P13
音響場で浮遊する多成分液滴の蒸発に伴う自然乳化・相分離Emulsification and phase separation associated withevaporation of multicomponent droplets in acoustic
levitation〇光野海祥 1, 長谷川浩司 2〇Misaki MITSUNO1, Koji HASEGAWA212

工学院大学大学院工学研究科機械工学専攻，Graduate School of Engineering, Kogakuin University
工学院大学工学部機械工学科，Department of Mechanical Engineering, Kogakuin University

1. Introduction
In recent years, contactless sample manipulation technology in midair has been attracting great attention
in the fields of biochemistry and pharmaceutical science. One of the promising methods is the acoustic
levitation 1). This method enables us to levitate, transport, mix, and evaporation of samples in resonant acoustic
fields 2). The acoustic levitation method is also expected to be applied in microgravity environments, where it
can levitate larger sample with lower acoustic pressure than that in 1G 3). However, it has been pointed outthat this method causes nonlinear and dynamic behaviors such as unsteady translational motion 4) andevaporation behavior 5) in the levitated sample. Although there are many previous studies on the evaporation
behavior of one or two component droplets, experimental studies on the evaporation behavior of three or morecomponent droplets are still in the development stage. In this study, we aim to understand the evaporation
dynamics of spontaneous emulsification and phase separation of Ouzo droplets 6) premixed with water,
ethanol and anise oil upon evaporation. To achieve this, the spontaneous emulsification and phase separation
process of Ouzo droplets upon evaporation was visualized and analyzed using a droplet evaporation model.
2. Experimental method
Figure 1 (a) shows a schematic diagram of the experimental setup used in this study. A sinusoidal signal
generated by a function generator is input to the ultrasonic transducer. The ultrasonic wave is emitted fromthe tip of the lower horn connected to the ultrasonic transducer and is reflected by the upper reflector to form
an acoustic standing wave between the horn and the reflector as shown in Figure 1 (b). A liquid droplet isinjected using a syringe near the acoustic pressure node of the acoustic standing wave to levitate a droplet. A
high-speed camera and IR camera are used to capture the droplet motion using the backlight method and thesurface temperature of levitated droplets. The obtained images are processed using a computer and in-housecode.
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The reflector shape used in this experiment has a radius of curvature of 36 mm (R36). The frequency is
19.3 kHz and the wavelength λ is 18 mm. The width of the horn and reflector is 36 mm (≈ 2λ). The distance
between the horn and reflector is 48 mm (≈ 5λ/2). The ambient temperature is 23±2°C and the humidity is 55±5%.

(a)

(b)

Figure 1 Schematic of acoustic levitator: (a) experimental setup and (b) levitation principle.

3. Results and discussions
Figure 2 (a) shows the time series of Ouzo droplets with the ethanol concentration of 70%. Figure 2 (b)shows the evaporation process of Ouzo droplets. The left and right vertical axes represent normalized surface
area (𝑑 ⁄𝑑0 )2 and aspect ratio, respectively. 𝑑 and 𝑑0 are the instantaneous and the initial droplet diameter.
The highlighted area in Fig. 2 (b) indicates the time at which each droplet observed the Ouzo effect. From theresults, (𝑑 ⁄𝑑0 )2 decreased by approximately 50% in 200 seconds. With respect to the Ouzo effect, weconfirmed that each droplet emulsified for 20 to 30 seconds. When comparing the droplet diameter and aspectratio, both sets of experimental data demonstrated similar decreasing trends. The aspect ratio asymptoticallyapproached a spherical shape as the droplets evaporated.

(a) Snapshot of Ouzo droplet during evaporation(b) Time evolution of (𝒅⁄𝒅𝟎 )𝟐 and As

Figure 2 Evaporation process of Ouzo droplets (ethanol: 70 vol%, water: 29 vol%, anise oil: 1 vol%).
2 of 7Figure 3 (a) shows the evaporation process for ethanol concentrations of 50%, 70%, and 90%, with thediameter on the vertical axis. In contrast, Figure 3 (b) shows a normalized result with (𝑑 ⁄𝑑0 )2 for the verticalaxis and the Fourier number for the horizontal axis. The Fourier number can be expressed by the following
equation.
𝐹𝑜 =𝐷𝑡(1)

𝑑0 2

where 𝑡 is time and 𝐷 is the diffusion coefficient. The diffusion coefficient of ethanol was used in the presentstudy. These results illustrate a gradual change in droplet size during the evaporation depending on theethanol concentration. Notably, Figure 3 (b) shows the experimental data for evaporating droplets with thesame concentration converged when using nondimensional values.

(a) dimensional form(b) non-dimensional form

Figure 3 Evaporation process of Ouzo droplets for different concentration.

Figure 4 shows the start/end time (Ouzo time) and duration time of the Ouzo effect. Figure 4 (a) showsthe effect of the ethanol concentration on Ouzo time. The start time was defined as the moment theemulsification appeared in the droplet, and the end time was when it disappeared, as observed throughvisualization. Figure 4 (b) shows the the effect of the ethanol concentration on the duration time of the Ouzoeffect. These results represent that the Ouzo effect occurred during droplet levitation when the ethanolconcentration was between 40% and 90%. We also confirmed that the higher ethanol concentration delayed
the onset time and shortened the duration of the Ouzo effect.
These results indicate that there are four distinct phases of evaporation of levitated ouzo dropletsillustrated in Figure 5: 1) preferential evaporation of ethanol, 2) onset of the Ouzo effect, 3) generation of coreshell structure due to evaporation of ethanol component, and 4) Remaining presence of only oil droplets due
to evaporation of water component. These findings can facilitate our understanding of evaporation and phaseseparation for levitated ternary droplets, paving the way for contactless droplet manipulation. For a deeper
understanding of the evaporation and phase separation process, the instantaneous concentration condition for
the Ouzo effect needs to be explored based on the experimental results and evaporation model.
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(a) Start and end time of Ouzo effect (anise oil: 1 vol%)(b) Duration time of Ouzo effectFigure 4 Characteristic time of Ouzo effect associated with evaporation.

Figure 5 Evaporation, spontaneous emulsification and phase separation process of Ouzo droplet in acoustic levitation.

Therefore, the next step is to estimate the concentration of each component in evaporating Ouzo droplets.
Here we predict the concentration of ethanol, water, and anise oil components based on the experimental
results. The mass fraction of each component can be obtained by equations (2) - (4).
𝑌𝑒 =𝑚𝑒𝑚𝑒 + 𝑚𝑤 + 𝑚𝐴

(2)

𝑌𝑤 =𝑚𝑤𝑚𝑒 + 𝑚𝑤 + 𝑚𝐴

(3)

𝑌𝐴 =𝑚𝐴𝑚𝑒 + 𝑚𝑤 + 𝑚𝐴

(4)

where 𝑌 is the mass fraction, 𝑒 is ethanol, 𝑤 is water, 𝐴 is anise oil, and 𝑚 is droplet mass. The masses
shown here can be calculated by finding the theoretical droplet diameter by classical the droplet evaporation
model (well-known d2-law7)). d2-law can predict the droplet evaporation by considering the droplet masstransport by diffusion widely used in estimates of single droplet evaporation. d2-law is expressed by equation(5).

4 of 7𝑑 2 = 𝑑0 2 −8𝐷𝑀 𝑝𝑠 𝑝∞( − )𝑡𝜌𝑙 𝑅 𝑇𝑠 𝑇∞(5)

where 𝑀 is molar mass, 𝜌𝑙 is liquid density, 𝑅 is gas constant, 𝑃 is vapor pressure, 𝑇 is temperature and𝑡 is time. This evaporation model was used to obtain the theoretical droplet diameter 𝑑𝑡ℎ as described inequations (6) and (7). With the theoretical droplet diameter, we can estimate the mass of the ethanol component
in the evaporating droplet with equation (8).
𝑑𝑡ℎ = √𝑑0 2 − 𝛽𝑡(6)8𝐷𝑀 𝑝𝑠 𝑝∞( − )𝜌𝑙 𝑅 𝑇𝑠 𝑇∞(7)4𝑑𝑡ℎ 3𝑚𝑒 = 𝜌𝑒 𝜋 ( )32(8)𝛽=Meanwhile, the instantaneous droplet mass can be quantified from the experimental data with the equation(9). The mass of water and non-volatile anise oil was calculated by each volume obtained from the droplet
diameter and each density as expressed by the equations (10) and (11, where 𝑉 is droplet volume.. Inacoustically levitated droplets, water vapor from the ambient air can condense on the droplet surface overtime. As a result, the mass of the water in the evaporating droplet can be estimated by the difference between
the experimental and theoretical diameters.

𝑚𝑒𝑥𝑝 =4𝑑 3
𝜌𝑒𝑥𝑝 𝜋 ( )32(9)𝑚𝑤 = (𝑚𝑒𝑥𝑝 − 𝑚𝑒 ) + 𝜌𝑤 𝑉𝑤(10)𝑚𝐴 = 𝜌𝐴 𝑉𝐴(11)

Figure 6 shows the results of concentration estimation of Ouzo droplets with 70% ethanol concentration.
Figures 6 (a)-(c) depict the time evolutions of normalized droplet surface area, the mass fraction in the droplet,
and the droplet surface temperature. The highlighted area in Figure 6 (b) indicates the time at which the Ouzo
effect occurred. In Figure 6 (b), the concentration of water rises over time. This is attributed to the condensationof surrounding water vapor as the ethanol evaporates. The surface temperature of the droplet interface plays
a role in facilitating this water condensation. The droplet surface temperature was a few degrees colder than
the ambient air, causing a temperature gradient that promotes condensation.
Onset condition of Ouzo effect and three-component phase diagram are shown in Figure 7. In Figure 7(a), the concentration of the anise oil was constant at 1%. In Figure 7 (b), 〇, △, □ represent the results for
1%, 5%, 10% anise oil. The results confirm that with an increase in anise oil concentration, the onset of theOuzo effect was observed at higher ethanol concentrations. We used the three-component phase diagram toclarify the onset region of the Ouzo effect in acoustic levitation. In our future study, we aim to incorporate ourexperimental findings into this three-component phase diagram for a more systematic comparison and indepth investigation.
5 of 7(a) Evaporation process(b) Mass fraction in the droplet(c) Droplet surface temperature
Figure 6 Evaporation process, mass fraction, and surface temperatu of Ouzo droplet.
(ethanol: 70 vol%, water: 29 vol%, anise oil: 1 vol%)(a) Onset condition of Ouzo effect with different ethanol concentration(b) Phase diagram for Ouzo effectFigure 7 Onset condition of Ouzo effect and three-component phase diagram.

4. Conclusions
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The aim of this study is to understand the evaporation dynamics of spontaneous emulsification and phaseseparation associated with evaporation of Ouzo droplets. To achieve this, we visualized and quantified thespontaneous emulsification and phase separation processes using the droplet evaporation model. The
obtained results are summarized as follows:
1)

The Ouzo effect was observable even in a non-contact condition via an acoustic field.

2)

The Ouzo effect occurred when the ethanol component ranged between 40% and 90%. It was also
observed that a higher ethanol concentration delayed the onset of the Ouzo effect and reduced its
duration.

3)

Through experimental data, we explored evaporation, spontaneous emulsification, and phase separationof Ouzo droplets in an acoustic field, indicating a four-step process.

4)

We were able to estimate the concentration of droplets at which the Ouzo effect occurs. The threecomponent phase diagram of water, ethanol, and anise oil was also created to show the region where theOuzo effect occurs.

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© 2023 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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