Dynamics of a highly viscous core flow in a microfluidic flow focusing system
Creators: name: Kurdzinski, Michael
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400899
subject: Electrical engineering not elsewhere classified
classificationCode: 401210
subject: Microfluidics and nanofluidics
classificationCode: 401705
subject: Microelectromechanical systems (MEMS)

description: Multiphase microfluidic devices offer precise control of the flows in a wide range of applications such as chemical reactions, biological samples, mixing and droplet formation by applying different flow rates and channel geometries. However, in the majority of these cases, fluids used in these devices can have similar physical properties such as viscosity, density and surface tension.<br><br>Interaction of fluids with different physical properties can generate multifaceted fluid patterns in confined systems. In particular, applying fluids with a large viscosity contrast into a microfluidic system can generate quite complex instabilities arising from localised velocity and shear stress gradients. High viscosity contrast interfaces have been investigated within microfluidic flow focusing systems. However, in these works, the ratio of core/sheath flows as well as the diffusion coefficient between the core/sheath flows is such that the width of the core reduces very quickly at the entrance of the flow focusing channel. This motivated the author to change the set the operating conditions of the system to study the stability of the core flow such that the core flow can maintain its width at the entrance of the flow focusing channel.<br><br>As the first research contribution, the author investigated the dynamics of a highly viscous glycerol core confined between water sheath flows. Experiments indicate that the structure of core flow is determined by the ratio of core/sheath flows. At high flow rate ratios, parallel streams of core and sheath flows move alongside each other throughout the channel. At lower flow rate ratios, a tapered core flow is formed at the upstream of the channel before converting into a thin ‘viscous thread’. In this case, the stability of tapered core flow is determined by the magnitude of the sheath flow rate. At low sheath flow rates, the tapered core remains stable. In contrast, at high sheath flow rates, ‘surface waves’ are induced at the interface of core/sheath flows. Such surface waves can buckle the core or even break it into two parts, which is referred to as ’core pinch off’ phenomenon, which has not been reported before.<br><br>As the second research contribution, the author characterised the dynamics of pinch off phenomenon under various combinations of core and sheath flow rates. In particular, using high speed imaging the author investigated the variations of core length and pinch off frequency. Experiments are conducted under a constant core flow rate but varying sheath flow rates as well as under varying core flow rates but a constant sheath flow rate.<br><br>As the third research contribution, the author explored the role of viscosity contrast between the core and sheath flows on the stability of the core. This has enabled the author to obtain the flow map of the core, representing the extent of the core not only under various combinations of core and sheath flows but also under two different viscosity contrasts.