Regimes of miscible fluid thread formation in microfluidic focusing sections

Abstract

We experimentally study the formation and stability of miscible fluid threads made of high-viscosity liquids using hydrodynamic focusing sections. Miscible core annular flows are useful for transporting viscous materials and can be destabilized for enhancing mass transfer. We delineate phase-diagrams of the generation of lubricated threads from low to large viscosity contrasts with various di↵usion coe cients. Depending on fluid properties and flow rates of injection, stable microflows are classified into engulfment, thread, and tubing regimes. For low Péclet numbers, we examine thread dynamics when di↵usive e↵ects strongly alter basic flow structures and induce new flow configurations, including ultra-di↵usive and di↵usive instability regimes. Another unstable flow arrangement is investigated for moderate Reynolds numbers where small threads are rapidly destabilized in the inertial flow field of the sheath fluid near the fluid junction. This study provides an overview of stable and unstable flow regimes and their transitions during the formation of miscible viscous fluid filaments in square microchannels.

Pressure-driven flow focusing of two miscible liquids

“…Here, the Schmidt number Sc = µ e /(ρ e D) is always larger than 5000, indicating that diffusive mass transfer is negligible with respect to diffusive momentum transfer. This is in agreement with the fact that we do not observe diffusive instabilities [23]. In our experimental conditions, the Péclet number Pe remains within the range of 2 − 30 × 10 3 ; therefore, the system operates in the convective regime, as seen in Figure 2(c).…”

Effective interfacial tension in flow-focusing of colloidal dispersions: 3-D numerical simulations and experiments

“…The fit of the threading regime as a function of flow rate ratio , represented by pink solid lines in figure 10, gives for the width and for the height. A power law in is predicted for a circular thread only controlled by the ratio of the flow rates (Cubaud & Mason 2009) and has been verified experimentally only by measuring the width of the thread assuming a circular cross-section (Cubaud & Mason 2008; Cubaud & Notaro 2014). The coefficient 0.58 in front of the power law is also in good agreement with previous studies (Cubaud & Mason 2008, 2009).…”

“…These time scales can be compared using the dimensionless Péclet number which is the relevant dimensionless number to describe convective diffusive flows (Atencia & Beebe 2005; Cubaud & Mason 2006). When miscible fluids are subjected to flow-focusing, at high Péclet number, the main flow regime observed is a threading regime with common features similar to immiscible fluid threads while, at low Péclet number, the thread can be destabilized by diffusion instabilities (Cubaud & Notaro 2014). However, as the existence of such thread structures are short-lived and limited to short time scales before complete mixing, little is still known about the evolution of viscous threads in miscible environments.…”

“…Here, we quantitatively investigate the characteristics of thread development such as 3-D topology and velocity fields as a function of interfacial tension and the ratio of the flow rates. The previous experimental studies by Cubaud & Mason (2008 and Cubaud & Notaro (2014) for both immiscible and miscible high-viscosity threads in flow-focusing channels were limited to a top view and did not resolve the full 3-D shape of the thread. Indeed, there are only a few 3-D studies of the threading regime (Knight 1998;Xu et al 2016) but their focus are not on the effect of interfacial properties on the thread shape.…”

From droplets to waves: periodic instability patterns in highly viscous microfluidic flows

“…When an inner stream is unsheathed with another fluid, the destabilization of miscible and immiscible core-annular flows has been studied in small tubes with the appearance of interfacial waves (Cao et al 2003), including bamboo waves (Joseph & Renardy 1993) and pearl-mushroom waves (D'olce et al 2008), which result from the development of absolute instabilities (Salin & Talon 2019). In microchannels, a variety of instability patterns were examined based on fluid and flow properties during the formation of miscible viscous threads (Cubaud & Notaro 2014). Overall, the development of periodic flow patterns allows one to relate flow characteristics to control parameters and provides insight into hydrodynamic instabilities.…”