is solved using the electrokinetic flow application mode in the Chemical Engineering Module and the General Form in COMSOL Multiphysics

3. Geometry and assumptions
Electrokinetic valve – Problem Definition
Geometry and assumptions
Focusing
Model solved in two stages
Electric field and flow profile solved at steady-state in the focusing stage.
The concentration profile is calculated as a function of t for the injection and separation stages.
Supporting electrolyte is assumed, i.e. the concentration of the species does not influence the conductivity of the medium.
Injection

4. Domain Equations, steady-state solution Focusing stage
Electrokinetic valve – Problem Definition
Domain Equations, steady-state solution Focusing stage
Momentum and global mass balance:
Global charge balance for supporting electrolyte:
Mass balance for diluted species:

5. Mass balance for solute:
Electrokinetic valve – Problem Definition
Domain Equations, time-dependent problem Injection and separation stages
Mass balance for solute:
Global charge balance:

6. Boundary Conditions, focusing stage
Electrokinetic valve – Problem Definition
Boundary Conditions, focusing stage
Momentum and global mass balances:
all inlets
outlet
Global charge balance:
all boundaries
Mass balance, solute:
main inlet
vertical inlets
outlet

7. Conditions, time-dependent injection and separation stages
Electrokinetic valve – Problem Definition
Conditions, time-dependent injection and separation stages
buffer inlet
all other boundaries
Initial condition obtained from the steady state solution

8. Electrokinetic valve – Results
Concentration profile in the focusing, injection, and separation stages. Field configuration 1
Cross section plot
Poor detachment gives pulse broadening

9. Electrokinetic valve – Results
Concentration profile in the focusing, injection, and separation stages. Field configuration 2
Cross section plot
Good detachment gives well-defined pulse

10. Electrokinetic valve – Concluding remarks
The model is very simple to define and solve in COMSOL Multiphysics.
The results agree with similar models from literature.
The model is able to predict the field configuration that gives a well defined sample pulse in the sample channel.