Department of Chemistry and Institute for Biophysical Dynamics, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States.
Langmuir. 2012 Jan 24;28(3):1931-41. doi: 10.1021/la204399m. Epub 2012 Jan 10.
This Article describes the use of capillary pressure to initiate and control the rate of spontaneous liquid-liquid flow through microfluidic channels. In contrast to flow driven by external pressure, flow driven by capillary pressure is dominated by interfacial phenomena and is exquisitely sensitive to the chemical composition and geometry of the fluids and channels. A stepwise change in capillary force was initiated on a hydrophobic SlipChip by slipping a shallow channel containing an aqueous droplet into contact with a slightly deeper channel filled with immiscible oil. This action induced spontaneous flow of the droplet into the deeper channel. A model predicting the rate of spontaneous flow was developed on the basis of the balance of net capillary force with viscous flow resistance, using as inputs the liquid-liquid surface tension, the advancing and receding contact angles at the three-phase aqueous-oil-surface contact line, and the geometry of the devices. The impact of contact angle hysteresis, the presence or absence of a lubricating oil layer, and adsorption of surface-active compounds at liquid-liquid or liquid-solid interfaces were quantified. Two regimes of flow spanning a 10(4)-fold range of flow rates were obtained and modeled quantitatively, with faster (mm/s) flow obtained when oil could escape through connected channels as it was displaced by flowing aqueous solution, and slower (micrometer/s) flow obtained when oil escape was mostly restricted to a micrometer-scale gap between the plates of the SlipChip ("dead-end flow"). Rupture of the lubricating oil layer (reminiscent of a Cassie-Wenzel transition) was proposed as a cause of discrepancy between the model and the experiment. Both dilute salt solutions and complex biological solutions such as human blood plasma could be flowed using this approach. We anticipate that flow driven by capillary pressure will be useful for the design and operation of flow in microfluidic applications that do not require external power, valves, or pumps, including on SlipChip and other droplet- or plug-based microfluidic devices. In addition, this approach may be used as a sensitive method of evaluating interfacial tension, contact angles, and wetting phenomena on chip.
本文描述了使用毛细压力引发并控制通过微流道的自发液-液流动速率。与由外部压力驱动的流动不同，由毛细压力驱动的流动主要由界面现象控制，并且对流体和通道的化学组成和几何形状极其敏感。在疏水 SlipChip 上，通过将含有水相液滴的浅通道滑入与充满不混溶油的稍深通道接触，来引发毛细力的逐步变化。该动作诱导液滴自发流入较深的通道。基于毛细净力与粘性流动阻力之间的平衡，使用液-液表面张力、三相水-油-表面接触线处的前进和后退接触角以及器件的几何形状，开发了预测自发流动速率的模型。接触角滞后、是否存在润滑层以及表面活性化合物在液-液或液-固界面的吸附的影响被量化。跨越 10(4)倍流速范围的两种流动状态得到了定量模拟，当油可以通过流动的水溶液逸出而取代时，获得较快的（mm/s）流速，而当油逸出主要受到 SlipChip 板之间的微尺度间隙限制时，获得较慢的（μm/s）流速（“死端流动”）。提出了润滑层破裂（类似于 Cassie-Wenzel 转变）作为模型与实验之间差异的原因。可以使用这种方法来流动稀释盐溶液和复杂的生物溶液，如人血浆。我们预计，由毛细压力驱动的流动将有助于设计和操作不需要外部电源、阀门或泵的微流控应用中的流动，包括在 SlipChip 和其他基于液滴或塞子的微流控器件上。此外，这种方法可用于评估芯片上的界面张力、接触角和润湿现象的敏感方法。
