Most lab-on-a-chip systems have a microfluidic channel to study dispersed particles. Optical trapping electrophoresis allows to accurately measure the surface charge of a microparticle, by combining optical tweezers with an AC electric field. The measured amplitude of the oscillatory motion is directly related to the particle charge.
A common issue in microfluidics is electro-osmotic flow. The origin of this flow lies at the finite surface charge of the channel, which attracts free moving ions of the opposite polarity. By applying an electric field, this layer of ions starts moving and it drags the fluid in the bulk along. Therefore, the measured particle movement is a superposition of the electrokinetic motion and the flow. This effect limits the accuracy in aqueous media of all commercially available charge measurement systems.
Figure: A 3D impression of a patterned OTE measurement cell and a corresponding 2D simulation of the flow.
A novel approach to counter electro-osmotic flow is to design the surface of the channel to have an alternating pattern of strips with positive and negative surface charge. Each two neighboring strips direct a flow in the opposite direction, effectively cancelling each other out. As a result, the bulk of the channel is flow free. Preliminary simulation results, as shown in the figure above, reveal that this method can reduce the flow velocity by a factor of 150.
This thesis includes simulating a microfluidic channel, has a technological aspect in manufacturing the designed microfluidic device and has a hands on part in which you will perform your own experiments with the optical trapping electrophoresis setup. Furthermore, this thesis fits in a bigger project, in which the LCP group develops a new technique to study charging mechanisms of particles in water.
The staff of the LCP group will provide you with all the necessary help and know-how, but we also encourage you to take the initiative to come up with your own ideas to tackle the project. Where feasible and within the scope of the project, we'll support you to develop these ideas.