Characterization of Pressure-Driven and Electro-Kinetically Driven Flow in a Micro-Fluidic Chip Using Particle Imaging Velocimetry

The flow profiles of pressure-driven and electro-kinetic driven flows were compared for a microfluidic chip. It was found that the pressure-driven flow had a parabolic profile while the electro-kinetic flow had a plug shaped flow profile. The measured velocities were similar to those determined by the Poiseuille flow model and the Helmholtz-Smoltchowski equation. Flow uniformity is very important for control in microfluidic mixers. Parabolic flow profiles lead to inconsistent reactions while the more uniform plug shape flow allow for a more steady reaction across the channel. Previous work had been performed to measure the flow of a solution of fluorescent polystyrene beads in PDMS channels using a laser confocal microscope. This showed that particles easily stuck to the channel making it difficult to measure over time. In addition, bubble formation in the channel made measuring velocities difficult. Current work used a LabSmith Video Synchronized microscope with software to measure the flow rates at different areas of the channel. Solutions of fluorescent polystyrene beads were used to visually observe the flow within a channel under a microscope. Four different channels were used for the pressure-driven flows of varying dimensions and materials. The channel with the best measured profile was also measured under electro-kinetic flow. A LabSmith High Voltage Sequencer was used to apply a voltage across the channel for electro-kinetic measurements. This research confirmed the different flow profiles under pressure-driven and electro-kinetic driven flow. Future work can be done to determine how this effects mixing in the channels

Characterization of Pressure Driven and Electro-Kinetically Driven Flow in a Micro-Fluidic Chip Using Particle Imaging Velocimetry

The goal of this research is to compare electro-kinetic and pressure driven flow rates and velocity profiles (near wall vs. middle) in a microfluidic chip made of PDMS using particle imaging velocimetry (PIV) of an aqueous solution of fluorescent polystyrene (PS) particles using a laser confocal microscope (LCM). Microfluidic channels were fabricated out of PDMS using a SU-8 mold to be 25mm long and 180um by 1000um. Pressure-driven data did not show the expected parabolic profile because of the large width to depth ratio. In addition, data showed a calculated average significantly higher than the projected particle velocity through the channel. Unexpected results were hypothesized to be caused by inaccuracies with the syringe pump or interactions with the tubing. The accuracy of the syringe pump was tested and found to be 20% lower for 0.5uL of flow and 4% lower for 0.75uL of flow, making the data even stranger. However, at the low pump rates the syringe pump was pulsing instead of having a consistent flow. This could have led to the inconsistencies seen. Other issues occurred when measuring electro-kinetic flow. Often flow would switch directions at random intervals, possibly due to some kind of charge build up. In addition, bubbles, possibly introduced into the system when connecting tubing or when moving the channel, were very problematic. Changes in temperature, pressure, surface properties of the channel, and properties of the fluid within the channel can cause bubbles to grow. Research suggested a bubble trapping method for temporary relief of keeping bubbles out of the channel. This research will be continued as my master’s thesis next year