Studies of droplets formation regime and actual flow rate of liquid-liquid flows in flow-focusing microfluidic devices
Introduction
Liquid-liquid flows in small channels, with dimensions less than 1 mm, have been found in many applications in the droplets production of emulsions [1], [2], [3]. The droplets are generated individually, allowing high control of the emulsification process and reduction of the coefficient of variation or droplets with low polydispersity. An additional advantage of the emulsification through microfluidics is the capacity to overcome the difficulties inherent to high-pressure homogenizers and high-speed mixers that use intense energy to breakup the droplets. In microchannels, the droplets generation is smoother than in conventional methods, thus, microfluidic devices could be used for the encapsulation of compounds that are sensitive to pressure and temperature, such as vitamins and probiotics [4], [5], [6], [7]. Some benefits can be obtained from the changes of macro to microscale. The ratio of the flowing surface area to volume increases and laminar flow is the characteristic regime in microfluidics devices. Thus, viscous and interfacial forces should be higher than the inertial ones to occur droplets breakup and the surface effects become more important than in macroscale [8].
Different techniques (laser ablation, micromachining and soft lithography) and materials (glass, PDMS, silicon and thermoplastic) has been used to produce a wide range of microfluidic devices [9], [10]. For the droplets generation, there are shear-induced and interfacial tension-induced geometries, such as terrace geometry. In this device, only interfacial forces drive the droplets formation, resulting in a very smooth droplet detachment and consequently a high energy efficiency. However, the fabrication costs of this device are higher than mostly shear-induced geometries, such as those based on capillaries assemblies and planar geometries [11], [3]. The microcapillary devices are assemblies of coaxial capillary tubes normally produced using glass. In this emulsification device, the droplets are surrounded completely by the outer phase, which increases the effects of interfacial and shear forces between the two phases favoring the droplet formation. On the other hand, intensive hand labor in the fabrication and alignment of the tubes is a disadvantage of this type of microfluidic device [12], [3]. The planar microfluidic devices are channels with rectangular cross section, which are easily designed and produced. These devices differ from each other according to the type of junction between the channels. T-, Y- and cross-junctions are the most common planar devices used for droplet generation. Compared with T- or Y-junctions, a higher shear is imposed by the continuous phase in a cross-junction caused by the flow at both sides of the dispersed phase stream, in a technique known as hydrodynamic flow-focusing, leading to the production of uniform and smaller droplets [13], [14], [15], [3].
The input flow rate of the phases is usually controlled by positive displacement pumps (syringe pumps), which are responsible for injection of the fluids inside the microchannels. Two different liquids are introduced to form droplets (disperse phase) surrounded by the other immiscible liquid (continuous phase). The velocity of the droplets detachment defines if the droplet formation regime is dripping, squeezing or jetting. In “model” systems (low viscosity fluids) the droplet formation regime can be accurately controlled through the adjustment of the geometry of the microfluidic devices and flow rate of the two phases [16]. However, to our knowledge few studies were conducted to verify the droplet formation regime of the emulsion generation using more complex materials, as biopolymers [4], [17], [15]. In these systems, the properties of both liquid phases (interfacial tension and viscosity) can change flow velocity of the phases, ie., the actual flow rates of the fluids within the microchannels are different to nominal input flow rate (pump set point), and thus determine the limitations of the process that are associated to clogging, expansion and, in some cases, disruption of channels. In order to perform an adequate scale-up of microfluidics processes aiming droplets production is necessary studying the flow conditions inside the microdevices. Therefore, implementation of this technology needs a careful analysis of the relation between process conditions and the fluid dynamics within the channels, which is reached by the control of the process variables during the droplets generation in microchannels.
The dimensionless numbers as Reynolds, Weber, Capillary and Eotvos are relevant to describe the effects of the dominant forces during droplets generation, which are associated with the phase velocities and the droplets formation regime in microfluidics devices [18]. Thus, physical properties and the actual velocity of the phases within the microfluidic devices are essential to determine the correct dimensionless number values and ensure that the forces during droplets formation will remain similar during scale-up. Therefore, the aim of this paper was to study the process conditions and limitations in the droplets generation of more complex materials in microchannels. For this, the effect of flow rates of the dispersed and continuous phases on the droplets formation regime were studied. The physical properties and the actual flow rate of the phases were evaluated to obtain a more accurate calculation of the dimensionless numbers aiming the scale-up of microfluidic processes for emulsification.
Section snippets
Microfluidic devices
The microfluidic devices were produced using polydimethylsiloxane (PDMS), by the mixture of Sylgard 184 silicon elastomer base and curing agent (Dow Corning, USA) at 10:1 (w/w) ratio. Glass slides (Perfecta, Brazil) were used to seal the PDMS chips.
Emulsions
Low-acyl gellan gum (Kelcogel® F) was donated by Kelco Biopolymers (USA) and deionized water was obtained from a Milli-Q system. Soybean oil (Bunge Alimentos, Brazil) was purchased in the local market and polyglycerol polyricinoleate (GRINDSTED®
Physical properties of the dispersed and continuous phase
Fluids can be exposed to high shear rates (∼103 s−1) during the flow into microchannels and the droplet detachment occurs when the shear forces exceeds the interfacial tension force [5]. Organic phase and water are Newtonian fluids (Table 1), while aqueous solution of gellan (0.6% w/w) showed shear-thinning behavior at medium and high shear rates and Newtonian behavior at low shear rate (Fig. 3). Shear rate was calculated from nominal flow rate of the phases (Eqs. (1), (2), (3)). Within process
Conclusion
Our results allowed to evaluate the droplets formation regime and hence, their size, which depend strongly on the physical properties of the phases (viscosity and interfacial tension) and process conditions (input flow rate of the phases). High Weber number, which is related to low interfacial forces and high inertial forces, and high Capillary number, which is related to high viscous forces, favored the formation of smaller droplets. Smaller droplets (or higher droplets surface area) can be
Acknowledgments
The authors thank CAPES – Brazil (DEA/FEA/PROEX) and FAPESP – Brazil (FAPESP 2007/58017-5 and 2011/06083-0) for their financial support. Ana Letícia Rodrigues Costa thanks CNPq – Brazil (CNPq 130534/2013-7) and Andresa Gomes thanks CNPq – Brazil (130544/2013-2) for the fellowship; Rosiane Lopes Cunha thanks CNPq (CNPq 305477/2012-9) for the productivity grant. The authors appreciate the technical support given by Professor Angelo Gobbi and Maria Helena de Oliveira Piazzeto of the
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