Abstract
In a flow-focusing microdroplet generator, by changing the flow rates of the two immiscible fluids, production speed can be increased from tens to thousands of droplets per second. However, because of the nonlinearity of the flow-focusing microdroplet generator, the production speed of droplets is difficult to quantitatively study for the typical flow-focusing geometry. In this paper, we demonstrate an efficient method that can precisely predict the droplet production speed for a wide range of fluid flow rates. While monodisperse droplets are formed in the flow-focusing microchannel, droplet spacing as a function of time was measured experimentally. We discovered that droplet spacing changes periodically with time during each process of droplet generation. By comparing the frequency of droplet spacing fluctuations with the droplet production speed, precise predictions of droplet production speed can be obtained for different flow conditions in the flow-focusing microdroplet generator.
Highlights
The design of microdroplet generators requires that a wide range of droplet production speeds can be tuned for different experiments
While some new methods of droplet formation have been provided for monodisperse droplet production [24,25,26], to the best of our knowledge, the nonlinear relationship between droplet production speed and fluid flow rate is mainly qualitatively discussed in experiments of droplet generation in the flow-focusing microchannel [27]
Since all of the experiments of monodisperse droplet production were conducted at low capillary numbers (Ca ≤ 0.1), this nonlinear relationship between droplet production speed and flow-rate ratio was mainly determined by the geometrical parameters of the flow-focusing microdroplet generator, and the influence of fluid viscosity on this nonlinear relationship can be neglected [19]
Summary
The design of microdroplet generators requires that a wide range of droplet production speeds can be tuned for different experiments. An experimental method that can achieve high accuracy for droplet production speed prediction is quite meaningful for improving the applications of droplet microfluidic systems [10,11,12]. For high capillary numbers, the production speed of droplets varies with the viscosity of fluids for a specific flow-rate ratio [19,20]. While some new methods of droplet formation have been provided for monodisperse droplet production [24,25,26], to the best of our knowledge, the nonlinear relationship between droplet production speed and fluid flow rate is mainly qualitatively discussed in experiments of droplet generation in the flow-focusing microchannel [27]
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