Microfluidic nanoprecipitation makes use of hydrodynamic focusing (HF) to accurately control the diffusive mixing of reactants. Both stability and precise handling of flow streams are essential for this application. However, flow stability is hardly attained when fluids are supplied by syringe pumps, due to the unavoidable fluctuations associated to the driving mechanical system. The alternative use of hydrostatic pressure is constantly increasing in microfluidic laboratories, though precise mathematical descriptions have not been reported so far. This paper presents a quantitative model for the HF driven by gravity in slit microchannels. The model analytically predicts the focusing width of the sample stream from the relative heights of the sample and sheath reservoirs. Fluids with different densities and viscosities are considered, which impact on the pressure provided by the hydrostatic columns, as well as on the flow pattern of HF. Theoretical predictions were successfully validated against experimental data. Flow-focusing experiments were carried out in hybrid PMMA/OCA chips with slit microchannels, using fluids with different physicochemical properties. Finally, a color reaction induced by pH-shift was implemented as a practical example. Different levels of diffusive mixing and reaction were attained along the focused stream by varying the relative heights of the fluid columns, precisely as predicted by calculations. The model thus provides a rational basis for the design of HF experiments using hydrostatics.

微流控纳米沉淀利用流体动力学聚焦（HF）来精确控制反应物的扩散混合。流量的稳定性和精确处理对于此应用都是必不可少的。然而，由于与驱动机械系统相关的不可避免的波动，当通过注射泵供应流体时，很难获得流动稳定性。在微流体实验室中，静水压力的替代用途正在不断增加，尽管到目前为止尚未报道精确的数学描述。本文提出了狭缝微通道中由重力驱动的HF的定量模型。该模型从样本和鞘层储层的相对高度分析性地预测样本流的聚焦宽度。考虑了不同密度和粘度的流体，这会影响静水柱提供的压力以及HF的流态。理论预测已成功地针对实验数据进行了验证。在具有狭缝微通道的混合PMMA / OCA芯片中，使用具有不同理化特性的流体，进行了流聚焦实验。最后，以pH变化引起的显色反应为实例。精确地如通过计算所预测的，通过改变流体柱的相对高度，沿着聚焦流获得了不同水平的扩散混合和反应。因此，该模型为使用静水压的HF实验设计提供了合理的基础。理论预测已成功地针对实验数据进行了验证。在具有狭缝微通道的混合PMMA / OCA芯片中，使用具有不同理化特性的流体，进行了流聚焦实验。最后，以pH变化引起的显色反应为实例。精确地如通过计算所预测的，通过改变流体柱的相对高度，沿着聚焦流获得了不同水平的扩散混合和反应。因此，该模型为使用静水压的HF实验设计提供了合理的基础。理论预测已成功地针对实验数据进行了验证。在具有狭缝微通道的混合PMMA / OCA芯片中，使用具有不同理化特性的流体，进行了流聚焦实验。最后，以pH变化引起的显色反应为实例。精确地如通过计算所预测的，通过改变流体柱的相对高度，沿着聚焦流获得了不同水平的扩散混合和反应。因此，该模型为使用静水压的HF实验设计提供了合理的基础。精确地如通过计算所预测的，通过改变流体柱的相对高度，沿着聚焦流获得了不同水平的扩散混合和反应。因此，该模型为使用静水压的HF实验设计提供了合理的基础。精确地如通过计算所预测的，通过改变流体柱的相对高度，沿着聚焦流获得了不同水平的扩散混合和反应。因此，该模型为使用静水压的HF实验设计提供了合理的基础。