1、微流体驱动 电渗驱动,姓名：王玲 学号：121630,微米乃至纳米尺度构件中流体的驱动和控制是微电子机械系统（MEMS）中经常要遇到的问题，也是MEMS发展需要解决的关键技术之一。近几年来生物芯片技术的进步和“Lab on a chip”概念的提出更是迫切要求实现微量流体的自动、精确的驱动和控制。,内容概要,微米尺度下流体基本特征,微流体驱动分类,电渗驱动,实例,一、微米尺度下流体基本特征,1.尺度效应,在微尺度下，支配流体的各种作用力的地位发生变化， 原来宏观流动中的主导作用力地位下降，表面力的作用超过体力。同时， 随着器件的特征尺度减小到微米甚至纳米级， 微流动出现许多宏观经典理论无法解释
2、的现象。,2.表面效应,当尺度减小时，微流体器件的表面积/体积比大大增加。这大大影响了质量、动量和能量在微流体器件表面的传输。很明显，表面效应将会在微小器件中起主导作用。表面的微小变化就会对流体的行为产生大的影响。,二、微流体驱动分类,welcome to use these PowerPoint templates, New Content design, 10 years experience,一类是从宏观流体驱动移植过来的驱动方式，主要包括机械压差驱动、外部压力驱动、离心力驱动、电水力（EHD）驱动。,另一类是根据微尺度下流体特征设计的驱动方式。主要包括表面张力驱动、电渗式驱动、热气泡驱
3、动、磁流体驱动。,由于尺度减小，流体的流动特性发生了变化。这种流动特性的变化使得宏观流体驱动与控制技术在微流体中的简单移植往往不成功或者效果不好。,三、电渗驱动,广义地讲，电渗是一种流体相对于带电管壁移动的现象，电渗的产生和偶电层有关。在溶液pH3的条件下，微通道内壁通常带负电（缘于表面电离或吸附），于是在表面附近的液体中形成了一个带正电的偶电层（包括Stern层和扩散层），在平行于内壁的外场作用下，偶电层中的溶剂化阳离子或质子引起微通道内流体朝负极方向运动，这种运动即被称之为电渗。,电渗原理示意图,电渗驱动具有如下优点：,流速大小可由外电场线性调节施加外电场的电极可以集成在芯片上，从而缩小了
4、芯片流体驱动系统的体积各种芯片材料均可诱导电渗流流体前沿为扁平状,电渗驱动存在的问题,电渗流对管壁材料和被驱动流体的物理化学性质敏感，因此它只适用于一定范围的流体和管壁材料。一些有机化合物和溶剂就不能形成双电层，而且表面杂质也会影响双电层的形成。产生电渗流所需要的高压电源会带来安全、功耗和所占空间大的问题，这不利于系统的微小型化。电渗流的实现要求流体在管道中保持连续性，这使得当管道中存在气泡时该驱动方法不再有效。电渗流尽管适于驱动和控制狭窄管道（1ul/s）驱动更宽管道中的流体，而这一能力在许多的微流体应用中是必要的。,四、实例,Fabrication and Characterization
5、 of a FritlessMicrofabricated Electroosmotic Pump withReduced pH Dependence,In this paper, an electroosmotic pump with both anion and cation exchange beads packed in separate channels that pump toward an intersection is presented. Combining the two flow streams results in higher flow rates for the p
6、ump over a wide pH range. This pump can be used to deliver solutions ranging from a pH of at least 2-12.,(a) Schematic diagram of the initial design of the microfluidicelectroosmotic pump. With the polarity indicated, the devicepumps solution from reservoirs 1 and 2 into the field-free channel and r
7、eservoir 3. The pumping channels with packed beads are each 15 mm in length. The direction of flow in the field-free channel can be reversed by reversing the polarity. The magnified area shows the cross section of the weir used to contain the beads in the channel.(b) Relationship between the volumet
8、ric flow rate and applied voltage for microchip EOF pump design in (a). The EOF pump reaches a flow rate of 2L/min at an applied voltage of 3kV at pH 6.8.,Most electroosmotic pumps are limited in the pH range in which they can operate.For microfluidic devices based on a native glass surface, EOF is
9、produced by the deprotonation of acidic surface silanol groups.Therefore, the EOF in glass devices is nearly zero below a pH of 4 and reaches a maximum above a pH of 8 where nearly all of the ionizable silanol groups are deprotonated. In this device, the pH dependence is reduced by the use of two pu
10、mping channels with oppositely charged surfaces. Conventional EOF pumps use either negatively or positively charged surfaces, but the pump reported here reduces the pH dependence by using both negatively and positively charged surfaces.,The pump utilizing anion exchange beads achieves amaximum flow
11、rate below a pH of 6, and the cation exchange beads achieve a maximum flow rate above a pH of 6. It is important to note that, for both pumps, the flow rate nearly goes to zero at high or low pH.,The channel with the cation exchange beads producesmost of the flow at high pH because the poly(aspartic
12、 acid) functional groups are largely deprotonated providing a negatively charged surface at pHs above its pKa. The pumping channel with the anion exchange beads produces flow at low pH becausepolyethyleneimine functional groups are predominantly positively charged at pHs below the pKa of 9.1 of the
13、conjugate acid. Thus,the opposite surface charges ensure that one of the channels is pumping at high or low pH.,The design shown in Figure 1a was found to provide the highest maximum flow rate, but made only minimal improvements in extending the useful pH range. The pH range of this design was limit
14、ed because, at low pH, the channel with the cation exchange beads did not pump and did not provide adequate resistance to hydrodynamic flow. Therefore, most of the flow from the anion exchange beads would go through the cation exchange bed, bypassing the field-free channel. At high pH, the situation
15、 was reversed.,To reduce this leakage problem, an improved design was employed that restricts the flow between the two pumping channels by narrowing the pumping channel to 50 m before its intersection with the fieldfree channel, as shown in Figure 3a. The 50-m channel section provides a much greater
16、 resistance to hydrodynamic flow back through the nonpumping bed and forced most of the solution out of the field-free channel. As shown in Figure 3b, the pump produced flow rates ranging from 0.1 to 1.0 L/min over a pH range of 2-12, with a maximum flow rate of 1 L/min obtained at a pH of 6.8. This
17、 maximum flow rate is less than the flow rate of 2 L/min obtained using the design in Figure 1a at pH 6.8.,Figure3,(a) Schematic diagram of an improved version of the microchip EOF pump shown in Figure 1a. The design shown in Figure 1a did not exhibit improved flow rates over the entire pH range and
18、 was modified to reduce the dependence of the flow rate on the pH.The 50-m constriction of the pumping channels reduces backflow through the nonpumping section at pH extremes by increasing the resistance to hydrodynamic flow into the nonpumping section.(b)Relationship between pH and flow rate at an
19、applied voltage of 3 kV for the microchip design in (a) packed with 5-m anion and cation exchange beads. (c) The design shown in (a) was further improved by the addition of a 1.5-mm section of 0.5-m beads to further prevent leakage into the nonpumping channel, and the relationship between pH and flow rate at an applied voltage of 3 kV is shown. The improved flow rates at both high and low pH produce the reduced pH dependence, compared to conventional designs shown in Figure 2b.,Thank you!,
