Separation of Oil from an Oil/Water Mixed Drop under a Lamb Wave Field: A Review

Abstract

Oil separation from oil/water mixed drop under a Lamb wave field is one of the emerging acoustofluidic technologies that integrate acoustics and microfluidics. In recent years, this technology has attracted significant attention due to its effective, fast, contactless, and pollution-free. It has been validated in the separation of oil/water mixture on different non-piezoelectric substrates and shows great potential in incompatible liquids applications. Here, we summarize our recent progress in this exciting field and show great potential in different applications. This review introduces the theories and mechanisms of oil/water mixed drop separation induced by Lamb waves, the applications of this technology in the separation of oil/water mixed drop, and discusses the challenges and prospects of this field.

1. Introduction

With the rapid development of modern industrial technology, environmental protection is also facing more threats, such as the indiscriminate discharge of industrial oily sewage, serious chemical pollution, large oil leakage, resource shortage, and many other problems, which make the oil/water separation method have received corresponding attention and research [1,2,3,4]. Over these years, there have been many approaches to separating oil/water mixture. The most traditional method is to heat the oil/water mixture [5]. According to the different physical properties of oil and water, such as boiling points [6], gravitation, or lyophobicities [7]. During the heating process of the mixture, due to the low boiling point of water, water first becomes water vapor and separates from oil. According to the difference in gravitation, the oil/water mixture can be separated into two layers. The corrugated plate oil/water separation technology, which organically combines gravity separation and coalescence separation, is highly feasible and effective and has long been paid attention [8,9]. In addition, a wide variety of mesh structures have been developed to separate two immiscible liquids based on their different lyophobicities [10,11,12,13,14], such as carbon nanotubes [15], nanostructured hydrogel coatings [16,17], or TiO₂ nanowires [18,19,20]. According to hydrophobicity, there are also many materials that can achieve separation, such as superhydrophobic magnetic reduced graphene oxide-decorated foam [21,22] and superhydrophobic aerogels [23,24,25,26]. Aerogel is a highly absorbent material that is widely used in large-scale oil/water separation because it can achieve greater benefits through simple steps. Traditional aerogels are composed of inorganic materials, and their preparation process and application effect are not ideal [27]. Yang et al. [28] proposed a more environmentally friendly bio-based aerogel, which created a vision for the future development of aerogel. Later, it was found that the oil/water mixture could also be separated by filtration using metal mesh [29], natural textile [30], organic synthetic polymer membrane [31,32], carbon base membrane [33], cellulose-based membrane material [34,35] and other materials. Liu et al. [36] attempted to pretreat the cotton fiber surface with NaOH aqueous solution first, then coat a layer of SiO₂ film nanoparticles by sol-gel process. The prepared cotton completely repels water while selectively absorbing various oils, and can reach up to 50 times the weight of its own cotton. Qing et al. [37] prepared mixed polyvinyl alcohol (PVA)/PTFE nanofiber membrane which was treated by electrospinning, and left pure PTFE nanofiber membrane after sintering, revealing the formation mechanism of the membrane and verifying the effective separation effect of the membrane. However, according to their large mesh sizes which the super-wetting filtration membranes cannot be used for various emulsions.

Covalent organic skeletons (COFs) are generally used as coating materials because of their strong chemical stability but extremely complex preparation process. Wang et al. [38] found a way to produce COF/GO hybrid materials without using any special equipment. During the preparation process, II.SERP-COF2-β COF particles with high hydrophobicity and acid-resistance and alkalinity are grown on GO sheets through a covalent connection, further broadening the application of COFs in oil/water separation. Based on the surface modification technologies, various nano-materials and nanotechnologies have been widely applied to separate the oil/water mixtures with super wetting surfaces [39,40,41,42], and superhydrophobic/superoleophilic and underwater superoleophobic materials [43,44,45,46,47] belonging to these materials. The surface with a water contact angle greater than 150° and an oil contact angle close to 0° is called superhydrophobic-superoleophilic surface [48,49,50]. This means that the oil in the oil/water mixture will easily penetrate the membrane and isolate the water at the same time, thus achieving efficient separation. For example, Liu et al. [51] manufactured a strong magnetic response super hydrophobic polyurethane (PU) sponge. Layered double hydroxide (LDH) and iron Fe₄O₃ nanoparticles are fixed to the sponge surface, so the sponge exhibits excellent repeatability and separability in the test. Yang et al. [52] proposed an innovative method to prepare a bionic super-hydrophobic eggbeater structure with controllable surface morphology based on the 3D printing surface that can display super-hydrophobicity and structure-dependent water adhesion. It is also a highly efficient separation method due to the different interfacial effects of oil and water. However, conventional approaches are usually applied to separate a large number of mixtures, and these approaches have been proven not to be effective methods for micron-sized oil/water separation.

Therefore, many researchers spend a lot of time looking for a simple method that is easy to install into an ordinary device. Luo et al [53] squeezed the oil/water mixture droplets with two non-parallel plates to separate the oil/water-mixed drops by squeezing the mixture, but the coating of the plate should meet critical requirements. Since acoustofluidics is an emerging technology introduced by Henrik Bruus in 2008 [54], he made significant contributions to the theories of acoustofluidic separation [55,56,57]. The acoustofluidic method can separate different particles based on their physical properties, such as size [58], density [59], and compressibility [60]. It is considered a promising technology to separate and manipulate cells [61,62] because of its label-free [63], contactless [64], and biocompatible features [65]. Jin et al. [66] found that using the one-dimensional SSAW generated by two parallel digital converters (IDT), the application of acoustic flow on large particles will induce greater axial force so that different particles will laterally move to different areas in the cross-section of the channel to separate particles. At the same time, it was found that the force generated by the magnetic field can also realize the separation of cells and control the movement of droplets [67,68]. Shi et al. [69] changed the magnetic field gradient by changing the solenoid coil. In the experiment, the CCD camera was used to photograph the instantaneous change of the liquid drop and studied the change of the ferromagnetic liquid droplet under different gradient sizes. In recent years, surface acoustic waves (SAWs) have been applied to drive and manipulate microfluidic flow [70], such as Rayleigh [71,72,73,74] and Lamb waves [75,76,77]. With its inherent technical characteristics, the surface acoustic wave has been widely concerned and promoted in the field of microfluidics [78], such as acoustic microreactors [79], microseparators [80], and micropumps [81]. However, these technologies are rarely used in liquid-liquid separation. Some researchers try to use the Lamb waves to separate an immiscible liquid mixture, and interestingly, it works.

To sum up, there are many approaches that can separate the oil/water mixture, especially the Lamb waves, which could separate the emulsion mixture of microliter volume. The purpose of this work is to provide a selective overview of oil/water separation under the external Lamb wave field and provide more systematic and complete knowledge of separation theory and mechanism, to help readers, especially those who are new to this field, to understand this technology more comprehensively.

2. Theoretical Analysis

The droplet motion of Lamb waves satisfies the Navier-Stokes equation of the laminar incompressible fluid with a constant viscosity, which is expressed as

$$\rho \frac{D{\nu }_1}{Dt}=\rho F-\nabla p+\mu {\nabla }^2{\nu }_1$$

(1)

$$\nabla \cdot {\nu }_1=0$$

(2)

where ρ is the constant equilibrium density, ${\nu }_1$ is the acoustic streaming velocity, p is the steady state pressure, F is the driving force of acoustic streaming and μ is the shear viscosity respectively [82,83].

As is shown in Figure 1, by analyzing the forces applied on the oil/water droplet, they are an acoustic streaming force with a dominant function in the droplet motion, the force of gravity along the X-axis, and the viscous force between oil/water and substrate which hold back from dropping down. In the work, finding a law of motion of oil/water separation induced by Lamb wave on an inclined plate was the fundamental destination.

In addition, the acoustic streaming force could be derived from the acoustic streaming theory, and the equation was given by

$$-F=\rho [<{\nu }_2\cdot \nabla >{\nu }_2+<\nabla \cdot v_2>],$$

(3)

where, $v_2$ is the first-order wave velocity, and the symbol $\langle X\rangle$ indicates “the time average of X” [84,85,86,87,88].

According to the article written by Shiokawa et al. which substitute the velocity formula into Equation (3), the streaming force of Lamb wave is given by:

$$F_{\mathrm{sx}}=-\rho \left(1+{\alpha }_1^2\right)A^2{\omega }^2{k}_{imag}exp2\left(k_{imag}x+{\alpha }_1{k}_{imag}z\right),$$

(4)

$$F_{sz}=-\rho \left(1+{\alpha }_1^2\right)A^2{\omega }^2{k}_{imag}{\alpha }_{1}exp2\left(k_{imag}x+{\alpha }_1{k}_{imag}z\right),$$

(5)

where ${\alpha }_1=-j\alpha$, the streaming force can be calculated by $F_S=\sqrt{F_X{}^2+F_Z{}^2}$ and is given as [82]:

$$F_S=-\rho {(1+{\alpha }_1{}^2)}^{3/2}{A}^2{\omega }^2{k}_i\exp 2(k_{i}x+{\alpha }_1{k}_{i}z)$$

(6)

where A is the amplitude of the Lamb wave, $\omega$ is the angular frequency, $\alpha$ is an attenuation constant, and $k_i$ is the coefficient of the energy dissipation. The acoustic streaming force obtained above acts as a body force near the interaction area between the droplet and substrate, with the direction being at the same angle as the radiation of the leaky Lamb wave. Moreover, the acoustic radiation of Lamb wave during the propagation will appear through the diffraction of waves at an angle of ${\theta }_L$, which is called Lamb angle or Rayleigh angle. Accordingly, the Lamb angle ${\theta }_{{\mathrm{L}}_{-}\mathrm{Water}}$ and ${\theta }_{{\mathrm{L}}_{-}\mathrm{Oil}}$ in water drop and oil drop can be calculated as ${\theta }_{{\mathrm{L}}_{-}\mathrm{Water}}={\sin }^{-1}\left(v_{f_{-}\mathrm{Water}}/v_s\right)\approx 42°$${\theta }_{\mathrm{L}{-}_{\mathrm{Oil}}}={\sin }^{-1}\left(v_{f_{-}\mathrm{Oil}}/v_s\right)\approx 41°$. Therefore, ${\theta }_{{\mathrm{L}}_{-}\mathrm{Water}}>{\theta }_{{\mathrm{L}}_{-}\mathrm{Oil}}$, the direction of acoustic streaming force in the water drop is slightly more forward in the x-axis than that of an oil drop [89].

Assume at the entrance points of both oil and water drops in the propagating of Lamb waves are identical, with the same angular frequency $\omega$ and the same amplitude A. The mass density of water and oil is 1000 kg/m³ and 918 kg/m³. The leaky wave number of mirror/water and mirror/oil is 95 m⁻¹ and 70 m⁻¹, respectively, which are calculated by the process of Rose [90] and Lowe [91], and could also be derived from the method of Jiao et al. [92]. Therefore, the acoustic streaming forces in oil and water drops according to Equation (6) can be obtained as below:

$$F_{S_{-}\mathrm{Oil}}=-2.9\times {10}^5{A}^2{\omega }^2\mathit{exp}2\left(70x+92.8z\right)$$

(7)

$$F_{S_{-}\mathrm{Water}}=-4.1\times {10}^5{A}^2{\omega }^2\mathit{exp}2\left(95x+121.3z\right)$$

(8)

The exponential function exp (x) is an increasing function. The droplets at each point (x, z) are in the first quadrant, which will be the case as the following inequality is satisfied: $\exp 2\left(95x+121.6z\right)>\exp 2\left(70x+92.8z\right)$, and eventually $\left|F_{{\mathrm{S}}_{-}\mathrm{Water}}|>|F_{{\mathrm{S}}_{-}\mathrm{Oil}}\right|$ [89].

In addition, the impeding force indicated by $F_r$ between the substrate and oil/water mixture should be considered, which is given by [93,94]:

$$F_r=k\gamma L\left(\cos {\theta }_r-\cos {\theta }_a\right)$$

(9)

where k is the dimensionless constant determined by the experiment, γ is the liquid-vapor interfacial tension, L is the length scale related to the drop, θa and θr are the advancing and receding angles, respectively.

According to the formulas deduced, apparently, the force equilibrium equation of droplets can be established based on the consociation setting by Equations (4)–(6) and (9).

$$F_X+mg\sin \alpha -F_r=m{a}_X$$

(10)

$$F_Z-mg\cos \alpha =m{a}_Z$$

(11)

It could be found from Equations (10) and (11) that there are three main force impact upon the droplet motion, which are acoustic streaming force, droplet gravity, and inclination angle of substrate [93].

3. Experimental Requirements

3.1. Experimental Devices

The experiment is based on the Lamb wave device platform, which contains a DC power supply, a function waveform generator, an own-made amplification circuit unit, a micropipette, an oscilloscope, a high-speed camera, a DSA25 droplet contact angle measuring instrument and two single-phase transducers (SPTs) which are produced by PI Ceramics. The voltage range that DC-regulated power supply can provide is 0–30 V. The function waveform generator generates a continuous square electrical signal of 1MHz frequency. With the self-made amplifier applied to both ends of the SPTs which are glued on the substrate, Lamb waves are generated [95].

The experiment is illustrated by the example of a 1 mm thick mirror plate as the substrate. SPT generates a continuous square electric signal with a frequency of 1 MHz on the function waveform generator. The SPT is attached to the mirror plate, which can excite the Lamb waves to derive the oil/water mixture separation. The linear relationship between the input power (VPP) on the SPT and wave amplitude A was estimated in Figure 2 by using the laser-Doppler vibrometer (Polytec OFV-5000). Wave’s amplitude is changed by changing input power. Furthermore, the acoustic streaming force, which is the main force of the separation, is directly determined by the wave’s amplitude.

3.2. Experimental Approach

Cover a thin layer of hydrophobic coating on the surface of the mirror base to make the contact angle of the liquid drop reach 90°. Lamb wave is excited by SPT with a frequency of 1 MHz, a phase velocity of 2641 m/s, and a wavelength of 2.6 mm, which is determined by the electrode structure. Place water and oil droplets on the substrate (Figure 3a,b).

In this work, three approaches have been applied to form an oil/water mixed droplet on the substrate. Three approaches differ in the location or the order that oil and water droplets are placed on the substrate. In the first approach, a water drop is placed on the top of oil drop (Figure 3c). In the second approach, oil and water drops are placed on the substrate side by side (Figure 3d), while in the third method, an oil drop is directly put on top of a water drop (Figure 3e). It is obvious to observe the experimental phenomena that the oil/water mixed droplet in the first method is similar to the third method. Experimental phenomena demonstrate that the structure of a mixture does not mainly depend on the procedure of dropping droplets. In this work, the first approach is used as producing an oil/water mixed droplet.

3.3. Experimental Procedure

Figure 4 shows the process of the oil droplet being separated from an oil/water mixture. The oil/water mixed drop is placed on the plate (Figure 4a). When Lamb waves propagate on the substrate, the oil/water mixed drop can be propelled (Figure 4b). After that, the oil/water mixed drop accelerates along the substrate surface. The phenomenon as observed that oil/water mixture will be separated until in a critical separation time (Figure 4c). With the acoustic streaming force and the gravity of oil/water mixture effect on the inclined substrate, the oil is separated from an oil/water mixture (Figure 4d).

4. Experimental Results and Discussions

A series of experiments have been carried out. It is obvious to observe the phenomenon that the resulting oil/water mixture is separated on the inclined mirror substrate by using Lamb wave. With the help of the micropipettes we measure 10 μL olive oil and 10 μL, 20 μL, and 30 μL water to be mixed one by one, which form oil/water volume ratios from 1:1 to 1:3. And the mirror substrate inclination is placed in 10°, 15°, and 20° in this work. The excitation voltage of the SPT has been increased from 138 VPP to 194 VPP for eight measurements. Each measurement was repeated 10 times and the average values were taken for comparison.

In addition, the oil/water mixture obtained acceleration along the x-axis and would slide along the substrate when the Lamb wave was applied for this experiment. The major factors which influence the experiment results, such as the acoustic streaming force, the mixture gravity, the component of gravitational force along the x-axis, input power VPP, the plate inclination α, and the oil/water volume ratio V.

4.1. Effect of Input Power

Oil has been separated from an oil/water mixture on the substrate. The input power effects the oil/water mixture separation time with different plate inclination angles and oil/water volume ratios (shown in Figure 5). From (Figure 5a,b). It is found that the critical separation time decreases with the increase of input power when the volume ratio of oil to water and the inclination angle of substrate is fixed. For this reason, the oil/water mixture separation time significantly relates to the input power (VPP).

4.2. Effect of Inclination Angle

As shown in Figure 6, the separation time of the oil/water mixture extremely depends on the inclination angle. When the oil/water volume ratio and the input power are fixed, the larger the inclination angle, the faster the separation. Furthermore, for different input power, the trend lines are similar.

4.3. Effect of Oil and Water Volume Ratio

The relationship between the oil/water mixture separation time and the oil/water volume ratios varied from 1:1 to 1:3 with the substrate inclination angles are 15° and 20° has been explored in (Figure 7). According to the pictures with the same input power (Vpp), no matter what the inclination angles are 15° and 20°, the mixture separation time is the longest with oil/water volume ratio of 1:1, the mixture separation time is the shortest which the oil/water volume ratio is 1:3, and it can be seen that the mixture separation time with oil/water volume ratio 1:2 locates in the middle (Figure 7a,b). It is obvious to show that under a certain input power and a certain inclination angle, as the oil/water volume ratio becomes smaller and smaller, the separation time becomes shorter and shorter.

4.4. Separation Experiment of Other Non-Piezoelectric Substrates

In addition to mirrors, other non-piezoelectric substrates can also achieve the phenomenon of oil/water separation. For example, oil/water separation on a glass substrate (shown in Figure 8), as validated by experimental results, the actuation of acoustic streaming in liquid by Lamb waves allows rapid and non-contact separation of mixed drops in particular on a inclined non-piezoelectric substrate.

The same results are obtained in the test on the steel substrate (shown in Figure 9). From what has been discussed above, both experimental results show that there are three main factors that influence the oil-water separation, which are peak-to-peak voltage on the SPT, oil-water ratio, and plate inclination angle.

5. Conclusions and Perspective

In this review, we summarized the theories and mechanisms of oil/water separation under a Lamb wave field. This separation method is based on different materials of liquids, which could absorb different energy from the Lamb waves, because the attenuation of the waves for different materials is different. Therefore, the velocities of the droplet movement for different liquids are different, one moves forward and the other one drops behind. Since the droplets are incompatible, two incompatible droplets of different materials could be separated through this method. From the experimental results of oil/water separation on different substrates (mirror, glass and steel), it is found that the separation on glass is the easiest which could be separated in a fastest separation time with a lowest input power, the separation on the mirror is the most difficult and the separation on steel is in between in these three cases. Similar to the mirror, glass, and steel as the substrate, there are a mass of existing materials in our life or in nature, which can be easily obtained. The applications of this method have been demonstrated in the microliter-scaled oil/water separation fields. While this technology has the advantages of being contactless, relatively simple to operate, inexpensive, new challenges and opportunities coexist in future environmental improvement, basic research, and commercialization.

On the other hand, the volume of the droplet affects the power absorption of the drop. However, it is not that the bigger the droplet, the more power the droplet absorbs. Some researchers found that the effective actuation wavelength is around 1/5 of the droplet radius, at which the droplet absorbs the maximum incident surface acoustic wave energy [92]. It is also worth mentioning that the defects of the acoustofluidic approach are closely related to field configuration and material properties and need further investigations. Although the oil/water separation induced by acoustofluidic technology has shown great efficiency, the volume increase of the microscopic environment to macroscopic cases in certain applications would be another considerable challenge.