Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 3863−3869
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Origin, Evolution, and Movement of Microlayer in Pool Boiling An Zou, Manish Gupta, and Shalabh C. Maroo* Department of Mechanical and Aerospace Engineering, Syracuse University, Syracuse, New York 13244, United States
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S Supporting Information *
ABSTRACT: The microlayer thin film is visualized in situ in a vapor bubble during pool boiling. Contrary to current understanding, bubbles originate on hydrophilic and silanecoated hydrophobic surfaces without a three-phase contact line, i.e., the microlayer completely covers the bubble base. The occurrence of such a wetted bubble base is found to be dependent on the liquid−solid interaction. As the bubble grows in time, the film decreases in thickness, eventually forming the contact line and dry region. During this drying out process, curvature at the center of the microlayer shows a cyclical behavior due to competing Marangoni and capillary flows, and is characterized as a “dryout viscosity”. After the dry region forms, the mechanism of contact line/microlayer movement of a single bubble on the hydrophilic surface is experimentally determined, and a generalized expression of energy required for its unpinning and movement is defined.
microlayer is a thin, multiscale liquid film present underneath a vapor bubble next to the three-phase contact line (Figure 1) and is of great importance in pool boiling.1−3 Extremely high heat flux occurs from the microlayer and thus serves as a major heat transfer mechanism during the bubble growth period.2,4 Better understanding of the microlayer has led to novel ideas for enhancing boiling heat transfer, e.g., microstructures fabricated on pool boiling surfaces have led to early evaporation of microlayer5,6 as well as its augmentation,7 causing an increase of ∼120% in the critical heat flux. The effect of the microlayer in pool boiling was noticed from temperature fluctuations measured underneath a vapor bubble during the ebullition cycle,8 and has led to studies characterizing its role and impact in pool boiling.9−16 Visualization of the microlayer using laser interferometry17−22 has provided important insights into its span and thickness. However, despite volumes of research on the microlayer for over half a century, its origin, evolution, and movement have not yet been studied in situ in a vapor bubble during pool boiling. Here, we report such a fundamental study by conducting experiments and performing molecular dynamics simulations of bubble nucleation. We used laser heating to create a vapor bubble on a surface submerged in a pool of deionized (DI) water at room temperature. This technique is useful in nucleating a small bubble at a predetermined location necessary for in situ visualization of the microlayer. A blue CW laser beam (wavelength of 447 ± 5 nm) was introduced into an inverted microscope, passed through a 50× objective, and focused on the sample to generate a highly localized heating area corresponding to an equivalent beam diameter ∼15 μm. The same objective was used to image the bubble, which was illuminated from above with a 632 nm HeNe laser (Figure 1); this configuration created a bubble image with a dark annulus ring as light has to refract across multiple interfaces in that
A
© XXXX American Chemical Society
Figure 1. Schematic of the multiscale microlayer film at bubble base and fringe formation in the microlayer due to interference of light.
region. The sample consisted of multiple thin layers deposited on a fused silica substrate: a sandwiched 40 nm tungsten film between titanium films to absorb the laser and heat the sample, and a 1 μm SiO2 layer acting as a hydrophilic surface (static contact angle; 19.6 ± 0.9°; advancing contact angle, 26.4 ± 0.5°). A hydrophobic surface (contact angle, 109.8 ± 2.9°) was also made by depositing a single layer of tridecafluoro-1,1,2,2tetrahydrooctyltrichlorosilane (FOTS) molecules on top of the SiO2 layer. The laser wavelength does not directly create a bubble in water and was tested with a different sample without the sandwiched film. Detailed description of the experimental Received: May 27, 2018 Accepted: June 25, 2018 Published: June 25, 2018 3863
DOI: 10.1021/acs.jpclett.8b01646 J. Phys. Chem. Lett. 2018, 9, 3863−3869
Letter
The Journal of Physical Chemistry Letters
liquid film between vapor and surface acts as a lubricant (Supporting Information (SI) Video S1). In contrast, a larger bubble with a three-phase contact line (formed in regular DI water) does not move along with the laser on the same hydrophobic FOTS surface (SI Video S1), thus confirming that the smaller bubble has the microlayer completely covering the bubble base. The bubble does not depart from the surface as the buoyancy force is smaller than the capillary and disjoining forces in the microlayer pulling the bubble to the surface.21 To understand the underlying physics of the completely wetted bubble base, molecular dynamics simulations were performed in LAMMPS software23 with liquid argon present between two platinum walls. Even though properties of water− SiO2 are different from argon−platinum, the liquid−vapor phase change process of the fluids are similar based on their saturation curves, i.e., a bubble forming in argon or water will follow similar thermodynamic steps even though the absolute conditions under which they occur are different. Thus, the MD simulations here are meant to identify the physical mechanism for the occurrence of the wetted bubble case. In these simulations, the upper wall was moved outward at constant speed (Figure 3a) to decrease the pressure in the liquid and initiate nucleation. The lower wall was modeled as the hydrophilic surface by using a 12−2 Lennard-Jones potential between the wall and argon atoms (Figure 3b). The hydrophobic FOTS-coated surface was mimicked by including a single graphene layer on the lower wall. Argon was coupled to a thermostat at 90 K. Detailed information on the simulation domain and model parameters is included in the SI. Parts c and d of Figure 3 depict the steady-state simulation domain with the nucleated bubbles where, similar to the experiments, a liquid film is present between the bubble and each surface. Statistical analysis shows that due to strong interaction between the lower wall and argon atoms, high density liquid layers form next to the wall (Figure 3e,f), leading
setup can be found in the authors’ previous work.21 Experiments were conducted with two kinds of water: degassed DI water and regular DI water (with dissolved air). The bubble and microlayer were imaged using a high-speed camera attached to the microscope through a side port. In degassed DI water with constant laser power, a stable vapor bubble (∼10 μm diameter) formed on both hydrophilic and FOTS-coated hydrophobic surfaces as evaporation from the microlayer was balanced by condensation at the cooler liquid−vapor bubble interface.21 Fringes were obtained in the microlayer thin film due to coherent light interference (Figure 1) across regions of different refractive indices.18 Surprisingly, the fringes are seen throughout the bubble base on both surfaces (Figure 2), i.e., the microlayer liquid film covers the
Figure 2. Experimental observation of completely wetted bubble base. Vapor bubble movement in pool boiling experiments on both hydrophilic and silane-coated hydrophobic surfaces. Scale bar is 20 μm.
entire bubble base and the three-phase contact line is not present. This observation is further verified with the movement of the surface relative to the laser spot; on both surfaces, the bubble smoothly follows the laser spot as the
Figure 3. Underlying mechanism of completely wetted bubble base. Molecular dynamics simulations and statistical analysis of onset of bubble nucleation on hydrophilic surface without (c,e,g) and with (d,f,h) hydrophobic graphene coating. 3864
DOI: 10.1021/acs.jpclett.8b01646 J. Phys. Chem. Lett. 2018, 9, 3863−3869
Letter
The Journal of Physical Chemistry Letters
Figure 4. Evolution of bubble from wetted bubble base to formation of dry region at its center. High-speed images of bubble growth on hydrophilic SiO2 without (a1,a2,a3) and with (b1,b2,b3) hydrophobic FOTS coating and corresponding microlayer thickness and curvature profiles (a4,b4). Schematic in inset shows the side view of the bubble for the corresponding image, highlighting the completely wetted bubble base or presence of contact line and dry region. Scale bar is 50 μm.
covers the entire bubble base similar to the prior finding of Figure 2. However, as the bubble grows, the microlayer thickness decreases (Figure 4a2,b2) and ultimately dries out at the center (Figure 4a3,b3), creating the three-phase contact line when the liquid−vapor bubble interface meets the solid surface. As the F-2 fringes are separated by an optical path difference that is half wavelength (eq 1), the position of the fringes was used to estimate the microlayer thickness and curvature profiles on both surfaces (Figure 4a4,b4). The continuous red line represents the best fit curve to the measured points (red dots), and the curvature profile (blue line) is determined from this curve fit (eq 2).
to significantly high pressures in that region (Figure 3g,h). A bubble forms above these liquid layers as it is thermodynamically favorable to achieve lower pressures required for nucleation, thus resulting in a liquid film (i.e., the liquid layers) being present underneath the bubble. A liquid film, although of smaller thickness, also forms on the hydrophobic graphene surface as argon atoms can still interact with the underlying hydrophilic surface. This mechanism is amplified in experiments with water−SiO2 combination where polar atoms are involved (comparison in SI). Such atoms interact as a function of r−1, compared to r−2 used in the simulations (r is the distance between two atoms), thus leading to thicker high density liquid water films which can be measured in experiments (the minimum measurable microlayer film thickness in our experiments is ∼160 nm based on diffraction limit). In fact, experiments24 have shown that a glass surface causes long-range ordering of water molecules up to a distance of 800 nm from the surface due to increased proton transfer rates. However, such an observation will not likely occur on liquid−solid combinations with weak interactions (e.g., if either is nonpolar); this was confirmed with additional simulations and experiments (please see SI). Next, experiments were conducted with regular DI water containing dissolved air. Even though the laser was at constant power, the bubble grew in time (SI, Video S2) as air was released into the bubble along with vapor. The vapor condensed on the cooler liquid−vapor bubble interface; however, the air kept accumulating, causing the bubble to grow continuously. The growth rate can be varied by mixing regular and degassed DI water. Parts a and b of Figure 4 show the bubble growth and fringe profiles in time for the hydrophilic and hydrophobic surfaces, respectively. Two sets of fringes can be seen:18,21 F-1 (interference caused by the top curved interface of the bubble) and F-2 (interference due to the microlayer thin film). F-2 fringes show up immediately after bubble nucleation, suggesting the presence of the microlayer, while F-1 fringes become more prominent after the bubble has grown to a relatively larger size. At the early stage of the bubble growth (Figure 4a1,b1), the microlayer
tm + 1 − tm = κ=
λ0 2n cos θ
|t ″| (1 + t ′2 )1/2
(1)
(2)
where tm+1 and tm are thickness for (m+1)th and mth fringe, respectively, λ0 is wavelength of the illumination source, n is the refractive index of the medium, θ is the incident angle of the light source, and κ is curvature. The maximum error possible in fringe location due to bubble size and angle of light source was found to be minimal ∼5% (analysis in SI). In the case of the completely wetted bubble base, the first fringe was placed at zero, as the thickness at the center cannot be measured due to lack of reference to the surface position. The wavy aspect of the curvature is either due to (1) instabilities in thin films which extend over a larger width25,26 or (2) noise in the experimental data.27 The latter implies that the average curvature away from the center of the film is ∼0 μm−1, as represented by the dotted lines in Figure 4a4 (average value = −0.003 μm−1) and Figure 4b4 (average value = −0.005 μm−1); thus, in such a case, the film thickness varies linearly along the radial direction. However, in this work, we are interested in the curvature at the center of the microlayer, i.e., at radial distance = 0 μm. The curvature is largest at the center of the microlayer, thus implying the liquid pressure is most reduced from its equilibrium state at that location due to capillary effects; in 3865
DOI: 10.1021/acs.jpclett.8b01646 J. Phys. Chem. Lett. 2018, 9, 3863−3869
Letter
The Journal of Physical Chemistry Letters
flow away from the center. As an effect, the rate of film thickness decrease slows down at the center, leading to a decrease in its curvature (Figure 5B) and capillary force, with its lowest point occuring at the base of the cycle (Figure 5C). At this point, due to weakened capillary flow, Marangoni force instantaneously (∼in a few millisecond) decreases the film thickness most at the center (as temperature is highest there), leading to a sudden increase in its curvature which is the peak of the next cycle, and the next cycle repeats similarly with continuously decreasing microlayer thickness. The two drastically different observed time scales are attributed to competing Marangoni and capillary flows (analysis in SI). The cycle period is also of importance in Figure 5, as it lengthens in each cycle due to increase in the peak curvature and corresponding maximum capillary pressure (Pcap = σ · κ). Capillary pressures >10000 Pa are sustained in the microlayer during dryout. Resistance to dryout occurs as liquid flows toward the center due to capillary forces and can be seen to be similar to the role of viscosity (resistance of a fluid to deformation). This “dryout viscosity” μdryout (unit of Pa·s) can be defined as in eq 3 and can be estimated from Figure 5 by curve-fitting a cycle (occurring between times t1 and t2) and finding the area underneath the curve.
such a case, liquid will passively flow to the center to keep the surface wet. However, evaporation and Marangoni effects dominate the evolution process, causing the microlayer thickness to decrease and eventually dry out at the center, creating the contact line. The curvature at the center of the microlayer was analyzed in the time between bubble nucleation and contact line formation, as the center is the most dynamic location of the microlayer where capillary and Marangoni effects are most prominent. High-speed camera images (at 500 fps) were used from the experiment on the hydrophilic surface because the bubble base remains wetted for a longer duration of time, as seen in Figure 4a. Over 1400 images were analyzed using Matlab in which the fringe positions were located for each image, and the microlayer profile was determined using eq 1 and curve fitting (detailed procedure in SI). From the microlayer profile, the curvature at the center was calculated using eq 2, and was plotted with time in Figure 5. The plot
μdryout =
∫t
t2
(σ · κ ) d t
1
(3)
As the film decreases in thickness, the dryout viscosity increases over 4-fold, from ∼2700 Pa·s (for cycle when N = 6) to ∼11500 Pa·s (for N = 11) as the peak curvature increases from ∼0.25 to ∼0.35 μm−1. Further, a Marangoni number for dryout can be defined as Madryout =
ΔT dσ dT Vy − dryout
μdryout
(4)
where T is temperature and Vy−dryout is the interface velocity normal to the surface as the film thickness decreases and dries out. In the experiment, the microlayer dryout happens after ∼4 s. The initial microlayer thickness is not known but will be in the range of ∼200 nm to ∼5 μm for fringe patterns to occur at the given wavelength. Thus, average values of Vy−dryout will range < 500 nm/s, and the corresponding Marangoni flow resistance to wetting (numerator of eq 4) is expected to be >20000 Pa·s for ∼60 °C temperature difference. Thus, Madryout > 1 leads to eventual dryout of the microlayer and formation of contact line. Vy−dryout is also expected to vary and decrease with time as the dryout viscosity increases. After the microlayer thickness decreases and dries out at the center (as shown earlier in Figure 4), the liquid−vapor interface of the microlayer meets the solid surface, creating the three-phase contact line (Figure 6a1). The movement of contact line and associated microlayer is very relevant in pool boiling as the contact line first recedes and then advances prior to bubble departure. The time taken for its movement is directly related to the occurrence of critical heat flux.28 Using our experimental setup, the contact line/microlayer movement can be visualized and measured in situ for a single bubble by moving the laser beam relative to the hydrophilic surface, and is done next. Figure 6a shows the vapor bubble’s contact line and microlayer movement on SiO2 surface. At t = 0, the bubble is in an equilibrium state with the laser present at the center of
Figure 5. Time variation of curvature at center of microlayer determined from analysis of over 1400 experimental images during bubble growth on the hydrophilic surface (dotted lines only serve as guide to eye). Area under the curve of each cycle can be calculated to determine the “dryout viscosity”. Schematics (A,B,C) explain the underlying mechanism for the cyclical behavior.
shows a cyclic behavior of the curvature and is dependent on the number of observed fringes N as it increases from N = 3 to N = 12. In each cycle, the curvature decreases and then suddenly jumps to a peak for the next cycle. Further, for time >1 s, the curvature initially decreases slowly followed by a dramatic decrease before jumping to the next peak. This behavior can be explained using the schematics A, B, and C shown in Figure 5, which correspond to the peak, midpoint, and base of one cycle, respectively. When the curvature is largest at the center for the thin microlayer film (Figure 5A), capillary force is highest (Pcap = σ · κ, where σ is surface tension) and provides stronger liquid flow toward the center to counter the dominant Marangoni force that causes liquid to 3866
DOI: 10.1021/acs.jpclett.8b01646 J. Phys. Chem. Lett. 2018, 9, 3863−3869
Letter
The Journal of Physical Chemistry Letters
Figure 6. Time evolution and mechanism of contact line/microlayer movement of bubble on hydrophilic SiO2 surface. (a) Experimental images of the bubble, (b) sketch of associated bubble shape (blue and red lines represent steady (at t = 0 s) and dynamic shapes, respectively), (c) experimental data of the microlayer profile with x = 0 defined as the laser position, (d) microlayer contact angle variation in advancing/receding region during bubble movement, and (e) generalized form of energy required to unpin and move the contact line/microlayer.
bubble base. At t = 1.5 s, the laser beam is moved to the right with a constant speed ∼16.7 μm/s. As seen in SI Video S3, the contact line and microlayer (and hence the bubble) move along with the laser. This phenomena and the underlying mechanism (Figure 6b) can be understood based on the microlayer contact angle, which is different from the apparent bubble contact angle. The microlayer contact angle at the right and left sides of the bubble are defined as receding (θr‑ml) and advancing (θa‑ml) contact angles, respectively. The microlayer spatial profile (Figure 6c) is experimentally obtained from the parabolic curve fitting of the microlayer height variation with fringe location,21 and the microlayer contact angle is dδ
determined from the measured profile as θml = arctan dx
x = r1
the advancing contact line, thus causing the contact line and microlayer (and hence the bubble) to move along with the laser: F = σlv(cos θa − ml − cos θapp)
(5)
Once the bubble moves, the fringes in the advancing microlayer region reappear and the advancing contact angle recovers to its initial value θa‑ml (Figure 6d). Thus, the energy E required for unpinning and moving the contact line can be obtained as E = FLd, where L is the projected length of contact line on which the net force is exerted and d is the width of the microlayer and contact line region which is pinned on the surface. As this energy is directly related to the bubble diameter (d bubble ), a generalized definition is derived E (please refer to SI) and plotted against the E* = σ ·d·d
,
where r1 is the radial distance where the first fringe shows up in advancing or receding region. The apparent bubble contact angle θapp is the same21 as that of a water droplet on the SiO2 surface. The temporal variation of θr‑ml and θa‑ml is shown in Figure 6d. Initially, as the bubble is in an equilibrium state at t = 0 s, θa‑ml is similar to θr‑ml. During the contact line/ microlayer movement beyond t = 1.5 s, θr‑ml maintains a similar value ∼10° as during equilibrium. However, contact line pinning occurs in the advancing region as observed by the disappearance of fringes in the advancing microlayer region (Figure 6a3), and θa‑ml increases to advancing θapp (i.e., maximum possible contact angle ∼26.4°) during t ∼ 4−5 s (Figure 6b3,d); this change in the advancing contact angle generates a positive net force F (as 0 < θa‑ml < θapp < π/2) at
lv
bubble
easily measured apparent bubble contact angle (Figure 6e). The microlayer contact angle typically ranges between 0.1° to 10° (typically
