Investigation of Transport and Mixing in Microchannel Driven by Capillary Pumping：Experiments and Theoretical Analysis

In the present study, a power-free method is explored to perform mixing in a microchannel without any external active mechanisms such as pumps, valves or external energies like electrostatic or magnetic fields. Often a relatively large support is needed for the desired power, thus limiting the capability of system miniaturization and integration. The surface tension is the only mechanism for driving the fluids through the microchannel. The channel of this mixer is designed to have no sidewalls with the liquid being confined to flow between a bottom hydrophilic stripe and a fully covered hydrophobic substrate. It is found from theoretical analysis and experiments data that for a given channel width, the flow rate solely due to capillary pumping can be maximized at an optimal channel height. The flow rate is in the order of nanoliters per second, for example, the flow rate is 0.65 nL s−1 at the optimal channel height 13 μm, given the channel width 100 μm. It is most crucial to this power-free mixing device that two liquid species must be well mixed. For this purpose, asymmetric staggered grooved cavities are optimally arranged on the bottom substrate of the channel, which can generate three-dimensional helical recirculation and let two different liquid species mixing efficiently. In the experiment, the fluid can be achieved fully mixing within 1.3 cm. his device has also been applied to whole blood to analyze the characteristics of blood in a microchannel at different sloping angles. The channel is determined by a bottom hydrophilic stripe on a glass substrate for the purpose of cost effective. It is observed that increasing the sloping angle from (downward flow) to (upward flow) increases the blood flow rate monotonically. The trend of the velocity of blood flow under various sloping angles is totally opposite to that of the DI water. These peculiar behaviors on the micro scale are explained by a dynamic model that establishes the balance among the inertial, surface tension, gravitational, and frictional forces. The frictional force is further related to the effective hematocrit. The model is used to calculate the frictional force from experimental data, and thus the corresponding hematocrit, which is smaller when the blood flows upward. n order to enhance the driven efficiency of this design, a superhydrophobic surface was considered to replace the original Teflon surface. We can find out the optimal fabrication parameters of utilizing induced couple plasma method, which can successfully generate compact silicon grass on the bottom. This structure can sustain DI water on the grass top and keep the contact angle around . And the average velocity is 1.21 times that of the original design from experimental results. To make a thorough investigation, when fluid flowing in the hydrophilic channel, it may contact with Teflon surface on both sides, thus produce friction force. Nevertheless, as for the superhydrophobic surface, it can form stable air cushion to isolate fluid; therefore, it will effectively reduce the friction force from both sides, and improve the driven efficiency.