Controlling thread formation during tipstreaming through an active feedback control loop
Microscale tipstreaming is a hydrodynamic phenomenon capable of producing submicron sized droplets within a microfluidic device. The tipstreaming process results in the drawing of a thin thread from a highly curved interface and occurs as a result of interfacial surfactant concentration gradients that develop due to elongational flows generated within flow focusing geometries. However, in conventional microfluidic devices, the thread formation is periodically interrupted by the formation of larger primary droplets. This study presents an active feedback control loop capable of eliminating the production of primary droplets and producing a continuous thread, and therefore a continuous droplet stream. A proportional controller is used to successfully control the position of the interface and generate a continuous thread. A derivative component is incorporated in an attempt to increase controller stability, but this component is found to be ineffective. Analysis of the tip position as a function of time is performed to determine the optimal proportional gain constant and set point value for the proportional controller that minimize fluctuations in the produced droplet sizes. The generation of a continuous thread facilitates the use of tipstreaming in several applications, including nanoparticle synthesis, chemical detection, and enzyme activity studies.