Nanotechnology/Nanotribology at Nanotechnology/Nanotribology at Appalachian State UniversityAppalachian State University

The Physics of BubblesThe Physics of Bubbles

Tonya CoffeyTonya Coffey

Ongoing Projects: High magnification Scanning Tunneling Ongoing Projects: High magnification Scanning Tunneling Microscopy and Atomic Force MicroscopyMicroscopy and Atomic Force Microscopy

Sample characterization, rhododendron leaves, Adhesion of acrylic nails….

Ongoing projects: QCMOngoing projects: QCMNanobubblesAlcohols as Vapor Phase Lubricants for MEMSMeasurement of the Viscosity of the Martian AtmosphereUsing QCM to measure contact angles

The effect of The effect of nanonano and and macroscalemacroscalebubbles on viscositybubbles on viscosity

Department of Physics Department of Physics and Astronomyand Astronomy

Coffey Research Group Coffey Research Group –– Jon Jon JonesJones

How bubbles form in liquidsHow bubbles form in liquidsIn most liquids, there is some dissolved gas. In high surface tension liquids, like water, it is tough for bubbles to form, because water molecules like to be next to other water molecules (capillary forces).

To overcome this, a nucleation site is generally needed. Gas molecules congregate next to nucleation sites, which break up the network of water molecules. When enough are gathered, they form a bubble.

Due to capillary forces, the bubble will initially stay at its nucleation site. But usually, the buoyancy of the bubble will eventually cause it to rise, as more and more gas molecules collect in the bubble.

*Liger-Belair and Jeandet, Europhysics News

Why do we care about bubbles?Why do we care about bubbles?We like our drinks nice and fizzy.

Thanks, Henry’s Law!

P=Kc

Here P is partial pressure of gas above liquid,K is a constant, and c is the molar concentration of solute.

The partial pressure of the gas above the solution is directly proportional to the concentration of solute gas in the solution.

So when you pop the top, the equilibrium within the bottle or can is broken, and the concentrated carbon dioxide gas leaves the container. The partial pressure of carbon dioxide in the surrounding gas then drops, forcing the concentration of the solute gas to drop, so it bubbles out.

More fun bubble factsMore fun bubble facts……

When a soda is bottled, it is bottled under a relatively high pressure of CO2 that exceeds the solubility of CO2 in the rest of the formula (mostly water). When the can is opened without shaking high pressure CO2 above the liquid escapes, making the familiar hiss. The CO2 in the liquid slowly escapes until equilibrium is achieved. When the unopened can is shaken, some of the gaseous CO2 gets mixed into the liquid, forming a supersaturated solution. The mixed in gas also provide nucleation sites for the dissolved CO2. The nucleation sites allow the CO2 to escape much more rapidly-- hence the "explosive" evolution of CO2 gas.

Why do we care about bubbles?Why do we care about bubbles?Higher quality champagnes have smaller bubbles, that form at nucleation sites and rise slowly upward in “bubble trains.” In recent research articles, it has been found that the nucleation sites are usually cellulose particles, that probably get there when the glass is dried with paper towels.

Beer forms a head, whereas soda and champagne do not. Why? Surfactants in the beer reduce the surface tension of the liquid (capillary force), so the bubbles are more stable and longer lasting.

Bubble PhysicsBubble Physics

cRmNP )/073.0(2

=Laplace’s equation places restrictions on the size of a bubble. In water, for an internal pressure of 1 atm, a bubble’s minimum radius is 1-2 microns.

So nanobubbles can’t exist because the internal pressures would be too high, and the bubble would pop if the internal pressure got too high, right?

But look!

The bubbles in this AFM picture appear to have radii around 70-100 nm. This would give an internal pressure of as much as 21 atm!

*Simonsen et al., J Coll. Inter. Sci.

Bubble PhysicsBubble Physics

The radius of curvature of the bubble is sometimes bigger than it appears. Not all bubbles are spheres!

But this doesn’t explain all the problems with Laplace’s equation.

*Simonsen et al., J Coll. Inter. Sci.

Hydrophobic = Water fearingHydrophobic = Water fearingHydrophilic = Water lovingHydrophilic = Water loving

There are some alterations that need to be made to Laplace’s picture. Bubbles like to form on hydrophobic surfaces because the water network is broken up, so lots of nanobubbles form on them when submerged in water. Bubbles don’t form as readily on hydrophilic surfaces, because they don’t break the water network as effectively.

So there’s some competing surface effects that Laplace’s treatment doesn’t include.

People are currently working on this.

NanobubblesNanobubbles(a semi(a semi--hot topic)hot topic)

These tiny bubbles are responsible for These tiny bubbles are responsible for many interesting effects.many interesting effects.Hydrophobic surfaces in a liquid are Hydrophobic surfaces in a liquid are attracted to one another: bubble bridging.attracted to one another: bubble bridging.Measured viscosities of liquids are Measured viscosities of liquids are sometimes abnormally low.sometimes abnormally low.Applications: bubbles as lubricants? Applications: bubbles as lubricants?

NanotribologyNanotribologyWhat is What is ““NanotribologyNanotribology””??

1.1. Our research group studies nanotribology, the study of friction,Our research group studies nanotribology, the study of friction,lubrication, and wear at nanoscale length and time scales. lubrication, and wear at nanoscale length and time scales.

2.2. Friction is a fundamental concept in physics, but the origins ofFriction is a fundamental concept in physics, but the origins offriction, from nanoscale phenomena, are poorly understood. friction, from nanoscale phenomena, are poorly understood.

3.3. Advancements in nanotribology are necessary to advance Advancements in nanotribology are necessary to advance

nanotechnology.nanotechnology.

MotivationMotivation•• There are 3 main techniques for measuring There are 3 main techniques for measuring

nanoscale friction: the atomic force microscope nanoscale friction: the atomic force microscope (AFM), the quartz crystal microbalance (QCM), (AFM), the quartz crystal microbalance (QCM), and the surface forces apparatus (SFA).and the surface forces apparatus (SFA).In some recent SFA studies of lubricants, very In some recent SFA studies of lubricants, very small friction coefficients or effective viscosities small friction coefficients or effective viscosities were measuredwere measuredIt is difficult for the SFA community and the It is difficult for the SFA community and the theorists to agree on the causes of decreased theorists to agree on the causes of decreased viscosity. One explanation, suggested by Steve viscosity. One explanation, suggested by Steve GranickGranick, is that the , is that the reduced viscosity of fluids reduced viscosity of fluids may be caused by the formation of may be caused by the formation of ‘‘nanonano--bubblesbubbles’’ at the solidat the solid--liquid liquid interfaceinterface[ii[ii]]. This is a . This is a controversial idea. controversial idea.

We are testing We are testing GranickGranick’’ss theories with the QCM.theories with the QCM.Possibly this reduced interfacial viscosity could Possibly this reduced interfacial viscosity could be exploitedbe exploited——bubble lubricants.bubble lubricants.

[i][i] J. N. J. N. IsraelachviliIsraelachvili, , Intermolecular & Surface ForcesIntermolecular & Surface Forces, 2nd ed. , 2nd ed. (San Diego, Academic Press, 1992): 260.(San Diego, Academic Press, 1992): 260.[ii][ii] S. S. GranickGranick, Y. Zhu, and H. Lee, , Y. Zhu, and H. Lee, Nature MaterNature Mater. 2 (2003): 221.. 2 (2003): 221.

nanobubbles?

Steve Granick

QQuartz uartz CCrystal rystal MMicrobalance (QCM)icrobalance (QCM)

(Krim and Widom, PRB, v. 38, n.17, 1988)

Quartz is a piezoelectric Quartz is a piezoelectric material. material.

Metal electrodes are deposited Metal electrodes are deposited on either face of a thin quartz on either face of a thin quartz disk. disk.

An alternating voltage is applied An alternating voltage is applied to the electrodes and the quartz to the electrodes and the quartz crystal will oscillate.crystal will oscillate.

Our crystals are cut so that they Our crystals are cut so that they oscillate in transverse shear oscillate in transverse shear mode.mode.

QCMQCM’’s were used as decades s were used as decades as as microweighingmicroweighing instruments. instruments.

They are very sensitive to small They are very sensitive to small changes in mass. (changes in mass. (nanogramsnanograms))

They have recently been They have recently been adapted for many other adapted for many other applications.applications.

2

2 tfq q

tfvfρ

ρ−∆ =

We measure the viscosity of liquids with the QCM using the Kanazawa equation:

*

This equation is solved for viscosity, the other parts of the equation are all based on the physical aspects of the apparatus.

QQuartz uartz CCrystal rystal MMicrobalance (QCM)icrobalance (QCM)in liquidsin liquids……

(c)

(b)

(a)

The QCM is used to measure (amongst other things), the relative The QCM is used to measure (amongst other things), the relative frequency frequency shift of the oscillation of the crystal while bubbles form, pop,shift of the oscillation of the crystal while bubbles form, pop, or move along the or move along the crystal face.crystal face.The shear wave generated by the QCM in water is heavily damped, The shear wave generated by the QCM in water is heavily damped, with a with a decay length of about 250 nm. Therefore we are truly measuring decay length of about 250 nm. Therefore we are truly measuring not the bulk not the bulk viscosity, but an effective viscosity of the fluid near the surfviscosity, but an effective viscosity of the fluid near the surface.ace.

a – computer, b – Maxtek RQCM apparatus, c – crystal holder (in beaker).

qq

LLqff

ρµπρη⋅⋅

⋅−=∆ 2

3

MacrobubblesMacrobubbles & & NanobubblesNanobubblesWe are measuring the effective viscosity of the following liquidWe are measuring the effective viscosity of the following liquids: Seltzer s: Seltzer

Water (Water (macrobubblesmacrobubbles), ), SonicatedSonicated DI Water (macro and DI Water (macro and nanobubblesnanobubbles), and Vacuum Distilled Water (), and Vacuum Distilled Water (nanobubblesnanobubbles) )

We used QCMWe used QCM’’s with both hydrophilic (gold and silicon dioxide) and s with both hydrophilic (gold and silicon dioxide) and hydrophobic (hydrophobic (octadecanethioloctadecanethiol coated) surfaces. coated) surfaces.

We acquired video of the QCM in seltzer water, and correlated thWe acquired video of the QCM in seltzer water, and correlated the e video with the viscosity data.video with the viscosity data.

For the vacuum distilled water, we break vacuum and acquire visFor the vacuum distilled water, we break vacuum and acquire viscosity cosity data with increasing time (minutes, hours, days) of the water sidata with increasing time (minutes, hours, days) of the water sitting tting in air. in air.

SSelf elf AAssembled ssembled MMonolayersonolayers ((SAMsSAMs))

For this experiment we used For this experiment we used octadecanethioloctadecanethiol (C(C1818HH3838S) S) monolayersmonolayers. They . They are very hydrophobic. are very hydrophobic. This SAM is easy to prepare: The crystals were cleaned in PiranhThis SAM is easy to prepare: The crystals were cleaned in Piranha solution a solution (30% H(30% H22OO2 2 / 70%H/ 70%H22SOSO44) rinsed in research grade Ethanol, and then placed ) rinsed in research grade Ethanol, and then placed in the surfactant solution (ethanol and water + in the surfactant solution (ethanol and water + thiolsthiols).).The The SAMsSAMs have a head and a tail, the heads bond to the gold surface in ahave a head and a tail, the heads bond to the gold surface in atightly packed pattern. The tails do not bond to heads, or to ttightly packed pattern. The tails do not bond to heads, or to the gold, so he gold, so after a few hours of submerging the gold coated crystal in the safter a few hours of submerging the gold coated crystal in the solution you olution you have a onehave a one--molecule thick layer. molecule thick layer. Data was taken with both CData was taken with both C--18 18 ThiolThiol coated crystals and hydrophilic crystals coated crystals and hydrophilic crystals for all cases.for all cases.

Diagram of SAM’s on gold, *Taken from http://www.ifm.liu.se/applphys/ftir/sams.html

Macroscopic Bubbles Macroscopic Bubbles

Seltzer water bubbles are Seltzer water bubbles are visible to the naked eye, so no visible to the naked eye, so no special equipment is needed. special equipment is needed. We used a normal video We used a normal video camera. camera. Correlating the video with the Correlating the video with the QCM measurements allows us QCM measurements allows us to intuitively understand how to intuitively understand how bubbles affect viscosity bubbles affect viscosity measurements. measurements. Seltzer water goes flat rapidly. Seltzer water goes flat rapidly. In just a couple of hours very In just a couple of hours very few visible bubbles are left. few visible bubbles are left. This helped us to observe the This helped us to observe the changing behavior of the changing behavior of the viscosity with time. viscosity with time.

5MHz Au crystal (no thiols) in carbonated water.

MacrobubblesMacrobubbles

As Bubbles form onto, or leave the surface of the QCM it resultsAs Bubbles form onto, or leave the surface of the QCM it results in a in a very visual viscosity change. The fewer bubbles on the surface very visual viscosity change. The fewer bubbles on the surface of of the crystal, the higher the viscosity of the liquid is. the crystal, the higher the viscosity of the liquid is. It is easier for the crystal to vibrate under the dome of a gaseIt is easier for the crystal to vibrate under the dome of a gaseous ous bubble, hence, as the liquid loses bubbles, viscosity increases bubble, hence, as the liquid loses bubbles, viscosity increases to the to the known value of what that liquid is.known value of what that liquid is.

15 16 17 18 19

0.6

0.8

1.0

Vis

cosi

ty (c

entip

oise

)

Time (min)

Viscosity vs. Time Selt0001VIDEO - Jon Jones

(a)

17:12(min:sec) 17:13(min:sec)

SonicatedSonicated and Seltzer Water on Goldand Seltzer Water on Gold

As shown above, degassed water (left) has a much smoother curve As shown above, degassed water (left) has a much smoother curve towards the final towards the final value for the viscosity of water (0.89 value for the viscosity of water (0.89 cPcP as determined by a fit to dataas determined by a fit to data). The seltzer ). The seltzer water (right) is choppy because of all the large bubbles formingwater (right) is choppy because of all the large bubbles forming, popping, or moving , popping, or moving on the surface of the electrode, yet a trend towards the final von the surface of the electrode, yet a trend towards the final viscosity is still visible.iscosity is still visible.The nearThe near--surface viscosity is smaller than the known bulk viscosity of wasurface viscosity is smaller than the known bulk viscosity of water. The ter. The trend in both the trend in both the sonicatedsonicated and seltzer water is to gradually approach a limiting value and seltzer water is to gradually approach a limiting value of viscosity with increasing time. The reduced viscosity and thof viscosity with increasing time. The reduced viscosity and the trend toward the final e trend toward the final value both indicate the presence of value both indicate the presence of nanobubblesnanobubbles. .

4 6 8 10 12 14 160.60

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Viscosity vs. Time Degassed H20 6.10.05 run 0001

10 20 30 40 500.60

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Viscosity vs. Time Seltzer Water 7.19.05 run 0021

Known Viscosity of water: 1.003 Known Viscosity of water: 1.003 centipoisecentipoise @ 20@ 20°°C, C, Our measured nearOur measured near--surface viscosity of surface viscosity of sonicatedsonicated water: 0.89 water: 0.89 centipoisecentipoise @ 20@ 20°°CC

Distilled and Seltzer Water on Distilled and Seltzer Water on ThiolsThiols

10 20 30 40 50 60 700.80

0.82

0.84

0.860.88

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Distilled, Decanted DI H2O on Au + Octadecanethiol - JMJ, HKN, DAG - 7/13/06

With the presence of With the presence of SAMsSAMs on the Au electrode, viscosity measurements behaved on the Au electrode, viscosity measurements behaved much differently than without.much differently than without.Viscosity of distilled water is 1.003 Viscosity of distilled water is 1.003 cPcP @ 20@ 20°°C C –– our measured value 1.004 our measured value 1.004 cPcP+/+/-- 2 x 102 x 10--3 3 @ 20@ 20oo CCThe seltzer water viscosity curve never approached a limiting vaThe seltzer water viscosity curve never approached a limiting value because there lue because there were so many bubbles present on the crystal during the entire duwere so many bubbles present on the crystal during the entire duration of the data ration of the data run (remember, lots of bubbles run (remember, lots of bubbles →→ lower viscosity)lower viscosity)This means that the bubbles clung heavily to the This means that the bubbles clung heavily to the thiolthiol--coated Au surface!!!coated Au surface!!!

10 20 300.1

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Seltzer H2O on Au + Octadecanethiol - JMJ, HKN, DAG - 7/13/06

Distilled Water on Distilled Water on ThiolsThiols and Au/SiOand Au/SiO22

As time progresses and distilled water sits in air, air moleculeAs time progresses and distilled water sits in air, air molecules will s will dissolve back into the water. This causes the viscosity to decredissolve back into the water. This causes the viscosity to decrease. ase.

We measured the viscosity of vacuum distilled water minutes, houWe measured the viscosity of vacuum distilled water minutes, hours, rs, and days after breaking vacuum, in order to make recommendationsand days after breaking vacuum, in order to make recommendationsfor those doing viscosity measurements of lubricants.for those doing viscosity measurements of lubricants.

We conducted these measurements with both hydrophilic and We conducted these measurements with both hydrophilic and hydrophobic QCM surfaces.hydrophobic QCM surfaces.

0 5 10 15 20 25 30 35 400.00080

0.00085

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2 )

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Thiol Crystal

SiO2 Crystal

Bulk H2O Viscosity

Distilled Water on Distilled Water on ThiolsThiols and Au/SiOand Au/SiO22

MacrobubbleMacrobubble ConclusionsConclusions

We are able to measure viscosity quite well using the We are able to measure viscosity quite well using the QCM QCM –– Distilled water, and regular DI waterDistilled water, and regular DI waterWe have acquired video of bubbles forming on the We have acquired video of bubbles forming on the surface, and correlated that video with QCM surface, and correlated that video with QCM measurements to quantify the effect of the bubbles on measurements to quantify the effect of the bubbles on viscosityviscosityWe are able to see clearly that We are able to see clearly that macrobubblesmacrobubbles reduce the reduce the measured viscosity measured viscosity Bubbles attach to the Bubbles attach to the nn--AlkanethiolAlkanethiol coated Au very well, coated Au very well, as the seltzer water data shows.as the seltzer water data shows.Bubbles leave hydrophilic surfaces relatively quickly, but Bubbles leave hydrophilic surfaces relatively quickly, but they linger on the hydrophobic surfacesthey linger on the hydrophobic surfaces

NanobubbleNanobubble ConclusionsConclusionsFor the first week after distillation, the nearFor the first week after distillation, the near--surface surface viscosity measured with both the hydrophobic and viscosity measured with both the hydrophobic and hydrophilic crystals matches the bulk viscosity within hydrophilic crystals matches the bulk viscosity within experimental error.experimental error.After one week, too much air has diffused into the water, After one week, too much air has diffused into the water, and the nearand the near--surface viscosity as measured by the surface viscosity as measured by the hydrophobic crystal is reduced compared to the bulk hydrophobic crystal is reduced compared to the bulk value. This indicates value. This indicates nanobubblesnanobubbles are forming on the are forming on the hydrophobic surface.hydrophobic surface.The average value of the nearThe average value of the near--surface viscosity as surface viscosity as measured by the hydrophilic crystal is 1.12 +/measured by the hydrophilic crystal is 1.12 +/-- 0.07 0.07 cPcP. . The average lab temperature is 20.4 +/The average lab temperature is 20.4 +/--0.2 C, and at this 0.2 C, and at this temperature, the bulk viscosity of water is between 0.98temperature, the bulk viscosity of water is between 0.98--1.00 1.00 cPcP..The elevated nearThe elevated near--surface viscosity measured by the surface viscosity measured by the hydrophilic crystal could be due to tight packing of the hydrophilic crystal could be due to tight packing of the water molecules near the surface, or water molecules near the surface, or solvationsolvation forces.forces.

AcknowledgementsAcknowledgements

Thanks to the PRF, URC, and College of Thanks to the PRF, URC, and College of Arts and Sciences for the funding of this Arts and Sciences for the funding of this project.project.Thanks to Jon Jones, Heather Thanks to Jon Jones, Heather NemetzNemetz, , and Joe and Joe MoebusMoebus for their work on this for their work on this project.project.Thanks for inviting me to speak!Thanks for inviting me to speak!