Liquid-in-gas droplet microfluidics; experimental characterization of droplet morphology, generation frequency, and monodispersity in a flow-focusing microfluidic device

Abstract

Microfluidic techniques for production of uniform droplets usually rely on the use of two immiscible liquids (e.g. water-in-oil emulsions). It has been shown recently that a continuous gas flow instead of a second liquid carrier can be used as an alternative approach in droplet microfluidics. In this work we experimentally investigate the generation of liquid water droplets within air in flow-focusing configurations. Over a wide range of flow conditions we identify six distinct flow regimes inside the microchannel: Co-flowing, Threading, Plugging, Dripping, Multi-Satellite Formation, and Jetting. Flow regimes and their transitions are plotted and characterized based on the Weber number (We) of the system. We further investigate the impact of liquid microchannel size on the flow maps. Generation frequency, morphology, and monodispersity of the droplets are characterized in more detail in the Dripping regime. Generation frequency can be related to the product of the liquid and gas flow rates. However, droplet morphology (length and width) is more dependent on the gas flow rate. We demonstrate the production of monodisperse droplets (d < 100 µm and σ/d < 5 %) up to kHz formation rates in liquid-gas microfluidic systems for the first time. The results of this work provide practical and useful guidelines for precise, oil-free delivery of ultra-small volumes of fluid which can be integrated in lab-on-a-chip systems for a variety of applications in biochemical research and material synthesis.

Motion of Long Levitating Drops in Tubes in an Anti-Bretherton Configuration

“…Finally we explore a larger set of parameters and draw a phase diagram delimiting the regimes in which long levitating droplets are formed. While systems designed to synthesize bubbles or drops in liquids are ubiquitous in microfluidics [22][23][24], the formation of long levitating drop in air has only been reported in superhydrophobic channels [25], wherein contact with the walls is prevented by the specific surface treatment. This work provides a simple way to generate long drops of controlled length stably propagating in regular tubes, that may serve in digital microfluidics to transport liquids without any wall contamination.…”

Detection of magnetic tracers with Mx atomic magnetometer for application to blood velocimetry

“…Magnetic nanoparticle droplets separated by the continuous air injection when we applied 20 mm/h flow rate for magnetic fluid phase and 600 mm/h flow rate for air phase. So, Liquid droplets generated within a gaseous flow (liquid-in-gas systems) 40 . The velocity of magnetic droplets is constant and slow.…”

Numerical simulation of high inertial liquid-in-gas droplet in a T-junction microchannel

“…Droplet generation in the gaseous phase doesn't require using surfactants which makes droplet as a clean and contaminationfree medium for drug delivery and cell encapsulation. 48 The W/A system can also be used in the variety of applications such as detection of airborne particles, 49 purication of organic substances, 50 aerosol drug delivery, 51 point-of-care diagnostics 52 and simulating a lung pathway. 53 Despite its importance, there are only a few experimental works available in the W/A system, namely analysis of water droplet/slug detachment in air ow, 47,54 W/A system in co-ow 43 and ow focusing geometries, 45,48 droplet collision mixing, 55,56 and coalescence of small aerosol droplets.…”

“…48 The W/A system can also be used in the variety of applications such as detection of airborne particles, 49 purication of organic substances, 50 aerosol drug delivery, 51 point-of-care diagnostics 52 and simulating a lung pathway. 53 Despite its importance, there are only a few experimental works available in the W/A system, namely analysis of water droplet/slug detachment in air ow, 47,54 W/A system in co-ow 43 and ow focusing geometries, 45,48 droplet collision mixing, 55,56 and coalescence of small aerosol droplets. 57 Flow regime mapping of high inertial gas-liquid droplets in ow-focusing geometries has been done by Shahriari et al 45 They experimentally and analytically investigated the effect of the microchannel geometry on different ow regimes and droplet sizes.…”

“…Another experimental research on liquid-gas droplets has been performed by Tirandazi and Hidrovo. 48 Droplet size, generation frequency and ow regimes under the effect of a high speed gas ow have been studied. Jiang et al 43 used gas as the continuous phase to generate water droplets inside a capillary co-ow system.…”