Characterisation of Ultrafine Bubbles

★ Nanobubble Research Centre Department of Applied Mathematics

Research School of Physics and Engineering Australian National University

Guangming Liu§, Muidh Alheshibri★, Marie Jehannin★, Jing Qian★

§USTC, Hefei , China

Vincent Craig★

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Ultrafine Bubbles – diameter <1000 nm

In the academic community Ultrafine Bubbles or submicron bubbles are known as Bulk Nanobubbles and their existence is highly controversial !

To illustrate the level of controversy • In response to a paper we recently submitted on the

History of Nanobubbles a reviewer wrote;

“….it is obvious for any critical reader that there is no evidence of the long-term stability of bulk nanobubbles at all.” Why are Ultrafine bubbles so controversial?

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Theory of Bubble Growth and Shrinkage

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Cbulk Bulk Concentration of Gas

Cbubble Concentration of Gas Adjacent to the bubble

Considers a single stationary bubble Calculates the diffusion rate between the source and the sink (Assumes initial process of establishing the concentration around the bubble is rapid) Need to determine the concentration adjacent to the bubble

Cbubble is calculated by using the Total Pressure in the bubble (Pext +PLaplace) in conjunction with Henry’s law to determine the concentration of gas in equilibrium with the bubble

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1.00E-07

1.00E-05

1.00E-03

1.00E-01

1.00E+01

1.00E+03

1.00E+05

1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03

Bub

ble

Life

time

(s)

Bubble Radius (m)

Oxygen Nitrogen Hydrogen

For bubbles in solutions saturated at 1 atmosphere of the same gas

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1.00E-07

1.00E-05

1.00E-03

1.00E-01

1.00E+01

1.00E+03

1.00E+05

1.00E-08 1.00E-07 1.00E-06 1.00E-05 1.00E-04 1.00E-03

Bub

ble

Life

time

(s)

Bubble Radius (m)

Oxygen Nitrogen Hydrogen

Ultrafine

Theory predicts that Ultrafine Bubbles should dissolve in less than a millisecond ! - Epstein and Plesset J. Chem Phys Vol 18 p1505 (1950)

For bubbles in solutions saturated at 1 atmosphere of the same gas

Evolution of a nanobubble

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200

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0 0.005 0.01 0.015 0.02Nan

obub

ble

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m)

Time (s)

Calculated nanobubble radius versus time using EP theory2 for a nitrogen filled nanobubble of initial radius 1000 nm in a solution that is saturated with dissolved nitrogen gas. (T=300K, g= 0.072 Jm-2, D=2.0 x 10-9 m2s-1, Csat =0.6379 moles m-3, r1 atm=40.6921 moles m-3)

What about Supersaturation?

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The level of supersaturation and bubble size determines if a bubble will shrink or grow. Bubbles on the line enjoy an unstable equilibrium

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10

100

1.00E-08 1.00E-06 1.00E-04

Satu

ratio

n le

vel r

equi

red

for s

tabi

lity

Radius of curvature (m)

The supersaturation in this region will cause bubbles to

Bubbles in this region will shrink

Supersaturation does not resolve the issue of long-term stability of Ultrafine Bubbles

Influence of supersaturation on lifetime

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0.01

0.1

1

10

100

1000

1 1.5 2 2.5

Nan

obub

ble

lifet

ime

(s)

Saturation level

Bulk nanobubble lifetime calculated using EP theory for a N2 gas filled nanobubble of initial radius 1000 nm as a function of saturation level of N2 gas in solution. When the saturation level is matched to that required for the bubble to be stable the lifetime is effectively infinite. However, a very small deviation of saturation level below that required for equilibrium leads to very rapid dissolution of the bubble. Similarly a very small deviation of saturation above the level required for equilibrium would lead to rapid growth of the bubble (not shown) (T=300K, g= 0.072 Jm-2, D=2.0 x 10-9 m2s-1, Csat =0.6379 moles m-3, r1

atm=40.6921 moles m-3) .

Characterisation fundamentals • Size and size distribution • Concentration • Lifetime • Surface Charge (zeta potential) • Interfacial properties • Evidence that they are actually bubbles • For applications

– Evidence of effectiveness

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Can utilise existing techniques for nanoparticles

Techniques are under development

Ultrafine bubbles are measured using a variety of techniques • Examples include

– Light scattering – Single particle tracking – Cryo SEM freeze fracture imaging

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However, these lack direct evidence that the particles observed are gaseous.

Despite this Ultrafine bubbles are measured using a variety of measures

• Light scattering

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0 50

100 150 200 250 300 350 400 450 500

1 hour 2 hours

3 hours

4 hours

5 hours

6 hours

7 hours

8 hours

9 hours

10 hours

SIZ

E (N

AN

OM

ETE

RS

)

TIME

Bubble Size vs time

Despite this Ultrafine bubbles are measured using a variety of measures • Single particle tracking

– Uses scattered light to track the 2D diffusion of particles

– Determines size and concentration

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Despite this Ultrafine bubbles are measured using a variety of measures

• Cryo SEM freeze fracture imaging

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Panel A: Rapid cryogenic freezing and subsequent fracture of a solution of nanobubbles was used to obtain replicas of the surface which were subsequently imaged by scanning electron microscopy to reveal nanobubbles ~ 100 nm in diameter. Panel B: A higher magnification image of a single nanobubble. (Images taken from Ohgaki, K.; Khanh, N. Q.; Joden, Y.; Tsuji, A.; Nakagawa, T., Physicochemical approach to nanobubble solutions. Chem. Eng. Sci. 2010, 65, (3), 1296-1300 )

How to show Ultrafine bubbles contain gas? • Properties of gases

– Low density (<1 g/litre) – Highly compressible – CO2(g) has particular spectral properties – High Interfacial energy – Particular circumstances where other explanations are

not likely

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Low Density • Archimedes

– Measures the density in relation to the solvent

– Insensitive to bubbles below 100 nm (at best)

– Performance depends on the particular resonator used

– Requires significant concentrations

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Positively Buoyant Ultrafine Bubbles Kobayashi, H.; Maeda, S.; Kashiwa, M.; Fujita, T., Measurement and identification of ultrafine bubbles by resonant mass measurement method. In International Conference on Optical Particle Characterization, Aya, N.; Iki, N.; Shimura, T.; Shirai, T., Eds. 2014; Vol. 9232

How to show Ultrafine bubbles contain gas?

• Utilise the compressibility of gas – Soundwaves

• Velocity and attenuation of sound waves • Poor sensitivity

– External Pressure • Measure the size as a function of external pressure • No device currently available to do this • Should also reveal the surface tension

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How to show Ultrafine bubbles contain gas? • Spectral Properties of CO2

• Fine structure is only seen for gaseous CO2

• Dissolved CO2 (aq) shows a single Gaussian peak

• Has not been reported for Ultrafine bubbles • Sensitivity and interference from larger

bubbles and atmospheric CO2 need to be addressed

• Not suitable for other gasses 18

How to show Ultrafine bubbles contain gas?

• Interfacial Tension – Laplace Pressure – Can be evaluated from a study of the size as a function of

external pressure – No equipment currently available for this

• We are building an apparatus – Suitable for all types of fine bubbles - universal – Potentially quick and simple method

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How to show Ultrafine bubbles contain gas? • Methods only applicable to a certain sample

– Eg A particular example

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2000

0 20 40 60 80 100 120 140

Dia

met

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)

Time (minutes)

NH4Cl(aq) + NaNO2(aq) NaCl(aq) + H2O + N2(g)

No Particles Particles Form Logically the particles cannot be anything but gas

In conclusion • We need a Standard method for testing if a particular solution

of sub-micron particles (particularly <200 nm in diameter) is indeed gaseous

• A technique suitable for all samples is not yet available • But we expect to have these methods developed soon • It may be very challenging when some of the particles are not

gaseous

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Acknowledgments and thanks • Revalesio • Tennant • Australian Research Council

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