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Article

Dynamics of Nonmagnetic and Magnetic Emulsions in Microchannels of Various Materials

by
Dariya Kalyuzhnaya
,
Evgeniy Sokolov
,
Anastasia Vasilyeva
,
Irina Sutarina
and
Petr Ryapolov
*
Department of Nanotechnology, Microelectronics, General and Applied Physics, Faculty of Natural Sciences, Southwest State University, 50 Let Oktyabrya Street, 94, Kursk 305040, Russia
*
Author to whom correspondence should be addressed.
Fluids 2023, 8(2), 42; https://doi.org/10.3390/fluids8020042
Submission received: 13 December 2022 / Revised: 14 January 2023 / Accepted: 23 January 2023 / Published: 25 January 2023
(This article belongs to the Special Issue Recent Advances in Hydrodynamics and Magnetised Fluids)
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Abstract

:
The formation of droplets in microchannels (droplet microfluidics) has a large number of applications, such as in micro-dosing and gas meters. This paper considers the dynamics of direct and inverse emulsions based on water, polydimethylsiloxane, and synthetic and mineral oil in microfluidic chips based on two technologies: glass–parafilm–glass sandwich structures and removable scaffold in a silicone compound. It is shown that wettability, roughness and chip wall material; channel thickness; magnetic fluid flow rate; and magnetic field strength affect the size of emulsion droplets formed in a microfluidic chip. The addition of another mechanism for regulating the hydrodynamics of emulsions using a magnetic field opens up new possibilities for the development of promising devices.

1. Introduction

As shown in the latest reviews by E. Samiei et al. [1], G.P. Nikoleli et al. [2], and Y. Gao et al. [3], microfluidics is one of the rapidly developing areas of scientific research. A large number of works have been presented on this topic. Microfluidic devices help solve the problems of industry (Holtze C. [4]), biology (K.F. Sonnen et al. [5]), medicine (Yang, Y. et al. [6], A. Basiri et al. [7], Q.R. Guo et al. [8]), pharmaceuticals (Carvalho, B.G. et al. [9], R. Ran et al. [10]), etc. There are a huge number of different materials and technologies for manufacturing microfluidic chips, as discussed by M.K. Raj et al. [11], A.K. Au et al. [12], E. Roy et al. [13], S.M. Scott et al. [14], and Ren, K. et al. [15]. This makes it possible to obtain certain characteristics necessary to control the dynamics of liquids and particles, which is shown in the articles mentioned. The study of this issue is also relevant because the use of microfluidic systems can significantly reduce the cost, complexity, and energy intensity of research. This is confirmed by the work of Gale B.K. et al. [16].
S.Y. Teh et al. [17] and C.N. Baroud et al. [18] use droplet microfluidics in their studies to form droplets and manipulate them in a liquid medium in closed microchannels. The works of J.D. Wehking et al. [19] and Z. Liu et al. [20] prove that through the use of immiscible multiphase flows inside microchannels, it is possible to control the droplet size and the dynamics of emulsions by increasing/decreasing the flow rate of liquids or changing the channel configuration. Liquid active components can be used as such a factor as well; they can change their properties under the influence of external influences. This is confirmed by the works of P. Zhu et al. [21], F. Alnaimat et al. [22], and N. Pamme [23]. Magnetic fluids (MFs) are one of the types of such media, which is shown in the works of R.E. Rosensweig [24], S.A. Sokolsky et al. [25], A.S. Ivanov et al. [26], A.R. Zakinyan et al. [27], R.J. Yang et al. [28], and S.H. Tan et al. [29]. MFs are colloidal systems containing nanometer-sized magnetite particles dispersed in various liquid carriers and coated with surfactants. In his work, N.T. Nguyen [30] states that the control of magnetic fluids parameters and behavior under the influence of an external magnetic field is a promising area of scientific research in microfluidics.
The dynamics of magnetic fluid droplets in microchannels is considered in many works in this area. The characteristics of magnetic fluids multiphase flow under the influence of an external magnetic field are studied. C.W. Chang et al. [31] consider factors influencing the fluid flow velocity and the position of the permanent magnet.
In the works of M.A. Bijarchi et al. [32,33,34,35], droplets formation and the process of MF droplets shape evolution were studied in microchannels under the action of a magnetic pulse in the presence of high non-uniform magnetic fields, as well as under the influence of an inhomogeneous magnetic field with pulse-width modulation. The kinematics and deformation of MF droplets moving under the influence of an external magnetic field were considered in the work of N.T. Nguyen et al. [36]. According to the results of the study of A. Favakeh et al. [37], it was found that the volume of the formed ferrofluid droplets in the microchannel decreases, and the frequency of their generation increases with an increase in the magnetic field both at a constant and alternating current.
Recently, the phenomenon of magnetic levitation of non-magnetic inclusions in a magnetic fluid medium under the influence of a non-uniform magnetic field of a special configuration has been the subject of topical research (P. Dunne’s article [38] and Q.H. Gao’s review [39]). Magnetic field source creates an inhomogeneous magnetic field in which there is a region of “magnetic vacuum”, where the magnetic field strength tends to zero. The source of the magnetic field also creates an area of “magnetic levitation”, in which the isolines of the magnetic field strength module are closed. Such systems make it possible to control the behavior of nonmagnetic objects: bubbles and droplets in a magnetic fluid in tubes with a cross-section of 12 cm and flat channels 2–5 mm thick, which was shown in our previous works [40,41,42,43]. The effect of surface tension is minimal in these systems.
The works listed above have several unexplored problems; determining the most optimal conditions for creating droplet and bubble flows in immiscible media in microfluidic chips of various configurations under the influence of an inhomogeneous magnetic field, including the study of the effect of channel wall wettability, is among them. The issue of the influence of surfactants on the formation of emulsions in microchannels in a nonuniform magnetic field also remains unexplored. Solutions to these problems will make it possible to determine the optimal conditions for the controlled generation of stable nonmagnetic bubbles and droplets in magnetic fluids.

2. Materials and Methods

2.1. Experimental Setup

An experimental setup was designed to study droplets and bubble flow. Its block diagram is shown in Figure 1.
Self-made microfluidic device 1 (2 cm wide, 12 cm long) is fixed using a system of non-magnetic fasteners. The chip is located parallel to the axis of the ring permanent magnet composed of NdFeB alloy (material: neodymium–iron–boron; geometric parameters: Magnet 1: outer diameter is 60, inner diameter is 24, thickness is 10 mm; Magnet 2: outer diameter is 50, inner diameter is 25, thickness is 5 mm) 2. Continuous and dispersed phases are introduced into the chip using dual-channel syringe pump 3 and 4, which are made from a kit intended for the assembly of a 3D printer. Computer 5 controls the syringe pump. The dynamics of the multiphase system were recorded by shooting with microscope 6. Video recording took place in the transmitted light of illuminator 7. The results obtained were transferred to computer 5, where the files were further processed in the NI Lab View program.

2.2. Investigated Fluids

Three fluids immiscible with water, namely, polyethylsiloxane-1 (PES-1), mineral and synthetic oils were investigated in the work. The physical parameters of the samples under study are shown in Table 1.
A 5% aqueous solution of blue food coloring E133 or distilled water acted as a dispersed phase.
Next, a magnetic fluid (MF) based on magnetite stabilized with an oleic acid surfactant was used in the experiment. Synthetic oil acted as the carrier fluid. The studied magnetic colloid was obtained through chemical condensation. The physicochemical characteristics of the studied MF sample are shown in Table 2.

2.3. Microfluidic Device Manufacturing Technology

The influence of the material of the walls of the microfluidic chip on the formation of droplets in a microchannel was studied experimentally. For this purpose, two types of microfluidic devices with the same “flow focusing” channel configuration (Figure 2a) were produced. The sample acting as a continuous medium was supplied to inlet 1. The fluid forming the dispersed phase was introduced into inlet 2. Outlet 3 was used to remove the emulsion from the microchannel.
The chips of the first type were created using the technology of glass–polymer–glass sandwich structures. We described this method in [44]. The technology is as follows. A stencil cut from a Parafilm® film according to the required chip configuration is placed between two glass slides. Then, the sandwich structure is assembled, and the chip is uniformly sintered on the plate for 10 min at a heating temperature of 55 °C. After cooling, the microfluidic device, the sample and configuration of which are shown in Figure 2b, is ready for use.
The chips of the second type were made based on the Embedded SCAffold RemovinG Open Technology (ESCARGOT) method. This method involves the production of channels from ABS plastic using 3D printing. The resulting chip scaffolds were placed in a liquid two-component silicone compound (PDMS) and cured for 2 h at a temperature of 75 °C. The chips were placed in a vessel with acetone and subjected to ultrasonic cleaning for 8 h to dissolve the ABS plastic scaffolds. In the end, the finished microfluidic chips, a sample of which is shown in Figure 2b, were obtained after the final washing with acetone and fastening the connectors to the inputs and outputs of the device. The method is described in the work of V. Saggiomo et al. [45].

3. Results and Discussion

Direct and inverse emulsions of various liquids are the subject of research. A direct emulsion is a dispersed system in which the dispersed phase can be a non-polar liquid, such as oil, and the dispersion medium can be a polar one, such as water. An inverse emulsion is a dispersion of water droplets in oil.
Several experiments on the production of direct and inverse emulsions were carried out on the experimental setup shown in Figure 1 to determine how the material from which the walls of microfluidic devices are composed affects the formation of droplets in the channel.
The samples of PES-1, mineral or synthetic oil were used as a dispersion medium to form an inverse emulsion, which is a suspension of water droplets in oil. Two microfluidic devices were involved in the study. One chip was made using the glass–polymer–glass sandwich structure method, and the second using ESCARGOT technology. Based on the results of the study, Table 3 was compiled.
In all three cases, water droplets formation in the test liquid was not observed in the glass microchannel. This phenomenon can be explained by the fact that the walls of the glass channel have good wettability, which hurts the formation of emulsions.
A continuous flow of two liquids was also observed in the silicone compound channel when PES-1 is supplied. However, the droplet flow and the formation of an emulsion can be observed when mineral or synthetic oil is fed into the chip. The positive result of the experiment is that the silicone surface is completely non-wettable. It was also observed that as the oil feed rate increased, the size of the water droplets decreased.
The following results, presented in Table 4, were obtained from the formation of a direct emulsion, where a 5% aqueous solution of food coloring was used as a continuous medium, and PES-1 or oil were used as a dispersed phase.
The formation of oil droplets in water could not be detected in a microfluidic chip manufactured using ESCARGOT technology. Conducting the experiment in a glass microchannel makes no sense due to the good wettability of the device walls.
The influence of the microchannel size and surface was determined in a further study. To carry out the study, six types of silicone-based microfluidic chips of various thicknesses (d = 0.5 mm; 1 mm; 2 mm) were made with an untreated scaffold surface containing traces of 3D printing in the form of filaments, shown in Figure 3a, and with the treated surface kept in acetone vapor for 30 min (Figure 3b).
The next stage of our work was to identify how the microfluidic device scaffold thickness and its treatment affect the droplets formation in the channel. Mineral oil, the characteristics of which are presented in Table 1, served as a continuous phase, and a 5% aqueous solution of blue food coloring E133 was the dispersed phase. Table 5 was compiled based on the results of the experiment.
Initially, a jet flow of liquid was observed at a channel thickness of 0.5 mm with an untreated scaffold, and then the formation of water droplets occurred in the middle of the channel. Only jet flow can be traced in the chip with the treated scaffold.
The formation of an inverse emulsion occurs in a microchannel of 1 mm in size, the scaffold of which was not treated. The generation of water droplets was also observed in the treated 1 mm chip. However, the formation of an inclusion occurs a little farther from the meeting point of the studied liquids.
The formation of water droplets in oil was also noticeable in microfluidic chips with a thickness of 2 mm, regardless of the treatment of the ABS scaffold of the microfluidic device channel.
The dependence of the size of the generated inclusion on the feed rate of the continuous medium was observed in all cases where it was possible to achieve a droplet flow of water in mineral oil. The greater the rate of the supplied dispersed phase, the smaller the size of the resulting drop.
Sodium coco-sulfate (a surfactant) was used in the next experiment to solve the problem of the influence of surface tension on the generation of droplets in the microchannel. The substance was added to a solution with tinted water (mass fraction of the surfactant was 0.2%). The resulting mixture was then vigorously stirred. Further, the experiments aimed at investigating which type of emulsions is advisable to use with the given surfactant were carried out. An aqueous solution of sodium coco-sulfate was fed into connector 1. It was intended to be used as a continuous phase. To form a direct emulsion, mineral oil, which is a dispersed phase (Figure 4a), was supplied to inlet 2. In the second case, to form an inverse emulsion, the tubes with the supplied liquids were interchanged (Figure 4b).
The formation of water droplets in oil was observed in all cases. Stable droplets of the same size were formed in the case of inverse emulsion. Therefore, it is expedient to use the surfactant applied in silicone microfluidic devices to obtain reverse emulsions. Neither direct nor inverse emulsion could be obtained in a chip with glass walls. The formation of a jet flow was observed in the microchannel.
A series of experiments on the study of magnetic multiphase systems in microchannels under the action of magnetic fields (Figure 5) was carried out in the following study—namely, the influence of the flow rate of a continuous medium (q1 = 0.18; 0.37; 0.74 and 1.1 µL/s) and the magnetic field (Magnet 1 with the outer diameter of 60 mm, inner diameter of 24 mm and thickness of 10mm, and Magnet 2 with the outer diameter of 50 mm, inner diameter of 25 mm and thickness of 5 mm) on the size of the formed droplet. The magnetic field strength of the magnets was 214 kA/m and 85 kA/m, respectively. An experiment without the influence of a magnetic field on the system was also carried out.
Synthetic oil-based magnetic fluid was used in the work. The physical and chemical characteristics are shown in Table 2. This magnetic fluid was a continuous medium for the formation of non-magnetic inclusions in the channel in the form of water droplets. A microfluidic chip based on ‘glass-Parafilm®-glass’ sandwich structures was chosen for the study. In a similar experiment with pure oil, we did not obtain a droplet flow. The presence of magnetic particles coated with surfactants is the reason for the flow pattern difference. Another factor that affects this process is the magnetic field of a special configuration. Moreover, we managed to achieve an adjustable liquid supply, which is the advantage of this method. The dependence of the size of non-magnetic inclusions on the change in the flow rate of the magnetic fluid (in this case, the rate of the dispersed phase remained constant q2 = 0.37 μL/s) is shown in Figure 6a. The graph in Figure 6b shows how the magnetic field strength of the ring magnet affects the volume of the separated droplet.
It can be seen from the Figure 6a that the size of the resulting water droplets is inversely proportional to the flow rate of the magnetic fluid for different values of the magnetic field strength. At the same time, the frequency of their generation increases. This dependence can be explained by a change in the hydrodynamic pressure on the area of inclusion formation, which increases with an increase in the flow rate of the continuous phase, leading to an earlier detachment of water droplets.
Figure 6b shows that the magnetic field also affects the droplet sizes, which is inversely proportional to its intensity. This can be explained by the influence of the configuration and strength of the magnetic field acting on the system on the interfacial tension in the magnetic fluid.
Microfluidic chips made using the ESCARGOT method are also suitable for such experiments. The generation of nonmagnetic inclusions in an oil-based MF is shown in Figure 7a,b.
The presented images indicate that both bubbles and droplets are formed in PDMS devices.

4. Conclusions

An experimental setup for studying multiphase systems in microchannels is presented in this paper. Various manufacturing methods were used for microfluidic chips production: the Embedded SCAffold RemovinG Open Technology (ESCARGOT) method and ‘glass-polymer-glass’ sandwich technology.
A number of experiments were carried out. They were aimed at investigating the hydrodynamics and hydroaerodynamics of various non-magnetic and magnetic fluids in microfluidic chips.
The paper shows how the wettability of the walls of microfluidic devices affects the formation of droplets in the channel. Their formation was best traced in the chips based on a silicone compound, where mineral or synthetic oils were the continuous media. It was determined that the thickness of the microchannels and the roughness of their walls also affect the formation of emulsions in the microchannels. Sodium coco-sulfate is appropriate to use for making inverse emulsions in silicone microfluidic devices. Surfactant reduces surface tension at the interface boundary.
The results of studying the dynamics of non-magnetic inclusions in an oil-based magnetic fluid showed the possibility of the formation of droplet flows both in silicone microfluidic chips and in glass ones. The size of the droplets formed is inversely proportional to the flow rate of the magnetic fluid and the magnetic field. The addition of another mechanism for regulating the hydrodynamics of emulsions using a magnetic field opens up new possibilities for the development of promising devices for droplet microfluidics: counters and dispensers.

Author Contributions

Conceptualization, E.S. and P.R.; methodology, E.S., D.K., A.V. and P.R.; software, P.R.; validation, E.S., P.R. and D.K.; formal analysis, I.S. and D.K.; investigation, I.S. and E.S.; resources, E.S.; data curation, E.S. and D.K.; writing—original draft preparation, E.S.; writing—review and editing, P.R.; visualization, E.S. and A.V.; supervision, P.R.; project administration, P.R.; funding acquisition, P.R. All authors have read and agreed to the published version of the manuscript.

Funding

The research was carried out at the expense of the grant of the Russian Science Foundation No. 22-22-00311, https://rscf.ru/project/22-22-00311/ (accessed on 12 December 2022).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental setup block diagram.
Figure 1. Experimental setup block diagram.
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Figure 2. Microfluidic devices: (a) the configuration of the microchannel; (b) a sample of the chip made based on sandwich structures; (c) a sample of the chip produced by ESCARGOT technology.
Figure 2. Microfluidic devices: (a) the configuration of the microchannel; (b) a sample of the chip made based on sandwich structures; (c) a sample of the chip produced by ESCARGOT technology.
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Figure 3. The surface of the scaffolds for ESCARGOT chip with silicone walls (Figure 2c): (a) the original surface of the scaffold with traces of 3D printing; (b) the surface treated in acetone vapor.
Figure 3. The surface of the scaffolds for ESCARGOT chip with silicone walls (Figure 2c): (a) the original surface of the scaffold with traces of 3D printing; (b) the surface treated in acetone vapor.
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Figure 4. Formation of emulsions in ESCARGOT chip with silicone walls (Figure 2c): (a) direct; (b) inverse.
Figure 4. Formation of emulsions in ESCARGOT chip with silicone walls (Figure 2c): (a) direct; (b) inverse.
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Figure 5. Dynamics of water droplets in MF in sandwich chip with glass walls (Figure 2b).
Figure 5. Dynamics of water droplets in MF in sandwich chip with glass walls (Figure 2b).
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Figure 6. Dependence of the volume of separated droplets in sandwich chip with glass walls (Figure 2b): (a) on the flow velocity MF; (b) on the magnetic field strength.
Figure 6. Dependence of the volume of separated droplets in sandwich chip with glass walls (Figure 2b): (a) on the flow velocity MF; (b) on the magnetic field strength.
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Figure 7. Behavior of non-magnetic inclusions in the MF in ESCARGOT chip with silicone walls (Figure 2c): (a) bubble flow; (b) droplet flow.
Figure 7. Behavior of non-magnetic inclusions in the MF in ESCARGOT chip with silicone walls (Figure 2c): (a) bubble flow; (b) droplet flow.
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Table
Table 1. Physical properties of the studied fluids.
Table 1. Physical properties of the studied fluids.
Fluid ParametersPES-1Mineral OilSynthetic Oil
Density ρ, kg/m3877841851
Viscosity, η, mPa∙s317435.91
Table
Table 2. Physical and chemical properties of magnetic fluid.
Table 2. Physical and chemical properties of magnetic fluid.
Fluid ParametersMF on Oil
Density ρ, kg/m3853
Viscosity, η, mPa∙s68.72
Volume concentration, φ, %0.4
Saturation magnetization, Ms, kA/m1.8
Table
Table 3. The effect of the chip wall material on the formation of droplets in the microchannel.
Table 3. The effect of the chip wall material on the formation of droplets in the microchannel.
Continuous PhaseSandwich Chip with Glass Walls (Figure 2b)ESCARGOT Chip with Silicone walls (Figure 2c)
Inverse emulsionPES-1Fluids 08 00042 i001Fluids 08 00042 i002
Mineral oilFluids 08 00042 i003Fluids 08 00042 i004
Synthetic oilFluids 08 00042 i005Fluids 08 00042 i006
Table
Table 4. The results of the experiment in obtaining a direct emulsion.
Table 4. The results of the experiment in obtaining a direct emulsion.
Continuous PhaseESCARGOT Chip with Silicone Walls (Figure 2c)
Direct emulsionPES-1Fluids 08 00042 i007
Mineral oilFluids 08 00042 i008
Synthetic oilFluids 08 00042 i009
Table
Table 5. Formation of droplets in microchannels of ESCARGOT chip with silicone walls (Figure 2c) with different scaffold thicknesses and treatment.
Table 5. Formation of droplets in microchannels of ESCARGOT chip with silicone walls (Figure 2c) with different scaffold thicknesses and treatment.
d, mmUntreated Surface of the ScaffoldsTreated Surface of the Scaffolds
0.5Fluids 08 00042 i010Fluids 08 00042 i011
1Fluids 08 00042 i012Fluids 08 00042 i013
2Fluids 08 00042 i014Fluids 08 00042 i015
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MDPI and ACS Style

Kalyuzhnaya, D.; Sokolov, E.; Vasilyeva, A.; Sutarina, I.; Ryapolov, P. Dynamics of Nonmagnetic and Magnetic Emulsions in Microchannels of Various Materials. Fluids 2023, 8, 42. https://doi.org/10.3390/fluids8020042

AMA Style

Kalyuzhnaya D, Sokolov E, Vasilyeva A, Sutarina I, Ryapolov P. Dynamics of Nonmagnetic and Magnetic Emulsions in Microchannels of Various Materials. Fluids. 2023; 8(2):42. https://doi.org/10.3390/fluids8020042

Chicago/Turabian Style

Kalyuzhnaya, Dariya, Evgeniy Sokolov, Anastasia Vasilyeva, Irina Sutarina, and Petr Ryapolov. 2023. "Dynamics of Nonmagnetic and Magnetic Emulsions in Microchannels of Various Materials" Fluids 8, no. 2: 42. https://doi.org/10.3390/fluids8020042

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