Immiscible Two-Phase Parallel Microflow and Its Applications in Fabricating Micro- and Nanomaterials

Introduction

The term 'two-phase flow' in fluid mechanics is applied to simultaneous flow of: (a) a single fluid flowing as two different phases, such as steam and water; (b) two different fluids with different chemical properties occurring in the same phase, for example, the oil-water liquid-liquid system; or (c) two materials with different states or phases, for instance, water flow with air bubbles or sand grains inside.

In macroscale, no well-defined interfaces between two miscible materials can be maintained and detected due to rapid mixing through convection as well as diffusion. In this case, two-phase flow generally referrers only to flow systems composed of immiscible materials.

But when the characteristic length of the flow channel is small enough, the Reynolds number is usually small and the flow typically possess a laminar feature. The mass transfer in the system is geometrically confined and dominated by diffusion. That is why in situations when a two-phase micro-flow is concerned, mixing between the two phases is largely suppressed. An interface region can be clearly found even between miscible fluids flowing side by side, as shown in Figure 1a. The interface region disappears only when the two phases are put in contact with each other for long enough time. Thus in microfluidics, both miscible and immiscible two-phase flows are studied and used for various on-chip applications like chemical synthesis, material characterization, cell culture, extraction/separation, emulsification, and even Boolean calculations. In comparison with the blurred interface developed between the two miscible phases in Figure 1a, sharp interfaces are easily built and held between immiscible micro-fluids (Figure 1b). This results in more extensive applications of the immiscible two-phase micro-flows(Atencia & Beebe, 2005).

Figure 1.

Interfaces developed in two-phase microflow systems with (a) miscible and (b) immiscible fluids flowing side by side in microchannels

All the applications of the immiscible micro-flow systems are related to the mass and heat exchange across the interfaces between the two phases. The interface morphology is in many cases decisive while it is largely influenced by factors like the viscosities of the fluids, interfacial tension, flow rates, shape and characteristic size of the microchannel, and the chemical and geometrical states of the channel walls. Flow patterns, such as parallel flow, wavelet flow, annular flow, droplet flow, and segmented/slug flow, are defined accordingly (Yagodnitsyna, Kovalev, & Bilsky, 2015; Zhao, Chen, & Yuan, 2006a; Azarmanesh & Farhadi, 2016; de Menech, Garstecki, Jousse, & Stone, 2008; Salim, Fourar, Pironon, & Sausse, 2008). When the co-flowing two phases are running as laminar layers parallel to each other and the interface in between is even and straight, a parallel flow is formed. But if the two-phase interface is a wavy one, a wavelet pattern is built instead. Figure 2a and Figure 2b are parallel flow and wavelet flow recorded in an oil-water two-phase system. The annular flow is a pattern formed when one phase is moving continuously in the central part of the microchannel and enclosed by the other phase as if the enclosed phase is flowing in a pipe made by the outside phase. According to the above flow patterns, both phases are flowing continuously. A droplet microflow is formed when one of the immiscible phases (called the dispersed phase) is flowing in small discrete volumes while the other (called the continuous phase) is streaming continuously in a laminar way (Figure 2c). In a segment/slug flow pattern, one fluid phase is separated into segments by segments of the other phase (Figure 2d).

Figure 2.

Optical microscope images of typical two-phase flow patterns. (a)A parallel flow, (b) a wavelet flow, (c) a droplet flow, and (d) a segment/slug flow