Electro-osmotic Flows In Micro Irregular Channels

At present, with the rapid development of micromachining techniques, Micro-Electro-Mechanical System (MEMS) and microfluidics, the micro equipments were applied in microelectronic mechanical system, MEMS, micro instruments and so on. Among them, electro-osmotic flow (EOF) micro pump has been favored due to simple structure and continuous flow fashion. With the advantage of no mechanical moving parts and smart size, this pump can satisfy the requirement of two-phase flow refrigeration technique and medical devices. Along with the increasing in short of energy, the electro-osmotic flow is closely related with methanol permeation in direct methanol fuel cell which is a new type of cleanly energy source. Understanding deeply the transportation characteristic of electro-osmotic flows in micro irregular channels performs in micro domain is the fundamental of developing these new technology and a new question. It also plays a very important role in improving the performance and understanding the transportation characteristic of fuel cells.In this paper, the models in micro triangle channel and micro irregular channels composing with cylinders arranged as triangle and square with considering the thermal effects or not were set up. A theoretical calculational method was proposed for the electro-osmotic flow in micro channels in this paper. The differential equations were solved numerically using the Galerkin method and the electric potential field and electro-osmotic flow field in micro channels including triangle and irregular channels were determined, respectively. The effects of the electric field strength, the zeta potential, the concentration of electrolyte, the fluid temperature, and the size of the micro channels on electro-osmotic flow behaviors were discussed. We also got the variations of fluid dimensionless mass flux with different parameters in micro channels. The conclusions would be described as follows:1) The electric field strength caused by electric double layer in micro channels exists only nearly by the wall of channels. Where there is nearer the wall, the distribution of charge is much denser and the bulk force of liquid caused by the electro-osmotic flow is much bigger.2) With the effects of bulk force and viscosity force, the velocity of liquid in the middle of where the space between the walls is smaller is bigger. And the velocity of liquid in the middle of where the space between the walls is bigger is smaller.3) The mass flux increases as the zeta potential, the electric field strength, the temperature of liquid, and the concentration of electrolyte. Moreover, we also found that there exists an optimal channel size, at which the mass flux of liquid in microchannels reaches its maximum.4) The mass flux increases with the positive pressure drop, but decreasing with negative pressure drop. When the pressure drop is small which is said that the electro-osmotic flow is much bigger, the mass flux changed very little. When the pressure drop increased to some value which is said that the electro-osmotic flow is much weaker than pressure drop, the mass flux changed as linearity nearly.5) The effect of pressure drop on electro-osmotic flow is much greater when the size of the channel is bigger, but changed little under different zeta potential.6) When the negative pressure drop reached a certain value, the velocity of liquid is positive where the space between the walls is small, but it is negative where the space between the walls is big.7) The temperature and the velocity of liquid increased with Ju number. The mass flux is much bigger with larger Ju number under the condition of small negative pressure drop, but when the negative pressure drop is bigger, the mass flux under bigger Ju number is smaller.8) The mass flux increases with K when the pressure drop is not changed. When the negative pressure drop reach to some value, the mass flux is small with small K number, but when the K number is bigger, the mass flux under bigger K number is much bigger.