Study Of Capillary Burst Valve Used For CD-like Microfluidic Platforms

The CD （compack disk）-like microfluidic system is a kind of simple and practical microfabricated platform. It would be a valuable tool in biochemistry analysis applied in point-of-care diagnostic and pharmaceutical synthesis, etc. This dissertation focuses the implement and performances of the capillary-diriven liquid flow control in the CD-like microfluidic system with different wetting walls.The capillary burst valve （CBV） is used to stop the liquid flow based on the contact angle hysteresis. The liquid meniscus and the Laplace pressure are affected greatly by the wetting properties of the microchannels and the flat cover of the planar CBVs. The burst pressures of the valves are investigated by the simulation using the Surface Evolver algorithm. For the planar CBVs, the burst condition and the level set constraints and their energies are provided and the necessary steps are given to improve the computational stability.Considering three configurations of CBVs:（1） the hydrophilic cover and the hydrophobic channel; （2） the hydrophobic cover and the hydrophilic channel; （3） the hydrophilic cover and the hydrophilic channel. For the first configuration, e.g. the CBV with a PDMS microchannel and a glass cover, the experimental and simulation results show that the contact line flows past the entrance of the expansive section in side walls. The appropriate boundary conditions should be set up to complete the simulations. The rotating experiment is presented and the simulated critical rotating speeds have a great agreement with the experimental results. For the second configuration, e.g. the CBV with a Si microchannel and a PDMS cover, the simulation results show that most of the contact lines are stopped at the entrance of expansive section before the CBV bursts. The restriction or relaxation of the contact line has a little effect of the burst pressure. And the SE results are agreement with the publicized experimental data and the simulations of the Young-Laplace equation. For the third configuration, e.g. the CBV with a Si microchannel and a glass cover, the available condition of the planar CBVs is studied. And when the sum of the contact angles of both the side wall and the flat cover are smaller than 90°, the CBV can’t stop the flow.The through hole CBV is a solution for the small contact angle condition where the planar CBVs can’t work. The requirement of the through hole CBV with an expansive angle is analyzed. The 1D hydrodynamics equation with the constant corner pressure model is used to calculate the advancing contact angle. The experimental results show the simulations are valid when the coming channel’s length is larger than 4mm. The range of the allowable expansive angle for the CBV is investigated. A converging through hole CBV is provided, which has better flow stopping performance and the lower fabricated difficulty. The CBV can stop the liquid flow when the flow speed is smaller than 0.5m/s (Ca≤4.0×10^-3) and the hole depth can be as short as to 20μm and the converging angle is not limited.Finally, a CD-like microfluidic platform that was fabricated with the PDMS and glass was provided to implement the plasma preparation. The platform stops the liquid flow using the CBV, and transforms the sample liquid using the centrigual force, capillary force and siphon.