Electro-osmosis refers to the fluid flow that occurs when an electric field is applied to the electrical double layer, an intrinsic property of a fluid-solid pair. This phenomenon has proven to have significant practical importance for microfluidic applications. FLOW-3D enables the modeling of combined pressure- and electro-osmotic-driven flows, with or without a free surface, in one- or two-fluid configurations.
By creating a series of deep slots in a microchannel, and then applying a potential across the channel, fluid flow can be controlled in a MEMS-scale pump. By adjusting the applied potential, the flow rate can be controlled. The video below demonstrates the application of electro-osmosis for use with micro-pumps.
As illustrated in the example above, FLOW-3D accurately represents the streamlines at any point in time for a complex and evolving flow field. The streamlines and particles in this animation show the inefficiencies in this pump design by capturing recirculation zones. These zones reduce efficiency when not all the flow is driven forward effectively.
A micro-pump should be able to raise the system pressure in order to drive the flow. The pump in the simulation is doing precisely this. The flow in the micro-pump is completely electrokinetically driven in that both inlet and outlets are at atmospheric pressure. So, there is no external pressure gradient across the pump. The pressure gradient is created over time due to electro-osmosis inside the pump. The gradient overcomes the drag forces within the channel driving flow.
Velocity Profile Evolution of an Electro-Osmotic Micro Pump
The simulation below shows the evolution of the flow direction velocity between the slots. Velocities drop to zero approaching the slot walls because of a no-slip condition. Between every slot pair, a plug-like velocity is established starting with approximately parabolic profiles at the start of the simulation. Over time, the profiles become constant and linear due to higher flow velocities and mixing of fluid.
At the right end of the plot in the simulation, distorted plug-like structure can be seen due to substantial mixing and trapping of particles in that zone. The circulation and trapping of particles can be seen in the first simulation. Fluid is moving under the influence of electric potential due to zeta-potential of -0.15V at the slot walls. Inlets and outlets are at zero potential. Flow is pulled in towards the center (in the slots area) due to their potential difference and continues to move on and out of the outlet due to inertia.
In the animation above, a non-uniform potential is created on the walls of a microchannel to induce a helical flow inside the channel. FLOW-3D clearly shows the mixing effect as the fluid stretches and folds, indicated by marker particles.