Department of Bioengineering, Imperial College, London, UK SW7 2AZ.
Phys Chem Chem Phys. 2011 Mar 28;13(12):5298-303. doi: 10.1039/c0cp02810j. Epub 2011 Feb 23.
The behaviour of droplets entering a microfluidic chamber designed to house microelectrode detectors for real time analysis of clinical microdialysate is described. We have designed an analysis chamber to collect the droplets produced by multiphase flows of oil and artificial cerebral spinal fluid. The coalescence chamber creates a constant aqueous environment ideal for the placement of microelectrodes avoiding the contamination of the microelectrode surface by oil. A stream of alternating light and dark coloured droplets were filmed as they passed through the chamber using a high speed camera. Image analysis of these videos shows the colour change evolution at each point along the chamber length. The flow in the chamber was simulated using the general solution for Poiseuille flow in a rectangular chamber. It is shown that on the centre line the velocity profile is very close to parabolic, and an expression is presented for the ratio between this centre line velocity and the mean flow velocity as a function of channel aspect ratio. If this aspect ratio of width/height is 2, the ratio of flow velocities closely matches that of Poiseuille flow in a circular tube, with implications for connections between microfluidic channels and connection tubing. The droplets are well mixed as the surface tension at the interface with the oil dominates the viscous forces. However once the droplet coalesces with the solution held in the chamber, the no-slip condition at the walls allows Poiseuille flow to take over. The meniscus at the back of the droplet continues to mix the droplet and acts as a piston until the meniscus stops moving. We have found that the no-slip conditions at the walls of the chamber, create a banding effect which records the history of previous drops. The optimal position for sensors is to be placed at the plane of droplet coalescence ideally at the centre of the channel, where there is an abrupt concentration change leading to a response time ≪16 ms, the compressed frame rate of the video. Further away from this point the response time and sensitivity decrease due to convective dispersion.
描述了进入微流控室的液滴的行为，该微流控室设计用于容纳用于实时分析临床微透析液的微电极检测器。我们设计了一个分析室来收集由油和人工脑脊髓液多相流产生的液滴。聚结室创造了一个恒定的水相环境，非常适合放置微电极，避免了油对微电极表面的污染。使用高速摄像机拍摄了这些液滴通过腔室时的交替亮暗液滴的流视频。对这些视频的图像分析显示了沿腔室长度的每个点的颜色变化演变。使用矩形腔室中泊肃叶流的一般解模拟腔室中的流动。结果表明，在中心线处速度分布非常接近抛物线，并且提出了中心线速度与平均流速之比作为通道纵横比函数的表达式。如果该宽/高纵横比为 2，则流速比与圆管中泊肃叶流非常匹配，这对微流道与连接管之间的连接具有影响。由于界面处的表面张力主导粘性力，液滴混合良好。但是，一旦液滴与腔室内保持的溶液聚结，壁面的无滑移条件就允许泊肃叶流接管。液滴背面的弯月面继续混合液滴，并充当活塞，直到弯月面停止移动。我们发现腔室壁的无滑移条件会产生带状效应，从而记录先前液滴的历史记录。传感器的最佳位置是放置在液滴聚结的平面上，理想情况下是在通道的中心，在该位置存在突然的浓度变化，导致响应时间≪16ms，即视频的压缩帧速率。离该点越远，响应时间和灵敏度会由于对流扩散而降低。
