Aota Arata, Hibara Akihide, Kitamori Takehiko
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan.
Anal Chem. 2007 May 15;79(10):3919-24. doi: 10.1021/ac070031d. Epub 2007 Apr 18.
An interfacial pressure balance model was proposed and verified for the elucidation of the physical mechanism of micro countercurrent flow in a hydrophilic-hydrophobic selective-modification microchannel. We considered the conditions of the microflow phase separation, where the phase separation entails a single phase flow in each output of the microchannel. In this pressure balance model, the pressure difference between the two phases due to pressure loss in each phase is balanced by the Laplace pressure generated by the interfacial tension at the liquid-liquid interface between the separated phases. When the pressure difference between the two phases is sufficiently low, the contact line between the two phases is pinned at the boundary between the hydrophilic and the hydrophobic surfaces. Since the contact angle is restricted to values between the advancing and receding contact angles, the Laplace pressure has a limit. When the pressure difference between the two phases exceeds the limiting Laplace pressure, one of the phases leaks into the output channel of the other phase, and the phase separation fails. In order to experimentally verify this physical picture, a microchip with an asymmetric cross section, whose hydraulic diameters were 19 and 102 mum, was used. In the microchip, a phase separation of a water-toluene micro countercurrent flow was achieved under pressure differences between an upper limit of 6.9 kPa and a lower limit of -9.3 kPa. The upper limit agreed well with the proposed model. The model is also applicable to cocurrent flows, so that it is useful for general multiphase microflows in continuous-flow chemical processing.
为阐明亲水性-疏水性选择性改性微通道中微逆流的物理机制，提出并验证了一种界面压力平衡模型。我们考虑了微流相分离的条件，其中相分离需要微通道的每个输出端为单相流。在这个压力平衡模型中，由于各相压力损失导致的两相之间的压力差，由分离相之间液-液界面处的界面张力产生的拉普拉斯压力平衡。当两相之间的压力差足够低时，两相之间的接触线固定在亲水表面和疏水表面之间的边界处。由于接触角限制在前进接触角和后退接触角之间的值，拉普拉斯压力有一个极限。当两相之间的压力差超过极限拉普拉斯压力时，其中一相泄漏到另一相的输出通道中，相分离失败。为了通过实验验证这一物理图像，使用了一个横截面不对称的微芯片，其水力直径分别为19和102μm。在该微芯片中，水-甲苯微逆流在6.9 kPa上限和-9.3 kPa下限之间的压力差下实现了相分离。上限与所提出的模型吻合良好。该模型也适用于并流，因此对连续流化学处理中的一般多相微流很有用。
