Source : Nanotech Japan Bulletin 2010

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Source : Nanotech Japan Bulletin 2010

Green Nanotechnology: Special Topic 6.

Nanobubbles with hidden Potential
---

Interview with Dr.Masayoshi Takahashi, Senior
Researcher at National Institute of Advanced Industrial Science and Technology


Bubbles are common and familiar to us and are used for marine aquacul
ture (oysters, scallops, pearls, etc.), water
purification (in dams and reservoirs, etc.), and the improvement of oxygen
-
deficient seabeds in fishing zones. Moreover,
it has recently become clear that small bubbles have a potential for new applications tha
t will have a great impact on our
daily life. Bubbles can be used to purify water, improve the vitality of fish, animals, and plants, and could contribute to
solving problems associated with biology, medicine, and food in the future. We visited Dr. Masayos
hi Takahashi, chief
researcher of the Environmental Fluid Engineering Group at the Research Institute for Environmental Management
Technology, the National Institute of Advanced Industrial Science and Technology (AIST), who has developed a method
to produc
e ultrafine bubbles (nanobubbles), clarified their various characteristics, and publicized useful information
during his 10
-
years research on small bubbles. We have interviewed him about the content and applications of
technologies using nanobubbles and th
eir future prospects.




Nanobubbles have been introduced in newspapers and on the Internet as "magic" bubbles with a diameter of one ten
-
thousandth of a millimeter or less


enabling the coexistence of sea bream (a saltwater fish) and
koi

carp (a freshw
ater
fish) in the same aquarium. Many visitors were astonished by this "magic" aquarium, exhibited at the 2005 Aichi EXPO
in Japan. It was the result of a peculiar characteristic of ultrafine bubbles (nanobubbles) less than one ten
-
thousandth of a
millimet
er in diameter. Dr. Takahashi, senior researcher at AIST, has investigated the mechanism behind this magical
effect and is devoting considerable effort to apply nanobubbles as a technology that will be truly useful in our society. To
understand nanobubbles
, we must also understand various characteristics of microbubbles; so we firstly asked Dr.
Takahashi to explain the details of microbubbles.

1.Overall Picture of Bubbles


Table 1 summarizes the classification of fine bubbles according to the definition given by Dr. Takahashi from which we
can obtain an overall idea of the size and properties of bubbles[1].

Normal
-
sized bubbles rapidly moved upward in water, and burst and di
sappear at the water surface, as shown in Fig.1.
This phenomenon has been used to increase the oxygen concentration dissolved in water, which is known as aeration.

Microbubbles, on the other hand, shrink by the dissolution of gas within them, and finally d
isappear when the gas is
completely dissolved because of their low rise speed and large specific surface to volume ratio. The disappearance of
microbubbles involves two important phenomena. One is an increase in the pressure of the gas inside microbubbles,

and
the other is the condensation of electric charges (ions) on the surface of the microbubbles. Microbubbles are used in
various fields, including the environment improvement, cleaning, and agricultural, marine, and chemical industries.

Nanobubbles were
considered to exist only for a short time at the final stage of disappearance when microbubbles were
collapsed. However, it has been clarified recently that nanobubbles stably exist in water containing an appropriate
concentration of electrolytes, because
ions accumulate on the surface of shrinking bubbles and have an effect to stop the
shrinkage temporarily. Using this phenomenon as a guideline, researchers at AIST and REO Research Institute generated
microbubbles in water containing electrolytes such as N
a, Mg, Fe, and Mn, and subsequently broke the microbubbles by
applying physical stimulus to them. Thus, they succeeded the stable generation of long
-
lasting nanobubbles for the first
time in the world[2][3][4]. Such water containing nanobubbles, called fun
ctional water, can invigorate animals and plants.
Dr. Takahashi himself is still amazed at the significance of the properties and effectiveness of functional water.

Table 1

Classification of small bubbles and their production methods, properties, and appl
ications.




Fig.1

Characteristics of microbubbles.

2.Generation of Microbubbles and Their Characteristics


(1)Method to generate microbubbles


We can generate microbubbles by refoaming the gas dissolved in water under pressure (pressure
dissolution/decompression method: Fig.2(
a)) or by dispersing double phases of gas and liquid through rotation (gas
-
liquid
double
-
phase rotation method: Fig.2(b)). The distribution of the diameter of the microbubbles generated by the gas
-
liquid
double
-
phase rotation method (for low concentrations
) is shown in Fig.3(b), whereas that in the case of the pressure
dissolution/decompression method is shown in Fig.3(a) (The reason why two peaks appear at approximately 10 and 50
μ
m is not known).



(a) Pressure dissolution/decompression method. (b) Gas
-
l
iquid double
-
phase rotation method.

Fig.2

Method to generate microbubbles.




(a) Pressure dissolution/decompression method. (b) Gas
-
liquid double
-
phase rotation

Fig.3

Bubble
-
size distribution.

(2)Speed of ascent of microbubbles


We can obtain the speed of ascent of microbubbles in water by observing, using a microscop
e, by which microbubbles
introduced into a small transparent cell under the condition of no inner convection (Fig.4). Figure 5 shows the speed of
ascent of microbubbles in distilled water at room temperature under atmospheric pressure. The result is in goo
d
agreement with Stokes' sedimentation law (although ascending rather than sedimentation is considered in this case).



Fig.4(left)

Methods of measuring speed of ascent of microbubbles and
ζ

potential.

Fig.5(right)

Speed of ascent of bubbles.

(3)Self
-
pres
surization effect of microbubbles


The inside of microbubbles

with a gas
-
liquid interface is self
-
pressurized owing to the effect of the surface tension of
shrinking water, given by the Young
-
Laplace equation
Δ
P = 4
σ
/D
, where
Δ
P,
σ
, and
D

are the increase in pressure, the
surface tension, and the diameter of the
microbubble, respectively.
Δ
P

is 0.3 atm for D=10
μ
m and 3 atm for D=1
μ
m,
and becomes infinitely large when microbubbles disappear (Fig.6).

The gas in the self
-
pressurized microbubbles is highly soluble in water according to Henry's law(where the amount o
f gas
soluble in water is proportional to the pressure of the gas). As the diameter of the microbubbles slowly decreases, the
internal pressure of the microbubbles further increases thus, accelerating the shrinkage rate. Finally, microbubbles of 1
µm or le
ss in diameter are completely dissolved and disappear instantaneously.


Fig.6(left)

Increase in interior gas pressure.

Fig.7(right)

ζ

potential and diameter of microbubbles.

(4)Surface potential (
ζ

potential) characteristics of microbubbles [5]


Figure 4 also shows a method of measuring the
ζ

potential of microbub
bles. As shown in Fig.7, the
ζ

potential is
constant regardless of the microbubble diameter (i.e., the electric charge per unit area of the interface is constant
regardless of the microbubble size). Also, the
ζ

potential greatly differs depending on the pr
operty of the water. In
particular, it is significantly affected by the pH level (Fig.8). How are microbubbles charged? A water cluster (a network
of hydrogen bonds) comprises water molecules (H
2
O) and small amounts of H
+

and OH
-

ionized from H
2
O. The H
+

a
nd
OH
-

tend to concentrate at the interface of microbubbles, and thus an ion density exhibits higher than that of the bulk (the
water itself), which causes the interface to be electrically charged. The interface is negatively charged under normal pH
condit
ions because OH
-

tends to accumulate on the interface more easily than H
+
. Positive ions are therefore attracted
around the negatively charged interface and electrically stabilize the interface.


Fig.8(left)

The relationship between
ζ

potential and pH of the water.

Fig.9(right)

Change in
ζ

potential during shrinkage of microbubbles.

(5)Natural collapse of microbubbles: Generation of free radicals [5],[6],[7]


The collapse is a term used in ultrasonic engineering. Upon the irradiation of an ultrasonic wave into water, the sound
pressure fluctuates, dur
ing which bubble’s cavitation are generated under negative pressure and then rapidly compressed
by the subsequent high
-
pressure wave. The pressure in the bubbles increases in inverse proportion to their diameter;
therefore, the abrupt reduction bubble size

results in the collapse of the bubble disappear (collapse). The rapid increase in
the pressure of bubbles during shrinkage is due to adiabatic compression when the rate of increase is high, which also
causes the temperature in the bubbles to rapidly incre
ase. This results in the formation of a region with a temperature of
several thousand centigrade under a pressure of several thousand atm at the point where the bubble disappears. This
ultimate reaction site (hot spot) is limited to a very small region but

the reaction is sufficiently strong to decompose
internal gas molecules, generating free radicals such as •OH with strong oxidizing power. Free radicals are also generated
in the course of natural collapse of microbubbles by hydrodynamic engineering, alth
ough its mechanism is different from
that of the collapse from ultrasonic engineering[2][3][4].

Figure 9 shows temporal changes in the diameter and
ζ

potential of microbubbles during shrinkage. The
ζ

potential tends
to increase with decreasing bubble diame
ter. As discussed in Fig.6, the microbubbles shrinkage rate increases as they
become smaller. These two results reveal that the
ζ

potential changes negligibly during the stage of slow shrinkage and
starts to increase when the diameter is decreased to appro
ximately 15
μ
m or less, and this increase rate is accelerated
with decreasing bubble diameter. Importantly, the increase in the
ζ

potential indicates that electric charges (ions) become
condensed on the gas
-
liquid interface of microbubbles during their shr
inkage. That is, excessive ions become unable to
escape into the liquid when the microbubbles shrinkage rate increases.

According to Dr. Takahashi, electric charges reach to an ultrahigh
-
density state region with high energy at the moment
the microbubbles
disappear. As soon as microbubbles collapse (their interface disappears), the condensed ions are
released, i.e., energy is released, and the surrounding H
2
O molecules are accordingly forced to split and to generate •OH
free radicals. However, the relations
hip between the condensed ions and the radicals has not yet been clarified and much
more theoretical research must be carried out. Dr. Takahashi has, also, investigated the generation of microbubbles, as
shown in Fig.10 where the bubbles were sampled in a
beaker, and confirmed the generation of •OH free radicals from the
microbubbles by electron spin resonance (ESR) using a spin trap agent (Fig.11).


Fig.10(left)

Experimental setup of radical generation.

Fig.11(right)

ESR spectrum indicating the generation of •OH (Hydroxyl radical).

3.Nanobubbles


The followings are the method used by AIST and REO Research Institute to generate long
-
lasting stable nanobubbles for
the first time in the world[2][3][4].


(1)Microbubbles 10
-
50 µm in diameter are generated in an aqueous solution, into whic
h electrolytes including Na, Mg,
Fe, and Mn ions are added so that the electric conductivity is 3 mS/cm or higher. These ions are attracted to the
negatively charged surface of the microbubbles (Fig.12(1)).


(2)Physical stimuli are then applied to the wat
er containing microbubbles, by applying a shock wave through an electric
discharge in water, forcing the water through small holes of a punching plate to rapidly shrink the microbubbles. Electric
charges (ions) on the surface of the microbubbles cannot dis
perse into the solution during this rapid shrinkage, causing
the local charge density to sharply increase (Fig.12(2)). An electrostatic repulsive force acts among ions with the same
polarity outside of the interface, which starts to disturb the shrinkage o
f the microbubbles.


(3)The shrinkage rate of microbubbles tends to increase with decreasing size because they are pressurized by the surface
tension of the solution. For bubbles with a diameter smaller than 500 nm, the electrostatic repulsive force becom
es
significant and is sufficiently applied to the microbubbles to stabilize the bubbles by balancing the shrinking and
repulsive forces (Fig.12(3)). The diameter of the stabilized nanobubbles, which is dependent on the type of electrolyte
and the ion conce
ntration, is roughly 50
-
500 nm.


(4)In addition, an inorganic shell, which is mainly composed of electrolytic ions such as Fe, and formed around
nanobubbles by the effect of the condensed, high electric field, prevents the dispersion of the internal gas(t
he salting
-
out
phenomenon). This also greatly contributes to the stabilization of nanobubbles (Fig.13).


Fig.12

Generation and stabilization of nanobubbles.




Fig.13

Stabilization mechanism of nanobubble (schematic illustration).

Unlike surfactants or

organic shells, the above
-
mentioned inorganic shell of nanobubbles easily collapses when it comes
into contact with foreign substances such as bacteria, because of the resultant dispersion of electric charges around the
nanobubbles. The gas in the nanobub
bles is readily released into the solution upon the collapse of the shells. Depending
on the environmental conditions, free radicals that exhibit strong oxidative power, such as •OH, may also be generated.


"We predicted the collapse of microbubbles after

observing the changes in electric charges. We confirmed this
phenomenon from the results of an actual measurement of radicals and succeeded in defining the phenomenon. However,
there is a researcher who applied this phenomenon before us to wastewater trea
tment and even succeeded in producing
the nanobubbles introduced here. He is Kaneo Chiba, a researcher at REO Research Institute (Higashi Matsushima,
Miyagi). I respect him very much for his great insight because he established such technologies from his o
wn experience,
independent of theories. Since then, the AIST and REO Research Institute have cooperatingly developed and applied the
technologies related to nanobubbles in various fields from the viewpoints of both theory and practice under a joint
researc
h agreement," said Dr. Takahashi. Examples of such technologies are introduced below.

4.Various Nanobubbles and Their Potential for Applications


4.1 Oxygen nanobubbles


Oxygen nanobubbles can be generated by the production of oxygen micro
bubbles in water containing nearly 1% NaCl
followed by jetting the water towards a punching plate so that it passes through small holes in it.[8] "Oxygen
nanobubbles have a mysterious function that can invigorate living organisms. We have been clarifying t
he mechanism
behind such a phenomenon occurring between nanobubbles and living organisms but still need more time to clarify the
mechanism." said Dr. Takahashi. Let us now focus on the enigmatic findings obtained from his experiments.

(1) Coexistence of fr
eshwater and seawater fishes in one aquarium

It is possible to breed
koi

carp and sea bream together for several months in the same water tank containing 1% salt with
oxygen nanobubbles (Fig.14). Neither fish could survive without the nanobubbles, even th
ough the salt concentration of 1%
is almost equal to the electrolyte concentration of their body fluid. In general, red sea bream is vulnerable to changes in
salt concentration, and
koi

carp also has difficulty in surviving in water containing 1% salt. Gol
dfish, however, are not
vulnerable to changes in salt concentration.


Fig.14(left)

Coexistence of freshwater and seawater fish in a water tank.

Fig.15(right)

Phalaenopsis orchid in water.


"The fish in this tank would die from oxygen deficiency if we stop
ped regularly bubbling air through the water. This
suggested to us that oxygen nanobubbles act not directly on respiratory and metabolic systems but through a different
mechanism. We then tried to elucidate the mechanism with support from biological and me
dical researchers, and released,
jointly with REO Research Institute, a report on the experiment of the water tank with nanobubbles to the press, aiming to
promote the applications and development of nanobubbles[8]. We received many inquiries from research
ers in various
fields including medicine. We replied to them by providing a sample of water to use freely. Most of them failed to obtain
significant results from the sample, but some obtained excellent achievements by appropriately applying the sample to a

specific purpose at their site," said Dr. Takahashi. He also introduced the following examples to us.


(i) Yukihiro Hojo, Jichi Medical University in Tochigi: Effect of nanobubbles on vascular endothelium (Nanobubbles can
markedly suppress skin inflammat
ion and tumors in blood vessels and are expected to be used for the treatment of
arteriosclerosis and cardiac infarction).

(ii) Yoshihiro Mano, Professor Emeritus of Tokyo Medical and Dental University: Suppression of proliferation of
squamous cancer usin
g nanobubbles[9][10]. (Oxygen nanobubbles suppressed the adhesion of activated macrophages
onto endotherial cells in cellular
-
level experiments.)

(iii) Masashi Goto, Tohoku University: Application of oxygen nanobubbles to the storage of the islet of the p
ancreas.
(This study originated from the following experimental finding: Oysters cultured in water containing oxygen nanobubbles
were frozen at
-
20°C for 48 h. All of the oysters were successfully revived when defrosted at room temperature after 24 h,
but
died after 48 h. We hope that this result will be effective for storing body organs.)

(2)
Phalaenopsis

orchid in water Great care must be taken when watering orchids. Researchers, however, succeeded in
keeping an orchid in bloom for several weeks even when it was soaked in water in its flowerpot, owing to the presence of
oxygen nanobubbles (Fig.15).

4.2 Oz
one nanobubbles


Ozone nanobubbles are generated by collapsing, for
example, ozone microbubbles in underground water sampled from
coastal areas (containing about 1% electrolytes such as NaCl)[4]. The bactericidal power of conventional ozonated water
is effective only for a few hours, whereas water containing ozone nanobubb
les retains its power for months without
significant deterioration if it is preserved under UV
-
blocked conditions. The excellent bactericidal power is shown using
the example of an oyster. The surface of an oyster was sterilized using conventional ozonated

water and hypochlorous
acid, but the bacteria inside its body were not sterilized. For an oyster left for 8 h in water containing ozone nanobubbles,

on the other hand, bacteria such as norovirus were eliminated from the oyster's body while the oyster was
alive (Fig.16).
This sterilizing technology has already been adopted by some seafood companies, and has markedly reduced the number
of complaints about their products.


Fig.16

Sterilization of oyster using ozone nanobubbles.

4.3 Another nanobubble applica
tion: Antiseptic
-
free Kamaboko (Shiraken Kamaboko Co., Ltd.)


Figure

17 shows the process of producing
kamaboko
(boiled fish paste). Minced fish contains various bacteria, including
heat
-
resistant bacteria. Heat treatment is thus unsatisfactory for destroying such bacteria. Antiseptic chemicals, such as
sorbic

acid, are unfavorable because they give bitterness to
kamaboko

and deteriorate its flavor. When the oxygen
nanobubbles obtained by removing NaCl from water using a reverse
-
osmosis membrane were added to the raw materials
of
kamaboko

during mincing, myofib
rils became thick, and highly nutritious, delicious, and antiseptic
-
free
kamaboko

was
obtained.

Oxygen nanobubbles activate living organisms. Therefore, bacteria also proliferate during mincing. However, oxygen
nanobubbles are stimulated to collapse during

the subsequent molding, heating, and generating free radicals such as •OH,
which are considered to kill the bacteria. The resulting chemical
-
free, safe, and tasty
kamaboko

sold well. "You may
think that the result was very easily achieved because I explai
ned it so briefly, but in fact, it took workers more than two
years to produce tasty
kamaboko

through a trial
-
and
-
error process by adapting manufacturing conditions; for example,
they first eliminated bacteria using ozone nanobubbles with strong bactericid
al power, which also deteriorated the flavor
of
kamaboko
. They then used oxygen nanobubbles after removing NaCl from the water that went through a reverse
-
osmosis membrane."


Fig.17

Processing of chemical
-
free
kamaboko

using nanobubbles.

5.Final Remarks


From a scientific point of view, we have learned of the interesting
behavior and characteristics of bubbles, including
nanobubbles and microbubbles, associated with their shrinkage through this interview. Bubbles are clean and require no
special materials. We hope that they will be applied to various fields including food,

medicine, and health care, in
addition to environmental purification. We are confident that there is plenty of attractive potential yet to be revealed for
the use of nanobubbles, such as through their actions on living organisms.

In summary Dr. Takahashi
concluded, "Nanobubbles are only a raw material. What is important is how and where to
apply them. However, we can neither develop the applications of nanobubbles nor cope with unexpected results if we
don't know their mechanism. Nanobubbles are still far
from being a truly effective technology. AIST, a public institution,
can provide basic information about bubbles. We would like users to understand the fundamentals of nanobubbles and
build up their own know
-
how in accordance with each application. We hope

that nanobubbles will break new ground in
medicine and biology."

References


[1]
Dr. Takahashi's website
.

[2] Kaneo Chiba and Masayoshi Takahashi:"Oxygen
-
Nanobubble Water and Its Production Method," Patent No.
4080440.

[3] Kaneo

Chiba and Masayoshi Takahashi:"Production Method of Nanobubbles," Patent No. 4144669.

[4] Kaneo Chiba and Masayoshi Takahashi:"Ozone Water and Its Production Method," Patent No. 4059506.

[5]
Masatoshi Takahashi:“The
ζ

Potential of Microbubbles in Aqueous Solutions

-
Electrical property o


the gas
-
water interface
--



J.Phys.Chem. B

109
-


pp.21858
-
21864


2005/11
.

[6]
Masatoshi Tak
ahashi


Kaneo Chiba and Pan Li:“Formation of Hydroxyl Radicals by Collapsing Ozone
Microbubbles under Strongly Acidic Conditions”

J.Phys.Chem. B

111
-
39

pp.11443
-
11446

2007/10
.

[7]
Masatoshi Ta
kahashi


Kaneo Chiba and Pan Li:“Free
-
Radical Generation from Collapsing Microbubbles in the
Absence of a Dynamic Stimulus”

J.Phys.Chem.B

111
-
6


pp.1343
-
1347

2007/02
.

[8]
Press release, "World's First Establishment of Technologies for Producing and Stabilizing Nanobubbles"
.

[9]

京医科歯科大学
・株式会社
REO
研究所:

組織の修復又は再生用製剤


国際公開番号:
WO2008/072370A1
.

[10]
東京医科歯科大学
・株式会社
REO
研究所:

組織の殺菌又は消毒用製




国際公開番号:
WO2008/072371A1
.


Original source :
https://nanonet.nims.go.jp/english/magazine/index.php?Vol.%203%2C%20No.%201%2C%202010
-
08
-
30%2FGreen%20Nanotechnology%20Special%20Topic%206