Description
In-air droplet generation in microchannels using co-flow focusing configuration
M.Bulois¹-², A. Allemand¹, B. Sobac², B. Scheid¹
¹TIPs, Université Libre de Bruxelles, 1050 Bruxelles, Belgique
²LFCR UMR 5150, CNRS, Universite de Pau et des Pays de l’Adour, 64600 Anglet, France.
Multiphase microfluidics involving bubbles and droplets in microchannels is central to many applications in chemical and biological fields, such as micro-emulsification and micro-encapsulation. However, most studies involve systems with a liquid continuous phase, whereas in-air microfluidics remains largely unexplored owing to the difficulty of achieving stable configurations, despite the numerous advantages it could offer.
Here, by combining optical microscopy with high-speed visualization and using the co-flow focusing droplet generator previously developed in our group (Ref. [1]), we succeed, for the first time to the best of our knowledge, in generating droplets in air in a controlled manner within a microchannel. This setup enables a systematic experimental investigation of water droplet formation and transport dynamics in a confined air flow at the microscale.
By varying the flow rates of both phases, we analyze flow regimes, droplet size, generation frequency, as well as droplet velocity and position. The explored flow range corresponds to gas Reynolds numbers from 47 to 600 and Weber numbers from 0.11 to 22, which can involve gas velocities of 100 m/s in the outlet capillary. The liquid phase operates with Reynolds numbers ranging from 2 to 40 and Weber numbers from 0.0029 to 2. This wide parameter space allows observation of three distinct flow regimes: jetting, dripping, and a bistable transient regime characterized by hysteresis between dripping and jetting, as well as the transition from in-liquid to in-air microdroplets. Remarkably, droplet generation frequencies exceeding 10 kHz and droplet velocities of the order of 10 meters per second are measured. The whole results clearly evidence the dominant role of inertial effects in this in-air microfluidic configuration with Laplace number of about 3000.
At this stage, numerous additional phenomena remain to be fully explored. Depending on the operating conditions, oscillations of droplets or jets are observed in the outlet capillary. In the dripping regime, oscillatory modes may also appear during droplet detachment from the nozzle.
Overall, this work demonstrates for the first time the feasibility of generating water droplets with an air flow inside microchannels at ultra-high throughput, offering a new pathway for oil-free encapsulation applications.
Reference
[1] Dewandre, A. et al. (2020). Microfluidic droplet generation based on non-embedded co-flow-focusing using 3D printed nozzle. Scientific Reports, 21616.