This document discusses techniques for characterizing nanobubbles, including dynamic light scattering (DLS), nanoparticle tracking analysis (NTA), and resonant mass measurement (RMM). DLS and NTA can measure nanobubble size distributions but DLS results may be influenced by contaminants. RMM can distinguish nanobubbles from contaminating particles by measuring buoyant mass. The document also examines measuring nanobubble zeta potential to characterize surface charge and stability. A variety of techniques provide complementary characterization of nanobubbles but RMM can uniquely discriminate bubbles from other particle types of similar size.

1. Nanobubbles
Separating fact from fiction
By
Ciaran C. Murphy
Head of Product Management
30/10/14

2. Content
› What are nanobubbles
› Why the interest
› Characterisation challenges
› Characterisation techniques
 Dynamic Light Scattering (DLS)
 Nanoparticle Tracking Analysis (NTA)
 Resonant Mass Measurement (RMM)

3. Large bubbles (several mm)
Courtesy: FBIA 2014

4. Ultra fine bubbles (<1µm)
Courtesy: FBIA 2014

5. Behaviour different bubble types
Courtesy: FBIA 2014

6. Applications
Courtesy: FBIA 2014

7. Towards ISO standards – ISO/TC 281
Courtesy: FBIA 2014

8. Characterisation challenges
› Size of bubble
 Typically <150nm
› Optical properties
 Refractive index difference with media
› Stability
 Pressure and temperature changes
 Zeta potential (charge) measurements
› Contaminants
 Differentiate bubbles from “something else”

9. Dynamic Light Scattering (DLS)

10. DLS Instrument Components
Laser
Cuvette
containing
sample
Digital signal processor
(Correlator)
Photon counting device
(Avalanche photo diode)
Scattered light

11. Correlation Functions
Correlate
Time
Intensity
Small Particles
Time
Intensity
Large Particles
Correlate

12. Correlation Functions: Size distributions
Time when decay starts
indicates mean size
Gradient
indicates the
polydispersity
of sample
Baseline
Intercept

13. DLS Correlelograms: Ultra fine bubble suspensions
Correlelograms show “tails”
Sample contains large particle
contamination
2E+08/ml
5E+08/ml
10E+08/ml

14. Ultra fine bubble size measurements using DLS
Nanobubbles were detected in all three samples
Z-Average has good repeatability within sample subsets
Sample Name Temp Z-Average PdI Attenuator Mean Count Rate Derived Count Rate
°C d.nm kcps kcps
2E8/ml 1 25 200.3 0.328 11 489.1 489.1
2E8/ml 2 25 183.7 0.428 11 472.7 472.7
2E8/ml 3 25 198.0 0.375 11 442.6 442.6
2E8/ml Ave 25 194.0 0.377 11 468.1 468.1
5E8/ml 1 25 224.3 0.243 10 330.0 1174.5
5E8/ml 2 25 189.9 0.286 10 327.1 1164.2
5E8/ml 3 25 159.3 0.391 10 309.7 1102.1
5E8/ml Ave 25 191.2 0.307 10 322.3 1146.9
10E8/ml 1 25 130.1 0.249 9 223.3 2012.0
10E8/ml 2 25 127.7 0.281 9 221.8 1998.4
10E8/ml 3 25 129.0 0.239 9 226.3 2039.0
10E8/ml Ave 25 128.9 0.256 9 223.8 2016.5
2E8/ml filtered 0.45um 1 25 185.8 0.188 11 412.2 412.2
2E8/ml filtered 0.45um 2 25 205.0 0.217 11 402.5 402.5
2E8/ml filtered 0.45um 3 25 160.7 0.214 11 351.8 351.8
2E8/ml (0.45um filter) Ave 25 183.8 0.206 11 388.8 388.8
5E8/ml filtered 0.45um 1 25 135.3 0.233 10 233.6 831.4
5E8/ml filtered 0.45um 2 25 137.7 0.223 10 226.7 806.8
5E8/ml filtered 0.45um 3 25 163.8 0.209 10 290.3 1033.1
5E8/ml (0.45um filter) Ave 25 145.6 0.222 10 250.2 890.4
10E8/ml filtered 0.45um 1 25 108.9 0.169 10 427.6 1521.8
10E8/ml filtered 0.45um 2 25 111.3 0.163 10 429.9 1529.9
10E8/ml filtered 0.45um 3 25 110.0 0.123 10 421.8 1500.9
10E8/ml (0.45um filter) Ave 25 110.1 0.152 10 426.4 1517.5
blank 1 25 6614.0 1.000 11 135.1 135.1
Malvern Instruments: Zetasizer Nano

15. Ultra fine bubble size distribution summary
Size distribution was altered/
Large particles were removed by filtration
After filtration
Before filtration 2E+08/ml
5E+08/ml
10E+08/ml

16. Results of Ultra fine bubble size measurements
Evaluate Z-Average of Samples
(After filter)
0.0
40.0
80.0
120.0
160.0
200.0
Z-Average(d.nm)
2E8/ml (0.45um filter) Ave 5E8/ml (0.45um filter) Ave 10E8/ml (0.45um filter) Ave
Evaluate Z-Average of Samples
(Before filter)
0.0
50.0
100.0
150.0
200.0
Z-Average(d.nm)
2E8/ml Ave 5E8/ml Ave 10E8/ml Ave
The difference became clearer
after filtration

17. Zeta potential measurements
Sample Name Temp Zeta Potential Mobility Conductivity Attenuator Derived Count Rate
°C mV µmcm/Vs mS/cm kcps
2E8/ml 1 25 -23.7 -1.859 0.0012 11 5
2E8/ml 2 25 -21.0 -1.643 0.0241 11 8
2E8/ml 3 25 -24.8 -1.946 0.0013 11 10
2E8/ml Ave 25 -23.2 -1.816 0.0089 11 8
5E8/ml 1 25 -25.4 -1.991 0.0278 11 22
5E8/ml 2 25 -31.0 -2.426 0.0018 11 45
5E8/ml 3 25 -21.4 -1.677 0.0036 11 16
5E8/ml Ave 25 -25.9 -2.031 0.0111 11 27
10E8/ml 1 25 -32.9 -2.577 0.0026 11 2134
10E8/ml 2 25 -25.5 -2.000 0.0033 11 33
10E8/ml 3 25 -32.0 -2.507 0.0041 11 104
10E8/ml Ave 25 -30.1 -2.361 0.0033 11 757
Zeta potential was measured for all three samples and was negative in all cases.
Count Rate was not stable due to presence of contaminating particles.
Malvern Instruments: Zetasizer Nano

18. -35.0
-30.0
-25.0
-20.0
-15.0
-10.0
-5.0
0.0
ZetaPotential
2E8/ml Ave 5E8/ml Ave 10E8/ml Ave
Zeta potential shows differences
Results of Zeta potential measurements:
Zeta potential, distribution and Phase plot
1. Zeta potential is negative
2. Varies slightly with
particle concentration

19. Zeta Potential: Results Summary
› Zeta Potential of the Ultra fine bubbles measured was negative and
highly reproducible
 Origin of the surface charge is unknown
› Zeta potential is an indicator of stability in colloidal systems and
depends on the nature of the interface between the 2 phases
› Further work on additions of small quantities of anionic or cationic
surfactants to the liquid during production or by changing the pH
would be interesting to elucidate the mechanism as well as a means
of controlling the stability and surface charge

20. Nanoparticle Tracking Analysis
(NTA)

21. Nanoparticle Tracking Analysis (NTA)
› As shown in the schematic below, the
technology comprises:
 Proprietary optical element
 Illuminated by a specially configured
laser beam
 Scattered light is collected by a camera

22. • Fine bubbles were
generated in a solution
containing NaCl by
vigorous agitation
• Sample was loaded into
the NS500 instrument at
the native concentration as
drawn from the suspension
• High concentration of
scattering spheres
visualised.
• High degree of
polydispersity, with a large
range of sizes indicated in
video
Fine bubble sample

23. Modal particle size: 114 nm
Polydispersity: scattering spheres of sizes ~50 – 800nm measured
Concentration: 9.21 x 10^8 particles/ml
Fine bubble sample: Size distribution profile

24. Ultra fine bubble sample
› Mode 321 nm
› 0.2 x108 particles/mL

25. Resonant Mass Measurement
(RMM)

26. Archimedes:
RMM: The Concept
Microchannel Resonator
• Microfluidic channel embedded inside resonant
structure
• Particle passing through the resonator changes the
mass and shifts the resonant frequency
• Excursion in resonant frequency gives an accurate and
precise measure of particle’s buoyant mass
MEMS
Sensor

27. Measuring Particle Mass in Fluid
-150 -100 -50 0 50 100 150
-400
-300
-200
-100
0
FrequencyShift(mHz)
Time (msec)
200
1. 3.
2.
1.
2.
3.

28. ARCHIMEDES’ Instrumentation
Frequency Resolution: 25 parts per billion @ 1 kHz
Mass Limit of Detection: < 1 femtogram = 10-15 g
PC
Signal Processing
Frequency
Measurement
and
Feedback
Sensor
Optics
Pneumatics
Fluidics
sample
waste
pressure
source

29. Harnessing Archimedes’ Principle

30. Superior Particle Metrology
1% precision and accuracy
Mixture of NIST-traceable standards
Ultra-high resolution
Easily distinguishes particles
< 2% different in size

31. The buoyant mass of a particle is always measured relative to its surrounding
fluid. A particle of dust will therefore have a negative buoyant mass in water,
and a bubble will have a positive buoyant mass in water
Buoyant Mass Measurement

32. RMM: 10 E+08/mL samples (filtered)
› Negatively buoyant – “dust” contaminant particles?
Mode diameter = 283nm
10 E+08 1.6um Filtered per mL (12th October 2012)
Particle Statistics All Particles Selected Particles
Particle Range (um) 0.246 to 0.934 0.246 to 0.934
# particles 150 150
% in range 100% 100%
Mean Diameter (um) 0.358 0.358
Standard Deviation (um) 0.124 0.124
Mode Diameter (um) 0.283 0.283
Polydispersity 0.345 0.345
Standard Error (um) 0.01 0.01
Concentration (#/ml) 1.20E+07 1.20E+07
Limit of Detection (um) 0.1
Bin Size (um) 0.011
Cumulative Statistics
Low % 0% High % 100%
D0 0.090um D100 1.279um

33. RMM – 10 E+08/mL samples
› Positively buoyant – “Nanobubbles”
Mode nanobubble diameter:
112nm
10 E+08 1.6um Filtered per mL (12th October 2012)
Particle Statistics All Particles Selected Particles
Particle Range (um) 0.102 to 0.268 0.102 to 0.268
# particles 350 350
% in range 100% 100%
Mean Diameter (um) 0.133 0.133
Standard Deviation (um) 0.029 0.029
Mode Diameter (um) 0.112 0.112
Polydispersity 0.22 0.22
Standard Error (um) 0.002 0.002
Concentration (#/ml) 2.81E+07 2.81E+07
Limit of Detection (um) 0.1
Bin Size (um) 0.011
Cumulative Statistics
Low % 0% High % 100%
D0 0.095um D100 0.277um

34. RMM Results Summary
› Populations of particles with negative and positive buoyant masses in
their suspending fluid (water) were detected and measured using
RMM
› A distinct population of positively buoyant particles with a mode
diameter of 112nm were detected
› The diameter range of the positively buoyant particles was 102nm to
268nm
› These positively buoyant particles are thought to be ultra fine bubbles
› Ultra fine bubbles are distinguished from contaminating material such
as dust particles in the same suspension by comparison of their
buoyant masses in water
› RMM is uniquely able to discriminate between dust particles and ultra
fine bubbles of the same size in the same sample

35. Conclusion
› All techniques can measure ultra fine bubbles
› DLS, NTA and MEMS techniques are complementary for
the characterisation of Nanobubbles
› NTA / DLS: Measurements of size distribution
› Electrophoretic mobility : Determination of the Zeta
Potential and characterisation of the surface chemistry
and stability
› RMM – size measurement, particle concentration and
discrimination between bubbles and contaminants

36. You can view a recording of this
webinar presentation here:
bit.ly/nanobubble
For further information, please contact:
Ciaran.Murphy@malvern.com
www.malvern.com