KONA Powder and Particle Journal No. 41 (2024) 183–196/https://doi.org/10.14356/kona.2024004

Review Paper

Characteristics of Ultrafine Bubbles (Bulk Nanobubbles) and
Their Application to Particle-Related Technology †Keiji Yasuda
Department of Chemical Systems Engineering, Nagoya University, Japan
Colloid of gold nanoparticles
Ultrafine bubbles (bulk nanobubbles), small bubbles with a diameterwith UFB
without UFBof less than 1 μm, have attracted academic and industrial attention
Gold nanoparticles synthesized
because of their numerous advantages, including their chemical-free
without ultrafinewith UFB
bubbles (UFB)
nature and extraordinarily long lifetime. The long lifetime is related tothe much higher Brownian motion velocity compared to buoyancy. The
reason why ultrafine bubbles can endure under stable conditions is still
Immediately after synthesis
unclear, even though their inside is highly pressured. They have several
with UFB
without UFB
characteristics, such as pH-dependent surface charge and reduced
friction. They are also closely related to ultrasound. Ultrafine bubbles
are generated and removed by selecting the ultrasonic frequency.
Reaction and separation using ultrasonic cavitation and atomization,
100 nm
100 nmAfter two days
respectively, are enhanced by ultrafine bubbles. They can produce
hollow nanoparticles, enhance adsorption on activated carbon, andclean solid surfaces. This review discusses the fundamental and ultrasonic characteristics of ultrafine bubbles and their application inparticle-related technology, encompassing fine particle synthesis, adsorption, desorption, extraction, cleaning, and prevention of fouling.
Keywords: ultrafine bubbles, bulk nanobubbles, ultrasound, sonochemistry, fine particle synthesis, cleaning

1. Introduction

Submicron-sized bubbles have attracted academic andindustrial interest lately. Although these floating bubbles ina liquid have previously been referred to as “nanobubbles”
or “bulk nanobubbles,” the International Organizationfor Standards (ISO) decided in 2017 that these bubbles
less than 1 μm in diameter be termed “ultrafine bubbles”(ISO 20480-1, 2017). Therefore, this review will refer tonanobubbles and bulk nanobubbles as ultrafine bubbles.
Furthermore, bubbles between 1 and 100 μm in diameterare termed microbubbles, and ultrafine bubbles and microbubbles are collectively termed fine bubbles.
Ultrafine bubbles and microbubbles are liquid-floating
bubbles with the entire bubble interface surrounded by liquid. Microbubbles in water appear cloudy, like milk, while
ultrafine bubbles are transparent because they do not scatter
visible light. Surface nanobubbles, on the other hand, aresmall bubbles with one side of the bubble interface in contact with liquid and the other side attached to the solid wall
(Lohse and Zhang, 2015). The surface nanobubbles arespherical caps with heights ranging from a few nanometers
to tens of nanometers and widths of about 1 μm. They do
not move from a solid wall.

†

Received 17 October 2022; Accepted 19 December 2022
J-STAGE Advance published online 25 March 2023
Add: Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
E-mail: yasuda.keiji@material.nagoya-u.ac.jp
TEL: +81-52-789-3623

The rising velocity U of fine bubbles (microbubbles andultrafine bubbles) in liquid due to buoyancy depends on the
diameter of fine bubbles d and the physical properties ofliquid and is expressed by the following Stokes equation(Stokes, 1851),U

d 2  ρ L  ρG  g
18 μ

(1)

where ρL and ρG are liquid and gas densities, respectively,
μ is the liquid viscosity, and g is the gravitational accelera(1)The Stokes equation is known to accurately representtion.
the setting motion of a solid sphere in a liquid when the
Reynolds number is less than 2. Experimental results of therising velocity of microbubbles agree with those calculatedusing Eqn. (1) (Takahashi, 2005; Lee and Kim, 2005).
However, ultrafine bubbles are subject to intense Brownian
motion, like solid colloidal particles, due to collisions
caused by the thermal motion of liquid molecules. The
following Stokes–Einstein equation expresses the displacement of spherical particles using Brownian motion at low
Reynolds numbers (Einstein, 1905).
Δx 2  Δy 2  Δz 2 

2kTt
πμd

(2)

where ⟨Δx2 + Δy2 + Δz2⟩ is the mean square displacementof the fine particles, k is the Boltzmann constant, T is the(2)temperature,and t is the observation time. Fig. 1 shows thecomparison between the rising velocity using Eqn. (1) andthe average velocity of Brownian motion per 1 s and 100 s

Copyright © 2024 The Author. Published by Hosokawa Powder Technology Foundation. This is an open accessarticle under the CC BY license (https://creativecommons.org/licenses/by/4.0/).

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by Eqn. (2) in the range of bubbles diameters from 10 nm
to 10 μm. When the temperature is 298.15 K, the liquid andgas are considered to be water and air, respectively, and the
bubbles are expected to be perfectly spherical. In the caseof ultrafine bubbles less than 100 nm, the average velocityof Brownian motion is much greater than the rising velocity. This means that the ultrafine bubbles are dominatedby random motion in all directions compared to the rising
motion. Due to this interesting behavior, the removal of ultrafine bubbles from the liquid surface is nearly negligible.
If there is no collision between the bubbles or dissolution
into the liquid, ultrafine bubbles remain for a long time.
Thus, ultrafine bubbles have various interesting properties, such as a very long lifetime in liquid, a charged

Moving velocity [nm/s]

100000
10000
1000
100
10
1
0.1
0.01

Air, Water,
298.15 K, 0.1 MPa

10

100

1000

10000

Bubble diameter [nm]Fig. 1

Velocity comparison between Brownian motion and buoyancy.

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surface, and controllable bubble concentration using ultrasound. Therefore, they have been applied in many fields,
including water treatment, medicine, and the food industry(English, 2022). This review explains the fundamental andultrasonic characteristics of ultrafine bubbles and describes
their applications to particle-related technologies.

2. Fundamental characteristics

2.1 Size, concentration, and zeta potential

Like colloidal particles in solution, the surface of ultrafine bubbles is charged. The size and concentration ofultrafine bubbles in a liquid are closely related to the zeta
potentials of ultrafine bubbles. They are mainly affectedby the pH of solution, concentration, and several types ofinorganic ions and surfactants in water.
Nirmalkar et al. (2018b) prepared air-filled ultrafine
bubbles from pure water at pH = 6.5 using hydrodynamic
cavitation methods and then changed the pH of ultrafine
bubbles water by HCl and NaOH. The size distribution andzeta potential of ultrafine bubbles were measured using
nanoparticle tracking analysis (Patois et al., 2012) andelectrophoresis, respectively. Fig. 2 shows the effects of
pH on (a) size distribution, (b) number density (concentration), (c) mean diameter, and (d) zeta potential of ultrafine
bubbles. When pH was higher than 4, the bubble size
distribution did not significantly change. However, at pH
below 4, the peak of the bubble size distribution decreased
sharply. This is reflected in the sharp drop in the bubble

a

b

c

d

Fig. 2 Effects of pH on (a) size distribution, (b) concentration, (c) mean diameter, and (d) zeta potential of ultrafine bubbles. Reproduced with permission from Ref. (Nirmalkar et al., 2018b). Copyright: (2018) The Royal Society of Chemistry.

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number concentration (density) as the pH decreased from4 to 3. The mean bubble diameter increased significantly
as the pH decreased below 4. The negative zeta potentialof bubbles approached 0 with decreasing pH and became
positive below the isoelectric point between pH 3 and 3.5.
This is because the electrostatic repulsive force between ultrafine bubbles becomes weaker, bubbles agglomerate, andthe bubble diameter increases, as the zeta potential is close
to the isoelectric point. At the same time, excessively large
bubbles rise and collapse at the gas–liquid interface, andthe bubble concentration decreases. Other researchers havereported similar results with isoelectric points between
pH 3 and 5 (Calgaroto et al., 2014; Azevedo et al., 2016;
Bui et al., 2019; Ma et al., 2022).
Effects of NaCl concentration on properties of ultrafine
bubbles have been widely reported (Millare and Basilia,
2018; Nirmalkar et al., 2018a, 2018b; Meegoda et al.,2018, 2019; Ke et al., 2019). Most of the data indicated thatas the NaCl concentration increased, the bubble concentration decreased, the mean diameter increased, and the zeta
potential approached zero. This is because sodium ions areadsorbed at the surface of the ultrafine bubble, and the zeta
potential is close to zero. In the case of AlCl3, a trivalentcation, the zeta potential first approached zero and thenchanged to positive with increasing AlCl3 concentration(Nirmalkar et al., 2018b). Hewage et al. (2021a) investigated the mean diameter and zeta potential of ultrafine
bubbles in aqueous solutions for several inorganic salts.
The addition of cetyltrimethylammonium bromide, acationic surfactant, changed the zeta potential from negative to positive, as in the case of AlCl3 (Nirmalkar et al.,2018b; Agarwal et al., 2022). On the other hand, adding
sodium dodecyl sulfate, an anionic surfactant, further decreased the zeta potential (Phan et al., 2021b). The additionof nonionic surfactants had a little significant effect on thezeta potential (Nirmalkar et al., 2018b). These experimental results of surfactant addition were explained using theadsorption of surfactants on ultrafine bubbles due to hydrophobic attraction. Effects of gas types inside ultrafine bubbles have been reported (Meegoda et al., 2018; Zhou et al.,a

Review Paper2021). For air, nitrogen, oxygen, and ozone gases, it was
explained that the bubble size increased with increasing
gas solubility in water and the zeta potential depended on
the ability of the gas to produce OH⁻ ions at the water–gas
interface (Meegoda et al., 2018).
The ultrafine bubbles in water were concentrated without loss and size change when water was evaporated fromultrafine bubble water under reduced pressure using a rotary evaporator (Tanaka et al., 2020). Additionally, ultrafine
bubbles in water were diluted by gently adding pure water
without loss and size change of ultrafine bubbles. A facile
membrane-based physical sieving method was developed
to make the ultrafine bubbles in targeting size (Zhang R.
et al., 2022b). They used several membranes consistingof six different materials with pore diameters of 0.22 and0.45 μm. The size distribution of the ultrafine bubbles
could be intentionally adjusted by selecting the membrane
and filtration rate. The ability to control the concentrationand size of ultrafine bubble will be important for futureapplication development.

2.2 Stability

The unique property of ultrafine bubbles is their unusually long lifetime. Ultrafine bubbles, once formed, arehighly persistent (Zimmerman et al., 2011) and stable fordays (Ohgaki et al., 2010; Ushikubo et al., 2010; Liu et al.,2013) and even months (Duval et al., 2012; Nirmalkar etal., 2018a; Kanematsu et al., 2020; Soyluoglu et al., 2021;
Tanaka et al., 2021b).
Tanaka et al. (2021b) generated air-filled ultrafine bubbles from ultrapure water using the pressurized dissolution
method and stored the ultrafine bubble water at 298 K for
74 days. The results showed little difference in the stabilityof ultrafine bubbles between 30 mL glass vials, 1 L glass
bottles, and 20 L high-density polyethylene containers.
Fig. 3 shows the effect of storage temperature on the stability of ultrafine bubbles in 30 mL glass vials. Althoughnumber concentration decreased with time, ultrafine bubbles still existed on day 74. This is because the translational velocity due to Brownian motion is considerablybFig. 3 Effect of storage temperature on the stability of ultrafine bubbles in 30 mL glass vials: a) number concentration and b) median diameter ofultrafine bubbles. Reprinted with permission from Ref. (Tanaka et al., 2021b). Copyright: (2021) John Wiley and Sons.

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higher than the rising velocity due to buoyancy, as shownin Fig. 1. The stability became higher as the temperature
decreased, and median diameter slightly increased withtime. They explained that the Brownian motion of ultrafine
bubbles became more active at higher temperatures andultrafine bubbles tended to aggregate more easily. They
expressed the stability of the ultrafine bubbles in terms ofa second-order flocculation rate equation. Kanematsu et al.
(2020) investigated the stability of ultrafine bubbles for 9
months at 298 K and reported that ultrafine bubbles werestable and zeta potential did not change significantly for 9
months.
Effects of additives on the long-term stability of oxygen
ultrafine bubbles were investigated for 60 days (Soyluoglu
et al., 2021). The bubble stability decreased with decreasing pH in the range of 3 to 9. The addition of Na+ and Ca2+
ions reduced the stability. This is because the zeta potentialof the ultrafine bubbles became close to 0, and the ultrafine
bubbles tended to aggregate. Furthermore, the stability
became low when the concentration of organic substances
was high, but the effect of chlorine was small.
The boundary layer structure of ultrafine bubbles has
been studied to elucidate the reason for their stability.
Ohgaki et al. (2010) used attenuated total reflectance infrared spectroscopy to analyze and reported that the surfaces of
the ultrafine bubbles contain hard hydrogen bonds that may
reduce the diffusivity of gases through the interfacial film.
Weijs et al. (2012) used molecular dynamics and concluded
that ultrafine bubbles within a cluster of bubbles protect
each other from diffusion by a shielding effect. Zhang X.
et al. (2016) performed quantum computations and Raman
spectrometric measurements. They proposed that an ultrafine bubble is more dynamically stable because the bubble
is surrounded by a layer of supersolid skin. Hirai et al.
(2019) studied the structure of ultrafine bubbles using
small- and wide-angle X-ray scattering and explained thatan ultrafine bubble is surrounded by a diffusive boundary,and the electron density differs between the diffusive
boundary and the bulk solution. Zhang R. et al. (2022a)
quantified the boundary layer thickness of the ultrafine
bubbles using solvent relaxation of nuclear magnetic resonance. For effective gas diameters of 243.5, 358.5, and412.8 nm, the determined boundary layer thicknesses were41.5, 44.8, and 35.9 nm, and the ratios of the boundary
layer thickness to the effective gas diameter were 0.17,
0.125, and 0.087, respectively.
The bubble was surrounded by water, which has highintermolecular forces, and the inner bubble was pressurized
because it is a gas with a sparse density compared to theliquid. The pressure difference Δp is expressed using theYoung–Laplace equation as follows (Attard, 2014),
Δp  pin  pout 

(3)
186

2σ
r

(3)

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Fig. 4 Dynamic equilibrium model. Reprinted with permission from
Ref. (Yasui et al., 2016). Copyright: (2016) American Chemical Society.

where pin and pout are the inner and outer pressure of the
bubble, respectively and σ is the interfacial surface tension,and r is the bubble radius. Assuming that the interfacial
tension at the macroscale and nanoscale is the same, theinternal pressure of a bubble increases with decreasing
bubble radius and becomes significantly high in the case ofultrafine bubbles. The inner pressure of bubbles becomes
2.98 MPa, when the bubble radius is 50 nm, the interfacial
tension of water is 72 mN/m, and the outer pressure of the
bubble is 0.1 MPa. The inner pressure of surface nanobubbles obtained using electrochemical measurement (German
et al., 2016) and that of ultrafine bubbles estimated fromthe measurement of gas content in water (Kim E. et al.,2020) were consistent with the theoretical pressure values
in Eqn. (3).
However, ultrafine bubbles are expected to dissolve immediately into the surrounding liquid because the pressureof the gas dissolved in a liquid is usually in the order of
0.1 MPa. The time for the complete dissolution of a bubble
with 100 nm in radius is only about 80 μs according to the
Epstein–Plesset theory (Epstein and Plesset, 1950; Yasui,
2015, 2018). Therefore, the lifetime of ultrafine bubbles
should theoretically be very short, but in practice, as previously mentioned, it is very long.
Several stabilization models of ultrafine bubbles havebeen proposed to explain this discrepancy. Yasui et al.
(2018) reviewed these models and supported the dynamic
equilibrium model (Yasui et al., 2015, 2016; Brenner andLohse, 2008; Petsev et al., 2013) because images of thetransmission electron microscope showed stable ultrafine
bubbles partly covered with hydrophobic materials inaqueous solutions (Sugano et al., 2017). Fig. 4 illustrates
the dynamic equilibrium model. This model shows that ahydrophobic substance adheres to the bubble surface, andthe gas concentrated on the surface of the hydrophobic substance flows into the bubble and is balanced by the amountof gas flowing out of the bubble in the rest of the bubble.
However, the high stability of ultrafine bubbles must be
investigated further, both experimentally and theoretically,
to obtain a consistent conclusion in the future.

2.3 Other characteristics

Other characteristics of ultrafine bubbles include aKeiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196

reduction in solution viscosity (Phan et al., 2021a), a
reduction in friction (Ushida et al., 2012a; Nakagawa et al.,2022), and the generation of radicals.
Viscosities of apple juice concentrate and canola oil
were examined after they were injected with CO2 ultrafine
bubbles with sizes ranging from 50 to 850 nm (Phan et al.,2021a). A significant viscosity reduction was observed by
up to 18 % and 10 % in apple juice concentrate and canolaoil, respectively. The pressure drop across a micro-orifice
was measured using water and glycerol aqueous solutions
containing ultrafine bubbles (Ushida et al., 2012a). The
friction reduction was observed for orifice diameters of
50 μm or less. This was because the ultrafine bubbles adhered to the wall surface, forming a gas phase that resulted
in wall slip. The reduction of friction factor in an acrylic
pipe with glass beads increased with an increasing number
of ultrafine bubbles (Nakagawa et al., 2022). They explained that ultrafine bubbles filled more undulated spaceand made the rough surface smoother.
Free radicals, including hydroxyl radicals, were detectedin ultrafine bubble water (Minamikawa et al., 2015; Liuet al., 2016a, b; Atkinson et al., 2019). Free radicals are
believed to be formed through the collapse of ultrafine
bubbles (Takahashi et al., 2021). Soyluoglu et al. (2021)
reported that hydroxyl radicals in oxygen ultrafine bubble
solutions were detected at pH 3 but were little detected at
pH 6.5. Yasui et al. (2018) discussed the generation possibility of hydroxyl radicals from air ultrafine bubbles using
numerical simulations. They suggested that hydroxyl radicals formed due to hydrodynamic cavitation in producing
ultrafine bubbles.

3. Ultrasonic characteristics
3.1 Ultrasonic cavitation

When water is irradiated with ultrasound, fine bubbles
are generated from bubble nuclei, grow to a resonancesize through rectified diffusion (Ashokkumar et al., 2007;
Crum, 1980) and collapse by semi-adiabatic compression(Leighton, 1994; Yasui, 2015). This phenomenon is called
ultrasonic cavitation. By collapsing fine bubbles, the field
inside the fine bubbles becomes high temperature and pressure, generating various radical species, which produceschemical effects (Suslick, 1990). Simultaneously, thecollapse of fine bubbles generates jet flow and shock wave
(Lauterborn and Ohl, 1997), producing mechanical effects.
Examples of the chemical effects include the decomposition of harmful substances (Pétrier et al., 1994;
Yasuda, 2021) and the synthesis of metal nanoparticles(Gedanken, 2004; Sakai et al., 2009). However, examplesof the mechanical effects include ultrasonic cleaning(Muthukumaran et al., 2005; Mason, 2016), ultrasonicemulsification (Beal and Skauen, 1955; Yasuda et al.,2012), and ultrasonic atomization (Wood and Loomis,
1927; Sato et al., 2001). Mechanical effects have been used

Review Paper

to develop commercial products such as cleaners, homogenizers, and atomizers. Usually, plate-type transducers areused for ultrasonic cleaners and atomizers, while horn-typetransducers are used for ultrasonic homogenizers. Thus,
ultrasound is closely related to ultrafine bubbles, and manystudies using ultrasound have been reported.

3.2 Generation and removal of ultrafine bubbles

Ultrafine bubbles were generated using ultrasoniccleaner at 42 kHz (Kim J.-Y. et al., 2000) and ultrasonichomogenizer at 20 kHz (Cho et al., 2005; Nirmalkar et al.,2018a, 2019; Li et al., 2021; Lee and Kim, 2022b; Ma etal., 2022). Their generation rate increased with increased
dissolved air content in pure water (Nirmalkar et al.,2018a). Studies have used several plate-type transducers
to change ultrasonic frequencies ranging from 20 kHz to
1 MHz and to generate ultrafine bubbles in ultrapure water(Yasuda et al., 2019). Figs. 5(a) and (b) show the effectof ultrasonic frequency on the generation rate and mode
diameter of ultrafine bubbles. The generation rate increased
with decreasing frequency because cavitation collapse
became stronger at lower frequencies. However, the mode
diameters of ultrafine bubbles were 90–100 nm regardlessof the frequency. The diameter, zeta potential, and stabilityof ultrafine bubbles generated using ultrasound are almost
identical to those generated by hydrodynamic cavitation(Jadhav and Brigou, 2020), which is a commonly usedmethod. The ultrasound method has the advantage ofsmall sample volumes and the ability to generate ultrafine
bubbles in variety of liquids, including organic solvents(Nirmalkar et al., 2019), inorganic solutions (Ma et al.,2022), and surfactant solutions (Lee and Kim, 2022a).
Removing ultrafine bubbles from liquid media is important to distinguish them from nanoparticles. The freeze–
thawing method is often used to remove ultrafine bubbles
in pure water. However, this method little removes ultrafine
bubbles in the surfactant solution (Nirmalkar et al., 2018a),and it is time-consuming. Ultrasound has been irradiated atvarious frequencies to highly concentrated ultrafine bubble
water produced using the pressurized dissolution method(Yasuda et al., 2019). Fig. 5(c) shows the effect of ultrasonic frequency on the removal rate of ultrafine bubbles.
The removal rate increased with increasing frequency because the secondary Bjerknes force, an attraction between
bubbles, increased. We modeled the formation and removal
of ultrafine bubbles using ultrasound and discovered thatultrafine bubbles reach a frequency-dependent equilibrium
concentration (Fig. 5(d)), regardless of the ultrasoundintensity. Tanaka et al. (2021a) confirmed the ultrasonicremoval of ultrafine bubbles using indirect irradiation. The
ultrasonic frequency was 1.6 MHz, and ultrafine bubbles
dispersions were prepared using two different bubble generator methods: pressurized dissolution and swirling liquid
flow. Results showed that the indirect ultrasonic irradiation

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Keiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196

1.0
0.8

Mode diameter [nm]22 kHz Ultrapure water, 15 W
43 kHz129 kHz488 kHz
1 MHz

0.6
0.4
0.2
0.0

0

5

10

15

20

25

3.5

Ultrafine bubble water, 15 W

2.5
2.0

22 kHz43 kHz129 kHz488 kHz
1 MHz

1.5
1.0
0.5
0.0

0

5

10

15

20

25

Irradiation time [min]

30

5 min
30 minUltrapure water

100

100

1000

Frequency [kHz]

d

×109

3.0

150

50
10

30

Irradiation time [min]cNumber concentration [mL-1]

b

×109

1.2

Equilibrium concentration [mL-1]

Number concentration [mL-1]

a

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2.0

× 109

1.5
1.0
0.5
0.0
10

100

1000

Ultrasonic Frequency [kHz]Fig. 5 Effect of ultrasonic frequency on (a) generation rate, (b) mode diameter, (c) removal rate, and (d) equilibrium concentrations of ultrafine
bubbles.

for 30 min reduced the number concentration of ultrafine
bubbles by 90 % regardless of the generation method. Feasibility studies were also conducted for ultrafine bubbles in
an aqueous surfactant solution or solid particle dispersion.
The ultrasonic method selectively destabilized ultrafine
bubbles in the solutions. Ultrafine bubble removal using
ultrasound has been adopted as an international standard
by the International Organization for Standardization (ISO24261-2, 2021) because of its simplicity and quickness ofoperation.

3.3 Sonochemical characteristics

Sonochemistry is a field in chemistry, physics, andbiology related to ultrasonic cavitation. Some reports haveshown that ultrafine bubbles promote ultrasonic cavitation(Tuziuti et al., 2020, 2021; Tsuchida et al., 2022). Sonoluminescence, which is light emission from a liquid due toultrasonic cavitation (Brenner et al., 2002; Ashokkumar,2011), was enhanced by ultrafine bubbles when ultrasoundwas applied at 54 kHz with high intensity (Tuziuti et al.,2020). This phenomenon was observed for various bubble
concentrations (Tuziuti et al., 2021). They considered thatultrafine bubbles had the potential to provide nucleationsites for ultrasonic cavitation. The sonochemical degradation of acid orange was accelerated when the microbubble
solution was irradiated with ultrasound at 45 kHz (Tsuchida
et al., 2022). They concluded that ultrafine bubbles played
an important role in improving sonochemical reactions.

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An in-situ sediment remediation method using a combination of ultrasound and ozone ultrafine bubbles wasinvestigated to remediate both organic (p-terphenyl) andinorganic (chromium) materials in contaminated sedimentsof the lower Passaic River in United States (Batagoda etal., 2019; Hewage et al., 2020, 2021b). The ozone ultrafine
bubbles increased the solubility of ozone in water andreduced wastage. Also, the high-ozone concentration inwater also increases chromium oxidation (Batagoda etal., 2019). This proposed treatment method showed sufficient remediation success with removing these combined
contaminants, on average, 60 % and 71 % for p-terphenyl
and chromium, respectively. The chromium removal was
directly influenced by the chemical oxidation and sonication for chromium desorption from sediments. However,p-terphenyl degradation was more likely influenced by the
combined effects of chemical oxidation and ultrasoundassisted pyrolysis (Hewage et al., 2021b). Dark green Rit
dye was effectively decolored by combining ultrafine bubbles and ultrasonic irradiation (Bui and Han, 2020). The
color removal mechanisms were due to the electrostatic
attraction between the ultrafine bubble and dye and theoxidation of the dye using reactive species such as hydroxy
radicals generated through ultrasonic cavitation.
Ultrasonic atomization generates a mist, such as fine
droplets at room temperature (Wood and Loomis, 1927;
Yasuda et al., 2011). When an aqueous ethanol solution is
atomized using ultrasound, the ethanol is enriched in theKeiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196

mist (Sato et al., 2001; Yasuda et al., 2003; Matsuura et al.,2007). Ethanol enrichment is attributed to the hydrophobic
interaction of ethanol molecules in water (Yasuda et al.,2014). Separation using ultrasonic atomization has also
been reported on ketones (Yasuda et al., 2004), amino acids(Suzuki et al., 2006), surfactants (Jimmy et al., 2008), fineparticles (Nii and Oka, 2014), and ion liquid (Mai et al.,2019). The advantages of ultrasonic atomization separation
include simple operation, maintenance-free, and suitability
for heat sensitive materials. Ultrafine bubbles water wasused as a solvent to improve the enrichment performanceusing ultrasonic atomization (Yasuda et al., 2020b; 2022).
The ethanol concentration in the collected mist with ultrafine bubbles was higher than that without ultrafine bubbles(Yasuda et al., 2020b). When the ethanol concentrationin the solution was low and the carrier gas velocity was
slow, the enrichment effect of ultrafine bubbles was particularly remarkable. Next, the enrichment characteristics ofamino acids were investigated using ultrasonic atomization(Yasuda et al., 2022). Fig. 6 shows the effect of the concentration of amino acids in solution on the enrichment factor
with and without ultrafine bubbles for L-phenylalanine andL-tyrosine. Here, the ratio of the solute concentration in
the mist to that in the solution is defined as the enrichment
factor. The enrichment factor increased with decreasing
solution concentration and was enhanced by adding ultrafine bubbles. The authors explained the reason for thisEnrichment factor [-]13(a) Phenylalanine

11□ with ultrafine bubbles■ without ultrafine bubbles

9
7
5
3
1

0

0.2

0.4

Enrichment factor [-]

13

0.6

0.8

1

(b) Tyrosine

11〇 with ultrafine bubbles● without ultrafine bubbles

9
7
5
3
1

0

0.02

0.04

0.06

0.08

0.1

0.12

Solute concentration in solution [mmol/L]
Fig. 6 Effect of solute concentration in solution on enrichment factor
with and without ultrafine bubbles for (a) phenylalanine and (b) tyrosine.

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enhancement as follows. Amino acid bonds to the surface
of ultrafine bubbles in a solution because of their hydrophobic affinity. Under ultrasonic fields, ultrafine bubbles
containing amino acids aggregate or coalesce by secondary
Bjerknes force (Leighton, 1994), rise in the solution bybuoyancy force and burst at the solution surface. The concentration of amino acids increases at the solution surface,and many amino acids are contained in the droplets.
Freezing of foods is a complicated and time-consuming
process that requires the aggregation of molecules froma liquid to a solid crystal network (Kiani and Sun, 2011).
The influences of CO2 ultrafine bubbles and ultrasoundon the crystallization behavior of water during freezingof model sugar solutions were examined (Adhikari et al.,2019). The combination treatment facilitated in reducing
the supercooling degree, resulting in faster nucleation
during the freezing process. This method may improve thecharacteristics of frozen products such as ice cream andfrozen desserts.

4. Application to particle-related technologies
4.1 Fine particle synthesis

Synthesis of fine particles using ultrafine bubbles has
been reported in several papers. Authors (Yasuda et al.,2020a) used HAuCl4 aqueous solutions and synthesizedgold nanoparticles by ultrasonic irradiation at 500 kHz
with the aid of ultrafine bubbles without any capping andreducing agents. Figs. 7(a) and (b) show the electron micrographs and size distribution of gold nanoparticles synthesized with and without ultrafine bubbles. The particle
sizes prepared with ultrafine bubbles were much smaller
than those of the gold nanoparticles prepared without
ultrafine bubbles. The mean diameter of the spherical gold
nanoparticles synthesized with ultrafine bubbles decreased
with increasing concentration of ultrafine bubbles. This isbecause ultrafine bubbles accelerate the sonochemical reduction of gold ions and suppress the aggregation between
gold nanoparticles. Moreover, the gold nanoparticles werestable in a solution containing ultrafine bubbles (Fig. 7(c))
because gold nanoparticles were electrostatically adsorbed
on ultrafine bubbles (Fig. 7(d)), which have a very long
lifetime in water.
Nanocomposites of CaCO3 and pulp fiber were preparedby injecting ultrafine bubbles of CO2 into an aqueous
Ca(OH)2 solution containing pulp fibers (Fuchise-Fukuoka
et al., 2020). The precipitated CaCO3 nanoparticles formedon the pulp fiber surfaces were more stably attached tothe pulp fiber surfaces against shear force. Comparedwith mixing method, the specific surface area and surface
smoothness of nanosized CaCO3-containing handsheets
increased. Water containing ultrafine bubbles of hydrogenat high concentrations was used as the blending solvent for
preparing cement mortar (Kim W.-K. et al., 2021). The cement mortar had greater flexural and compressive strengths

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Keiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196

aReview Paperb

100 nmRelative number [-]without UFB

0.2

without UFB

0.1

d
UFB

UFB

0

100 nmcwithout UFB

0.3

UFB

without UFB

UFB

0

50

[Au(I)Cl2]−
AuNP
＋

＋

AuNP
＋

AuNP
＋

UFB

[Au(I)Cl2]−＋＋

After two days

200

AuNP
Electrostatic repulsion
AuNP
AuNP
between UFB＋

Immediately after synthesis

100
150
Diameter [nm]

＋

AuNP

＋

＋

＋

AuNP
AuNP

AuNP

＋
＋

＋

UFB

Fig. 7 Effect of ultrafine bubbles (UFB) on sonochemical synthesis and colloidal stability of gold nanoparticles: (a) Electron micrograph and (b)the size distribution of particles; (c) Photograph of colloids immediately and 2 days after synthesis; (d) Stabilization mechanism by ultrafine bubbles.

than plain water-based mortars. This was because ultrafine
bubbles enhanced the collision between the solvent andcement particles, accelerating the hydration and pozzolanic
reactions.
Jadhav and Brigou (2020) formed the hollow zinc phosphate nanoparticles using ultrafine bubbles as shown in
Fig. 8. First, zinc nitrate was added to the ultrafine bubble
water to adsorb Zn2+ ions on the negatively charged surface
of ultrafine bubbles. Next, by adding diammonium phosphate, the ultrafine bubbles served as nucleation sites forthe subsequent reaction of Zn2+ with PO43– ions. Finally,
the pH of the solution was adjusted to 8.5 to precipitate zinc
phosphate on the ultrafine bubble surfaces, forming hollow
particles. The advantages of using ultrafine bubbles for fineparticle synthesis are safety, low environmental loads, andno impurities in fine particles. Ultrafine bubbles, in particular, are optimal for creating fine hollow fine particles since
there is no need to remove the core templates, which are
generally employed with solid or liquid particles.

a

b

c

4.2 Adsorption, desorption, and extraction

Studies have been reported on the use of ultrafine
bubbles to promote adsorption onto particles, as well as
desorption and extraction from particles. The effect of ultrafine bubbles on the adsorption of lead ions onto activatedcarbon was investigated with and without agitation (Kyzas
et al., 2019; 2020). In the case of agitation, the effect ofultrafine bubbles on the adsorption capacity of lead ions
was small. However, in the case without agitation, the190

Fig. 8 Images of transmission electron microscopy of hollow zinc
phosphate nanoparticles obtained by encapsulation of ultrafine bubbles
shown on different scales: (a) 1000 nm; (b) 200 nm; (c) schematic
representation of the formation of the hollow nanoparticles. Reprinted
with permission from Ref. (Jadhav and Brigou, 2020). Copyright: (2020)
American Chemical Society.

Keiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196

adsorption capacity in the presence of ultrafine bubbles became much higher than that in the absence of ultrafine bubbles. Moreover, ultrafine bubbles largely accelerated theadsorption rate with and without agitation. They explainedthat the ultrafine bubbles accepted positively charged leadions onto their interface and assisted the diffusion and penetration phenomena of lead ions into the activated carbon
pores. The influence of ultrafine bubbles on the adsorptionof sodium oleate on calcite surface was examined (Wang etal., 2019). Ultrafine bubbles had a certain inhibitory effecton the adsorption of sodium oleate on the calcite surface,
and it significantly promoted the flocculation between particles of sodium oleate.
Hydrogen ultrafine bubbles were applied to soils contaminated with copper to desorb copper (Kim and Han,
2020). Ultrafine bubbles improved the desorption of copper. Additionally, when the ultrafine bubbles were used asdesorption enhancers in electrokinetic experiments, they
had better remediation effects than distilled water.
Ettoumi et al. (2022) generated ultrafine bubbles in
ion liquid using compression–decompression method andextracted polyphenols from Chinese Hickory. Compared tothe extract using ion liquid without ultrafine bubbles, theextract of this method showed significantly higher antioxidant activity and polyphenol yields. Electron micrographs
confirmed that nanojets due to ultrafine bubbles caused
morphological destruction of the husk powder. Their groupalso extracted phytochemical compounds from Camellia
Oleifera shells using water with CO2 ultrafine bubbles(Javed et al., 2022). Compared with ethanol extraction,
ultrafine bubble water led to a higher extraction yield of
total phenolic and flavonoid content. Collagen was extracted from tilapia scales by bubbling ultrafine bubbles ofthree different gases (O2, CO2, and O3) into an acetic acidsolution (Kuwahara, 2021). Using CO2 in the acetic acidsolution was the most effective method to obtain collagenin a relatively high yield. The ultrafine bubble method is a
simple, mild, cost-effective, and environmentally friendly
method for adsorption onto particles, as well as desorption
and extraction from particles.

4.3 Cleaning and prevention of fouling

The most typical application of ultrafine bubbles iscleaning. There are several reports on the use of ultrafine
bubbles to remove contaminants from solid surfaces. Minute particles with a diameter of about 50 nm were successfully removed from the wafer surfaces by impinging a jetof ultrapure water containing ultrafine bubbles (Morimatsu
et al., 2004). Removal of protein from the solid–liquidinterface was investigated using a quartz crystal microbalance to evaluate the removal amount (Liu et al., 2008; Liuand Craig, 2009). Bovine serum albumin was completely
removed from both hydrophobic and hydrophilic surfaces
using ultrafine bubbles. The cleaning efficiency compared

Review Paperto sodium dodecyl sulfate, a typical surfactant (Liu et al.,2008). Lysozyme was completely removed from hydrophobic surfaces by combing ultrafine bubbles and sodium dodecyl sulfate (Liu and Craig, 2009). Ushida et al. (2012b)
investigated the washing rate of cloth in an alternating flow
system. The ultrafine bubble water achieved a washingrate greater than that of ion-exchanged water. Moreover,the mixture with ultrafine bubbles and anionic surfactant
exhibited a washing rate higher than that of an aqueous
anionic surfactant solution without ultrafine bubbles. The
ceramic membrane completely clogged by humic acidfouling was effectively cleaned using ultrafine bubbles(Ghadimkhani et al., 2016). Dyed cotton towels were
washed without soap using oxygen-rich ultrafine bubbles
and with soap using the conventional method (Anis et al.,2022). The lowest color strength and highest fastness wereobtained after washing with ultrafine bubbles. Chemical
oxygen demand measurements of washing baths revealed
that ultrafine bubble washing was more environmentallyfriendly than conventional methods.
Terasaka (2021) investigated the cleaning process of salt
adhering to wall surfaces when ultrafine bubble water was
passed through a sample surface at a constant flow rate.
The cleaning rate was determined using the dissolved salt
concentration in water. Fig. 9(a) shows the progress of thecleaning rate when “hard deposits,” such as salts, were
washed with running water, as opposed to paste-like “softdeposits,” such as biofilm and starch. The cleaning process
consists of adhesion, dissolution, peeling, and dissolution.
The starting time of the peeling step is promoted by ultrafine bubbles, and consequently, the cleaning is completed
in a shorter time with ultrafine bubble water. Fig. 9(b)shows a hypothesis of the cleaning mechanism. It explainsthat the cleaning mechanism using ultrafine bubbles uses
the high permeability of ultrafine bubbles accompanying
water and the metastability of ultrafine bubbles that shift
to a stable state when the specific potential is exceeded. In
2011, West Nippon Expressway Company Limited started
cleaning toilets in expressway service stations and parking
areas using ultrafine bubbles. Ultrafine bubbles have beensuccessfully used to remove salts from bridge surfaces.
The necessary time for removing salt from the surface has
shortened to less than one-fourth using a water jet containing ultrafine bubbles (Yabe, 2021).
Ultrafine bubbles were found to prevent surface foulingof bovine serum albumin (Wu et al., 2008) and lysozyme(Zhu et al., 2016). Farid et al. (2022) demonstrated thebenefits of introducing ultrafine bubbles into the feed foralleviating flux reduction and membrane pore wetting
associated with mineral scaling in membrane distillation.
Introducing ultrafine bubbles reduced membrane scaling
propensity and offered excellent resistance to pore wetting
associated with scaling. This is attributed to high surface
shear forces due to flow turbulence and electrostatic191

Keiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196a(1) Adhesion(2) Dissolution(3) Peeling

Review Paper(4) Dissolution

100

Cleaning rate β [%]

(4)

(3)

0

(2)

(1)

0

tUFB

t0
Cleaning time t

b
Salt

Salt

Salt
Flat plate

Flat plate

Flat plate

UFB water flows over thesolid salt adhering flat plate.

UFB water permeates into
narrow gap and UFBs attachat the plate and the salt.

Local dissolved gas
concentration in the gap
is increasing.

MicrobubbleThe plate is cleaned by chain
peeling.

MicrobubbleSalt

Flat plate

Flat plate

By expanding microbubbles,
the adhered salt is lifted andpeeled off from the plate.

UFBs are growing tomicrobubbles by gas
absorption in the gap.

Fig. 9 (a) Cleaning process of salt adhering to wall surface with ultrafine bubble water and (b) hypothesis of removal mechanism of wall-attached salt
by flowing ultrafine bubble water. Reprinted with permission from Ref. (Terasaka, 2021). Copyright: (2021) Jenny Stanford Publishing.

attractions between the negatively charged ultrafine bubbles and the counterions. The potential use of ultrafine
bubbles as corrosion and scale inhibitors for geothermal
power plants was discussed (Kioka and Nakagawa, 2021).
The crystal growth rate of calcite, which causes scaling,
was found to be more retarded as the concentration of ultrafine bubbles increased (Tagomori et al., 2022). Ultrafine
bubbles inhibit the corrosion of low-carbon steels, with an
inhibition efficiency of 20–50 % in the acidic geothermal
fluid (Aikawa et al., 2021). They concluded that ultrafine
bubbles could act as a nanoscopic coating material in
mitigating corrosion, increasing the slip length on the solidinterface and preventing exposing the interface to acidic
geothermal fluids by (1) behaving as a bubble mattresscovering most of the steel surface or (2) promoting nucleation and aggregation of an insignificant quantity of silica
precipitation on the steel surface. Compared to commonlyused chemical products, ultrafine bubbles have shown to192

be a potent, chemically benign, environmentally friendly,
and inexpensive corrosion and scale inhibitor of calcium
carbonate.

5. Conclusions

This review discussed the fundamental and ultrasoniccharacteristics of ultrafine bubbles (bulk nanobubbles) andintroduced their application to particle-related technology.
The diameter of ultrafine bubbles is less than 1 µm, andthe bubble surface is highly charged. The rising velocity
due to buoyancy is much smaller than the Brownian motion velocity. The diameter and concentration are closely
related to zeta potential. The diameter, concentration,and zeta potential of ultrafine bubbles depend on pH of
solution, concentration, and several types of inorganic ionsand surfactants in water. Additionally, the concentration
can be increased using a rotary evaporator. They are stablein water for several months; however, there is no modelKeiji Yasuda / KONA Powder and Particle Journal No. 41 (2024) 183–196

to fully explain the stabilization mechanism. They formedhydroxy radicals at strong acid conditions and reduced infriction in the orifice.
Ultrafine bubbles can be generated and removed using ultrasonic irradiation at low and high frequencies,
respectively. Chemical and physical effects of ultrasoniccavitation, such as luminescence and degradation, were enhanced by ultrafine bubbles. It was considered that ultrafine
bubbles became the nucleation of cavitation. Separation
performance of ethanol and amino acids from aqueous
solutions using ultrasonic atomization was improved byultrafine bubbles.
Ultrafine bubbles have been applied to fine particle
synthesis, adsorption, desorption, extraction, cleaning, andprevention of fouling. Fine gold nanoparticles, CaCO3/pulp
fiber nanocomposites, and hollow zinc phosphate nanoparticles were synthesized by ultrafine bubbles. Adsorption oflead ions to activated carbon and desorption of copper fromsoil were enhanced by ultrafine bubbles due to electrostaticinteraction between ultrafine bubbles and metal ions. Bubbling CO2 ultrafine bubbles effectively extracted collagenfrom tilapia scales. Ultrafine bubble water without surfactant efficiently cleaned the protein, dye, and salt on solid
surfaces and prevented surface fouling. Thus, ultrafine
bubbles have been shown to be powerful, environmentallyfriendly, safe, clean, simple, and inexpensive methods.
Currently, ultrafine bubbles have many theoretically
unexplained characteristics, such as high stability andchargeability, and a great deal of academic research is still
being conducted. Clarifying the various characteristics ofultrafine bubbles is expected to lead to academic development and further social and industrial diffusion of ultrafine
bubbles through applied research, such as developing new
applications in various fields and reducing production
costs.

Acknowledgments

This work was partly supported by JSPS KAKENHI
Grant Numbers JP16H04560 and JP19H02505.

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Review Paper

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Author’s Short Biography
Keiji Yasuda
Keiji Yasuda is currently an Associate Professor of the Department of Chemical Systems Engineering, Nagoya University. He received Doctor degree of Engineering from Nagoya University in
1997. His research interests are in the area of fundamentals of fine bubbles and sonochemistry, andtheir application to material and environmental fields.

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