(441d) Interfacial Instabilities during Displacement in Microchannels: The Effect of Boger Fluid in Channels with Bends

In everyday chemical processes, displacement is a crucial phenomenon. Activities from the cleaning of medical devices to enhanced oil recovery involve the removal of a liquid by another often non-miscible liquid, in small channels of various configurations. In biological analysis, the body fluids used are non-Newtonian as they contain cells and other substances similar to long-chain polymers. Likewise, in enhanced oil recovery, polymers and surfactants are added to increase efficiencies, which change the rheological properties of the liquid. With the increase in applications involving microfluidic devices, the understanding of displacement in small channels is important. If the displacing liquid is less viscous than the initial one, the displacement is always inefficient, with a layer of the initial fluid remaining on the wall and the displacing phase flowing in the core of the channel. Furthermore, after the initial displacement front, the interface could become unstable due to viscosity and density differences of the two phases.

In our research, we are comparing the differences between a Newtonian and a Boger (pure viscoelastic) fluid on a displacement flow inside a microchannel. Specifically, we study the residual film thickness left behind by the initial liquid, as well as the interfacial instabilities occurring after initial displacement. To do so, we have used a mixture of polyethylene oxide (PEO), polyethylene glycol (PEG) and zinc chloride (ZnCl₂) to produce a viscoelastic fluid with no shear-thinning properties to displace an immiscible silicone oil. For both the Newtonian and the viscoelastic cases, the same viscosity ratio is kept, at 0.824 with the silicone oil being the more viscous liquid. PTFE tubing with an internal diameter of 500 µm was initially used to observe the displacement, and a microchannel with an internal diameter of 200 µm was later employed to study displacement in channels with bends, as often observed in applications as well as displacement in porous materials. High speed imaging was used to track the interface, including its deformation during the displacement process.

The displacement front observation showed that the Boger fluid produces a thinner residual film when compared to the Newtonian one at displacing flowrates above 0.08 ml/min (fig. 1). A correlation was produced for both cases, which was then compared with previous film thickness correlations. In the case of the Boger fluid, the Deborah number was included to account for the additional elastic force acting on the interface. We have also found that the interfacial instabilities that developed behind the displacement front had different wavy patterns (fig. 2), which switch freely from one shape to another. These instabilities, well-known for Newtonian fluids, are still poorly investigated for viscoelastic fluids. We were able to identify different modes of instability, including the shifting of the core fluid to minimise drag force, as well as a periodic instability. Using Fourier and wavelet analysis, it was possible to identify the frequencies that comprise the instability. Often, bands of different frequencies were observed, as the instabilities result from the superposition of multiple smaller interfacial waves. When the flowrate of the displacing phase is increased, the frequency of the instability increases, albeit at lower amplitudes due to the higher inertial force. Additionally, we have also observed an increase in the number of frequency bands in the wavelet maps at the high displacing phase flowrate cases, with the shapes of the interfacial instability becoming more chaotic. In the case of the microchannels with bends, we have observed a backwave effect when the displacement front reached a bend, changing the shape of the instabilities along the interface with a velocity approximately equal to the velocity of the displacement front (fig. 3).

In summary, we have shown that a pure viscoelastic fluid results in a thinner residual film, and thus higher displacement efficiencies when compared to a Newtonian one, particularly at high displacement flowrates. Additionally, the interfacial instability that appears after the displacement front is periodic for the viscoelastic case but not for the Newtonian one. Bends downstream the displacement from were found to affect the interfacial instabilities and led to more chaotic patterns compared to straight channels.