Global and local instability of flow focusing: The influence of the geometry
Global and local instability of flow focusing: The influence of the geometry
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DOI：
10.1063/1.3450321
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发表时间：
2010-06
影响因子：
4.6
通讯作者：
E. J. Vega;J. Montanero;M. Herrada;A. Gañán-Calvo
中科院分区：
文献类型：
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作者：
E. J. Vega;J. Montanero;M. Herrada;A. Gañán-Calvo
在流动聚焦技术中，液体流速Q通过微毛细管注入以形成附着于其边缘的弯月面。弯月面被拉伸，直到由于由压降Δp驱动的气流的作用，细射流从其尖端逐渐变细。液体射流和气体流都以距离H穿过位于毛细管前面的板的孔口。本文对锥形弯月面和射流的稳定性进行了实验分析。确定了三种制度：（i）稳定喷射状态，其中液体弯月面是稳定的并且射流是对流不稳定的;（ii）局部不稳定状态，其中液体弯月面是稳定的并且射流是绝对不稳定的;以及（iii）全局不稳定状态，其中液体弯月面是不稳定的。负责这些制度之间的过渡机制进行了说明。实验表明存在一个流量Q的最小值Qmin，低于该最小值时，流聚焦是全局不稳定的，与施加到气流上的压降Δp无关。考虑到不同的液体的稳定性阈值Qmin相对于毛细管到孔的距离H的依赖性进行了分析。如果其余的几何参数是固定的，则存在毛细管到孔口距离H的最佳值Hopt，对于该最佳值，稳定性阈值Qmin最小。我们还确定了Hopt和相应的最小流速Qopt相对于毛细管直径的依赖性。此外，我们发现，Qmin发散的毛细管孔距离H的减少，并接近一定的临界值，在该临界值，从流动聚焦到“流动模糊”发生的过渡。我们通过对上述三个区域进行数值模拟，证实了我们对实验结果的解释。在流动聚焦技术中，通过微毛细管注入液体流量Q，以形成附着在其边缘的弯月面。弯月面被拉伸，直到由于由压降Δp驱动的气流的作用，细射流从其尖端逐渐变细。液体射流和气体流都以距离H穿过位于毛细管前面的板的孔口。本文对锥形弯月面和射流的稳定性进行了实验分析。确定了三种制度：（i）稳定喷射状态，其中液体弯月面是稳定的并且射流是对流不稳定的;（ii）局部不稳定状态，其中液体弯月面是稳定的并且射流是绝对不稳定的;以及（iii）全局不稳定状态，其中液体弯月面是不稳定的。负责这些制度之间的过渡机制进行了说明。实验结果表明，存在一个最小值Qmin的流量Q低于流动聚焦是全局使用。
In the flow focusing technique, a liquid flow rate Q is injected through a microcapillary to form a meniscus attached to its edge. The meniscus is stretched until a thin jet tapers from its tip due to the action of a gas stream driven by a pressure drop Δp. Both the liquid jet and the gas stream cross the orifice of a plate located in front of the capillary at a distance H. In the present work, the stability of both the tapering liquid meniscus and the emitted jet is analyzed experimentally. Three regimes are identified: (i) the steady jetting regime, where the liquid meniscus is stable and the jet is convectively unstable; (ii) the local instability regime, where the liquid meniscus is stable and the jet is absolutely unstable; and (iii) the global instability regime, where the liquid meniscus is unstable. The mechanisms responsible for the transitions between those regimes are described. The experiments show the existence of a minimum value Qmin of the flow rate Q below which flow focusing is globally unstable independent of the pressure drop Δp applied to the gas stream. The dependence of the stability threshold Qmin with respect to the capillary-to-orifice distance H is analyzed considering different liquids. If the rest of the geometrical parameters are fixed, there is an optimum value Hopt of the capillary-to-orifice distance H for which the stability threshold Qmin is minimum. We also determine the dependence of Hopt and the corresponding minimum flow rate Qopt with respect to the capillary diameter. In addition, we find that Qmin diverges as the capillary-to-orifice distance H decreases and approaches a certain critical value, at which the transition from flow focusing to “flow blurring” takes place. We confirm our interpretation of the experimental results by conducting numerical simulations for the aforementioned three regimes.In the flow focusing technique, a liquid flow rate Q is injected through a microcapillary to form a meniscus attached to its edge. The meniscus is stretched until a thin jet tapers from its tip due to the action of a gas stream driven by a pressure drop Δp. Both the liquid jet and the gas stream cross the orifice of a plate located in front of the capillary at a distance H. In the present work, the stability of both the tapering liquid meniscus and the emitted jet is analyzed experimentally. Three regimes are identified: (i) the steady jetting regime, where the liquid meniscus is stable and the jet is convectively unstable; (ii) the local instability regime, where the liquid meniscus is stable and the jet is absolutely unstable; and (iii) the global instability regime, where the liquid meniscus is unstable. The mechanisms responsible for the transitions between those regimes are described. The experiments show the existence of a minimum value Qmin of the flow rate Q below which flow focusing is globally u...