Microfluidic systems or devices, such as inkjet print heads, DNA microarrays, biosensors, cell sorting devices, and lab-on-a-chip devices, rely on sub-millimeter plumbing components to manipulate fluids precisely. A microfluidic system includes various channels, also referred to as capillaries, through which fluids are processed, such as by moving, mixing, and separating the fluids. A microfluidic system may include various components, such as micropumps and microvalves, to control the processing. A micropump may be used to deliver fluids in a continuous or pulsed manner to a channel. A microvalve may be used to control flow rate and direction of fluids entering a channel.
Due to the low Reynolds flow characteristics within microfluidic channels, microfluidic systems are amendable to physics-based models that inform suitable geometric designs and/or operating conditions for a wide variety of microfluidic applications. In one application, microfluidic systems have been used to produce droplets containing an inner fluid that are microencapsulated by a middle (or shell) fluid and that are formed in the presence of and transported by an outer fluid. The fluids are immiscible. As the droplets of the inner fluid are formed, they are surrounded by the middle fluid. Such a microfluidic system may employ a double-capillary junction. (See, Utada, A. S., Lorenceau, E., Link, D. R., Kaplan, P. D., Stone, H. A., and Weitz, D. A., “Monodisperse Double Emulsions Generated from a Microcapillary Device,” Science, Apr. 22, 2005, vol. 308, iss. 5721, pp. 537-541, which is hereby incorporated by reference.) A double-capillary junction may consist of a square glass capillary that encompasses a tapered capillary and a uniformly cylindrical capillary, whose openings are separated by a gap. Using independently controlled syringe pumps, the middle fluid and the outer fluid are fed from opposite directions into the square capillary, and the inner fluid is fed into the tapered capillary. The tapered capillary is centered on an orifice of the cylindrical capillary (i.e., formation channel). The inner, middle, and outer fluids enter the cylindrical capillary via the center, middle, and outer portion, respectively, of the orifice. Under ideal operating conditions characterized by a “dripping” regime, as the three fluids exit via the cylindrical capillary, the middle fluid encapsulates the inner fluid and pinches off into droplets near the orifice that are swept downstream by the outer fluid. The middle fluid may be a photopolymerizable silicone that hardens when exposed to an ultraviolet light source that is placed downstream of droplet formation to form the microcapsules. Within the dripping regime, nearly monodisperse microcapsules form at rates of 1-100 Hz with tunable diameters ranging from 100-600 micrometers, depending on fluid viscosities, flow rates, and capillary sizes.
The efficient operation of a microfluidic system, especially for large-scale production campaigns, typically requires time intensive monitoring and intervention by humans to prevent or minimize disruptions in production that occur when a system enters a mode of abnormal operation (e.g., the middle fluid is not properly encapsulating the droplet) due to abnormalities in fluid flow. The abnormalities in fluid flow that may cause a disruption may include unexpected clogs, air bubbles, chemical impurities, particulates, pressure fluctuations in external pumps, fluid instabilities, and so on. Such disruptions may result in time and material loss, reduced production, reduced quality, and, depending on the severity, even damage to the microfluidic system. To minimize the effects of such disruptions, a person may use a microscope to view the fluid flow of the microfluidic system to assess the cause of the disruption. Depending on the cause, the person can then make various adjustments to the operation of the microfluidic system, such as adjusting the flow rate of a fluid, to return to normal operation. Because the operation requires human monitoring and intervention, the identification of the abnormalities and the determination of needed adjustments are both subjective and time-consuming, which may result in considerable loss of production, especially when the person is not highly experienced. In addition, a large-scale production environment may require highly parallelized arrays of microfluidic platforms that may be too complex to be effectively monitored and controlled by humans.