Stick and Oscillatory Behavior of Bubbles Due to TiO2 Nanoparticle Coating in Subcooled Pool Boiling on a Wire

Mehrdad Karimzadehkhouei1,2, Arzu Özbey1,2, Khellil Sefiane2,3 and Ali Koşar1,2,4,a

1 Mechatronics Engineering Program, Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, Istanbul 34956,

Turkey2 Institute for Materials and Processes, School of Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JL,

UK3 Tianjin Key Lab of Refrigeration Technology, Tianjin University of Commerce, Tianjin City 300134, PR China4 Center of Excellence for Functional Surfaces and Interfaces for Nano-Diagnostics (EFSUN), Sabanci University, Tuzla,

Istanbul 34956, Turkey

Abstract. Nanoparticles are found to alter the contact angle and wettability characteristics thereby inducing a significant

effect on pool boiling. Generally, boiling of nanofluids results in deposition of nanoparticles on the heating surface.

Therefore, it is difficult to distinguish whether nanoparticles coating or dispersion is dominating in boiling. The present study

reports on sticking and oscillatory behavior of bubbles in pool boiling on a platinum wire to further broaden our

understanding of these underlying phenomena. Four different cases, namely pure deionized water on both pristine and TiO 2

nanoparticle coated wires, TiO2 nanoparticle/water nanofluids at two mass fractions of 0.002% and 0.005% on pristine wire,

were tested to unravel bubble dynamics in pool boiling in the presence of nanoparticles. Moreover, particle-particle

interaction and nanoparticle coating effects on contact angle were investigated by comparing the results and describing acting

forces on bubbles. The presence of both coated surfaces and dispersed nanoparticles led to the stick and oscillatory behavior

of bubbles at high mass fraction, which was explained by the force balance analysis.

Since their introduction by Choi and Eastman,1 nanofluids have attracted great attention due to their potential in heat

transfer enhancement, patterning-based applications and drug delivery. Previous investigations indicate that nanoparticles

alter the thermophysical properties of base fluids and consequently affect heat transfer.2–6 One of the most promising

properties of nanofluids is enhanced thermal conductivity compared to the base fluids. 4,7 The effects of nanoparticles on heat

transfer were explained with ballistic phonon transport, Brownian motion of the nanoparticles, layering at the solid/liquid

interface and agglomeration/deposition of nanoparticles.8,9 Recently, a number of studies have focused on the viscosity of

nanofluids.7,10 Accordingly, the viscosity of nanofluids is found to increase with mass fraction and decreases with

temperature.11,12

Both enhancement and deterioration in boiling heat transfer with nanoparticles were reported due to the change in

nucleation site density and surface roughness/morphology13–15, while enhancement of critical heat flux (CHF) was achieved

due to modified surface wettability, structural disjoining pressure and contact line pinning. 13,15 Disjoining pressure, which has

three components; namely molecular, electrostatic and structural, is one of the important surface forces near the three-phase

a Corresponding author, Tel: (+90)216-4839621, e-mail: kosara@sabanciuniv.edu

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contact region and depends on the thickness of the thin film.16 The structural disjoining phenomenon arises from ordered

layering of nanoparticles inside the confined geometry of the wedge film forming between liquid drop or bubble and solid

surface.17 The reason for the pinning behavior is possibly structural disjoining pressure, which changes the force balance at

the contact line.

On the other hand, dispersed nanoparticles in nanofluids change the surface characteristics due

to deposition of nanoparticles on the surface and generate a thin layer, which affects boiling heat

transfer. Deposition of nanoparticles may decrease the nucleation site density18,19 and may

introduce an extra thermal resistance via affecting the surface roughness,18 wettability,19,20 and

receding and advancing contact angles.20 The mechanisms of wetting in nanofluids are not yet fully

understood because of additional particle-particle, particle-solid and particle-fluid interactions in

comparison to pure fluids. Furthermore, both nanoparticle concentration and size of nanoparticles

influence the contact angle.21 A possible reason for the change in contact angle is due to solid-like

layer ordering of nanoparticles in the contact line region.22,23

The aim of this study is to investigate and explain the stick and oscillatory behavior in bubble dynamics on a platinum

wire in the presence of nanoparticles at two different mass fractions. This has not been reported in the literature when

investigating pure liquids.

The experimental setup, Fig. 1, consists of a platinum wire with a length and diameter of 80 mm and 250 µm,

respectively. The bulk temperature inside the chamber was controlled by a cartridge heater inside the chamber and a PID

unit.

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FIG. 1. Schematic of the experimental setup.

Nanofluids were prepared by dispersing TiO2 nanoparticles with mean diameters of 10-30 nm (Ionic Liquids

Technologies, io-li-tec GmbH, Germany) in deionized water, without using any surfactant, at two mass fractions of 0.002%

and 0.005%. The mixing was performed via a sonicator (Fisher Scientific Ltd., cat. no. FB15047) and magnetic stirrer (for at

least half an hour each).

Nanoparticles were analyzed with the Scanning Electron Microscopy (SEM) technique (Fig. 2a). Dynamic light

scattering (DLS) measurements were performed for both mass fractions (90PLUS/BI-MASS, Brookhaven Instruments

Corporation, USA). As shown in Fig. 2b, the average hydrodynamic diameters of prepared nanofluids are 214 nm and 117

nm for mass fractions of 0.002% and 0.005%, respectively. Additionally, SEM images of pristine (before experiment) and

TiO2-nanoparticles-coated surfaces, which were achieved during experiments with nanofluids of mass fractions of both 0.002

and 0.005 wt.%, are displayed in Figs. 2c and d, respectively. It can be clearly observed in Fig. 2d that there is a coated layer

of nanoparticles, which affects the bubble behavior on the wire.

FIG. 2. Scanning electron microscopy (SEM) image of: a) TiO2 nanoparticles, c) pristine platinum wire (before experiment), d) TiO2 nanoparticle coated platinum wire (after experiment). b) Dynamic light scattering (DLS) measurements of TiO2

nanoparticle/water nanofluids.

The experiments were conducted at atmospheric pressure by: i) filling the pool boiling chamber from the upper part with

working fluids up to a level of 2.5 cm above the wire, ii) adjusting the bulk temperature to the desired temperature using the

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PID controller starting from 30°C with intervals of 5°C up to 50°C, iii) applying heat flux to the platinum wire, and iv)

recording the videos utilizing a Phantom V4 high speed camera with the resolution of 512×512 and frame rate of 1000 fps.

Following the experiments, the captured images were analyzed using the PCC 2.7 software. The figures displaying the results

were prepared using ImageJ software.

The equilibrium contact angle, θ , is expressed according to Young’s equation as:

cosθ=γ SV −γ SL

γ LV(1)

where γSV −γ SL is the adhesion tension and γ LV is the surface tension. Surface tension of water at 25°C is approximately

72 mN/m, and its value decreases with temperature. It is approximately 69.6 mN/m at 40°C. 24 The surface tension of TiO2

nanoparticles/water nanofluid at mass fractions of 0.002 wt.% and 0.005 wt.% (without introducing any surfactant at the

ambient temperature25) is approximately the same as that of DI-water. However, a decrease in the contact angle is observed

for nanofluids (Fig. 3b). This can be attributed to the disjoining pressure and pinning effects or deposition of a layer of

particles, which affect the solid/liquid interfacial layer. With an increase in mass fraction, the structural disjoining pressure

and pinning effect become more obvious, and a further decrease in the contact angle can be observed (Fig. 3c). Moreover,

deposition and alteration of solid/liquid interfacial tension should be taken into consideration that by increasing concentration

the coverage increases. This trend in the contact angle can be also attained for pure water on nanoparticle coated wire (Figs.

3a and Fig. 3d) due to the effect of TiO2 nanoparticle deposition on the adhesion tension of the surface.

FIG. 3. Contact angle of vapor bubble during leaping on platinum wire is: a) 60° (DI-water, q´´=1096 kW/m2), b) 40° (φ=0.002 wt.%, q´´=1291 kW/m2), c) 35° (φ=0.005 wt.%, q´´=1222 kW/m2), and d) 48° (DI-water, deposited wire, q´´=1393

kW/m2). (Tb=40°C)

In what follows we attempt to describe the main forces acting on the bubble during growth on the wire, namely

Marangoni, buoyancy, surface tension (contact line), and drag forces shown in Fig. 4.

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FIG. 4. The main forces acting on a bubble

The Marangoni force, FMa , is expressed as:26

FMa∝∇ σ (2)

Surface tension, σ , varies with temperature, T , and concentration, c :27

dσ=∂ σ∂T

dT+∂ σ∂ c

dc (3)

By considering the temperature gradient only, the thermocapillary force, FMa , is described as:28

FMa=−23

πrb γΔT (4)

where rb is the bubble radius, γ=−∂σ /∂T i is a positive constant number for most of the fluids, T i is the temperature

along the bubble interface and ΔT is temperature gradient. Thus, vertical and horizontal Marangoni forces, FMa , y and

FMa , x , can be calculated by considering the temperature gradient between bubble’s top and bottom or front and backside,

respectively. It should be noted that although the density of the bubble is much smaller than that of the bulk fluid, bubbles

remain on the wire due to the fact that the Marangoni flow generates a thrust force29 on the bubbles, and bubble rising is thus

prevented because of the vertical component of the thermocapillary force.

The contact line force, Fσ , and the contact line force per unit length, Fσ'

, (projection along the wire) are given as:

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Fσ=(2πrb sinθ )σ cosθ (5)

Fσ' =σ cosθ (6)

where θ is the contact angle of the bubble on the surface. Contact angles of a bubble on the platinum wire for different

surfaces, and working fluids were measured (Fig. 3). As previously mentioned, the surface tension of the working fluids

remains almost the same. By implementing the measured contact angles to the contact line force equation and comparing the

results with pure water on the pristine platinum wire, increases in the contact line force are approximately 30%, 40% and

15% for nanofluids with mass fractions of 0.002 wt.%, 0.005 wt.% and for pure water with a nanoparticle coated surface,

respectively.

The drag force, which acts on the bubble can be categorized as viscous drag force, acting by the fluid on the bubble, and

friction force, acting by the surface on the bubble. The viscous drag force, FD , according to the Hadamard-Rybczynski law

is expressed as:30

FD=−4 πμ f r b u b (7)

where μ f is the liquid viscosity and u b is the bubble velocity. Besides, friction force which acts as a resistive force, also

plays a role during bubble motion on the wire. The parameters affecting the friction force are surface roughness, and

nanoparticle presence.

The buoyancy force, Fb , is given as:

Fb=( ρg−ρf )g cosϕ .V b (8)

where ρg and ρ f are the gas and fluid densities, respectively, ϕ is the plane orientation and V b is the bubble volume.

Figure 5 (see Supplementary Video) shows four cases; Case I) DI-water on a pristine platinum wire, Case II) nanofluid

with a mass fraction of φ=0.002 wt.%, Case III) nanofluid with a mass fraction of φ=0.005 wt.%, and Case IV) DI-water on a

wire with TiO2-nanoparticle coating. In Case I, slipping of bubbles upon nucleation is dominant due to thermocapillary

forces. After bubble nucleation, when thermocapillary force becomes greater than contact line force, the bubble starts

slipping. Additionally, collision phenomenon can be observed at t+234 ms owing to the fact that an adjacent bubble acts as a

heat sink. Hence, the temperature gradient becomes zero between the front and backside of the bubble. Besides, the drag

force acts as a resistive force. In Case II, with the introduction of nanoparticles in DI-water at the same heat flux, typically

coalescence takes place. After the coalescence of the two bubbles (Bubble I and Bubble II) (t+106 ms), interestingly, a small

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bubble (Bubble IV) is flung out of the newly formed bubble (Bubble III). Then, Bubble IV continues growing (t+177 ms) at

the same position, while Bubble III moves away from the growing bubble. According to these observations, it can be claimed

that not only momentum of initially moving bubbles but also remaining small bubbles which are flung out after coalescence

could determine the migration direction of the newly formed bubble. This is because the remaining small bubble can alter the

wire temperature and change the newly formed bubble direction by changing the force balance acting on the bubble. One of

the most remarkable phenomenon is stick behavior of the bubble in Case III. With a further increase in the nanoparticle mass

fraction, the deposition effect of nanoparticles on the surface becomes more than in Case II. Furthermore, sticking bubbles

induce periodic oscillations of the bubble. Deposition of nanoparticles increases the friction force and decreases the contact

angle, thereby increasing the contact line force as presented earlier. Thus, the oscillation process will be facilitated in the

presence of nanoparticles due to the formation of more stationary sticking bubbles. Solid-like ordering of the nanoparticles

can occur in the contact line region,16 and this effect is more obvious at a higher mass fraction.21 The bubble behavior of Case

IV is similar to Case II. When the measured contact angles are taken into account, it can be observed that the contact angle of

a bubble in Case IV is greater than that in Case II. Thus, contact line force is lower. On the contrary, the contact line force in

Case IV is higher than that in Case I, due to a lower contact angle in Case IV.

By measuring the contact angles (shown in Fig. 5) as approximately 55°, 50°, 40°, and 42°, respectively, the same trend

as in Fig. 3 can be observed. As mentioned in the manuscript, surface tension for pure water and both of nanofluids are

approximately the same. Thus, contact angles become dominant parameters in the contact line force. By implementing the

measured contact angles to the contact line force equation and comparing the results with pure water on the pristine platinum

wire, increases in the contact line force are approximately 15%, 35% and 30% for nanofluids with mass fractions of 0.002 wt.

%, 0.005 wt.% and for pure water with nanoparticle coated surface, respectively. Additionally, contact line force and drag

force for sample bubbles (in each case shown in Fig. 5) are calculated and displayed in Table 1.

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FIG. 5. Bubble behavior for applied heat flux of approximately q´´=845±45 kW/m2, bulk temperature of Tb=35°C and working fluid of: a) pure deionized water on pristine wire, b) nanofluid (φ=0.002 wt.%), c) nanofluid (φ=0.005 wt.%) (Black

arrows point the sticking bubbles as marked.), and d) pure deionized water on TiO2-nanoparticle coated wire.

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Table 1. Contact line and drag forces of sample bubbles marked in Fig. 5.

BubbleDiameter

(µm)

θ

(°)

Velocity

(mm/s)

Fσ

10-5 (N)

FD

10-8 (N)

(a) 270 55 34 2.77 5.77

(b) 350 50 43 3.77 9.47

(c) 290 40 28 3.12 5.12

(d) 520 42 39 5.65 12.7

Figure 6 shows the behavior of a bubble under the influence of TiO 2-nanoparticle deposition on the wire while having

oscillations for the nanofluid with a larger mass fraction, namely 0.005 wt.%. As can be seen, a moving bubble approaches a

stationary one at t ms. During lateral migration until it stops near the stationary bubble, there is a knoll formed as a result of

the nanoparticle deposition. Interestingly, sometimes the oscillating bubble climbs up the knoll, t+58 ms, and passes the hill

and continues with its motion. However, sometimes the bubble cannot climb up to the top and oscillates between the knoll

and the other stationary bubble (which is not seen in the figure). The major forces acting on the bubble in this situation are

Marangoni, contact line, and drag forces (Fig. 4). It should be mentioned that besides the nanoparticle deposition, structural

disjoining pressure and pinning should be taken into account. According to the contact line force calculation based on the

experimentally measured contact angles, contact line force increases approximately up to 35% for the nanofluid with a mass

fraction of 0.005 wt.% in comparison to pure water on the pristine wire. Thus, the required driving force, mainly the

horizontal component of Marangoni force, should be higher, which leads to the bubble exhibiting a more sticking behavior.

FIG. 6. Effect of nanoparticle deposition on bubble behavior on wire for nanofluid.(φ=0.005 wt.%, Tb=35°C, q´´=845kW/m2)

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Sticking bubbles are observed for nanofluids with a mass fraction of 0.005% at bulk temperatures of 35°C and 40°C at

applied heat fluxes of 213-2015 kW/m2 and 175-2005 kW/m2, respectively. Besides, oscillatory behavior of the bubbles for

the mentioned nanofluids at the mentioned bulk temperatures is seen at heat fluxes of 463-2015 kW/m2 and 497-2005 kW/m2,

respectively.

In summary, both deposition and the presence of nanoparticles alter the contact angle of a bubble on the wire and change

the wettability of the surface due to particle-particle interaction and deposition. By introducing nanoparticles into the base

fluid or on the surface as coating, stick and oscillatory behavior can be observed in different cases due to the change in the

force balance on bubbles. The most distinguishable result stems from the combined effect of nanoparticle deposition and

nanofluid dispersion and is the stick and oscillatory behavior possibly due to structural disjoining pressure and pinning

effects.

The authors would like to thank Prof. David Kenning for Oxford University for donating the pool boiling chamber for

their use. This work was supported by the Newton Research Collaboration Program with Contract Grant No.

NRCP1516/1/126. The authors thank Dr. Coinneach MacKenzie-Dover from Scottish Microelectronics Centre (SMC) for his

assistance in taking the SEM images, Mr. Douglas Carmichael, Mr. Kevin Tierney from the electrical workshop and Mr. Paul

Angus from the mechanical workshop of the University of Edinburgh for their valuable help in the preparation of the

experimental setup. KS acknowledges the support from the British Council, EPSRC under grant EP/N011341/1.

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