Proceedings of the 7th World Congress on Mechanical, Chemical, and Material Engineering (MCM'21)
Prague, Czech Republic Virtual Conference – August, 2021
Paper No. HTFF 144
DOI: 10.11159/htff21.144

A Novel Evaporator Integrated with Wettability Pattern for Improving
Thermal Performance of an Ultrathin Vapor Chamber
Yinchuang Yang1, Jian Li1, Dong Liao1, Xin Ye1, Huihe Qiu1,
1

The Hong Kong University of Science and Technology, Department of Mechanical and Aerospace Engineering
Clear Water Bay, Hong Kong SAR, China
yyangbv@connect.ust.hk; eelijian@seu.edu.cn; dliao@connect.ust.hk; xin.ye@connect.ust.hk; meqiu@ust.hk

Abstract - We investigated the thermal performance of a novel multiscale evaporator of a 0.23 mm thickness ultrathin vapor chamber
which combines a microstructured wettability patterned surface with a nanostructured woven mesh wick. The microstructured wettability
patterned surface underneath the nanostructured woven mesh wick can pin the three-phase contact lines on the hydrophilic/hydrophobic
boundaries, enlarging the area of thin-film evaporation. Besides, the nanostructured mesh not just provides a large amount of backflow
water for maintaining the evaporation on the three-phase contact lines, but also gives more thin-film evaporation area owing to its needlelike nanostructure. Microstructured wettability pattern is fabricated on the inner surface of the evaporator and the wick is pressed onto
the wettability patterned surface by a micropillar array to make them close contact with each other. The micropillar array fabricated byelectroplating on the inner surface of the condenser is also used to support a vapor core as a vapor flow path. The horizontal and vertical
thermal resistances are measured to evaluate the thermal performance of the ultrathin vapor chamber for different charge amounts ofwater when the input heat flux ranges from 8.59 W/cm2 to 23.91 W/cm2 and the heat sink is 22 ℃ forced air cooling with 6.875 cfm
flow rate. The measurement results show that the novel evaporator not just greatly reduce the horizontal thermal resistance, but also lower
the vertical thermal resistance compared with the evaporator without wettability pattern. The highest in-plane effectivity of the ultrathin
vapor chamber with the novel evaporator can reach 11499.5 W/(m·K) at 23.91 W/cm2, giving a 212.7% improvement compared withthe ultrathin vapor chamber without wettability pattern on its evaporator.
Keywords: Chemically patterned surface; Ultrathin vapor chamber; Wettability; Enhance heat transfer; In-plane effective

thermal conductivity

1. Introduction
Increasing demand of the cooling capacity for the thin electronic devices requires a heat spreader with a compact shapeand powerful cooling ability. Ultrathin vapor chamber (UTVC), which utilizes the phase change of working fluid to transfer
heat both in vertical and horizontal direction, becomes a promising solution for the increasing demand owing to its brilliant
cooling ability. Chen et al. [1] developed a 0.4 mm UTVC utilizing etching and diffusion bonding for the fabrications. Huang
et al. [2] adopted a composite wick structure which combines four spiral woven meshes and one bottom mesh for their
UTVC, greatly enhancing the temperature uniformity across the condenser surface. Struss Q et al. [3] investigated thefeasibility of an UTVC fabricated with silicon material for compact electronic cooling. Huang et al. [4] numerically analysed
the heat transfer characteristic of an UTVC which utilized a multi-vapor channel with a spiral woven mesh as a composite
wick. Li et al. [5] experimentally investigated the effect of vapor core thickness on UTVCs. Yu et al. [6] analysed the effectof the passage area ratio of liquid to vapor on the thermal performance of UTVC. As the thickness of UTVC commonly isless than 0.5 mm, enhancing the thermal performance by modifications of wick structures which can cause an increase in the
thickness is limited. While wettability has large effects on the evaporation and boiling, changing wettability with chemicalcoating to improve the thermal performance is more appreciable as it does almost not increase the thickness. Sun et al. [7]modified the condenser wettability of a vapor chamber to superhydrophobic, greatly enhancing the temperature uniformity
across the condenser surface. They also made the wick structure of the vapor chamber superhydrophilic, lowering the vertical
thermal resistance and increasing the critical heat flux [8]. Shaeri et al. [9] investigated the feasibility of a hydrophobic
evaporator for the vapor chamber but failed to enhance the thermal performance compared with a baseline vapor chamber.
Then they adopted a biphilic evaporator to the vapor chamber. Though the vapor chamber shows some enhancements

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compared with the vapor chamber with a hydrophobic evaporator, but still performed poor than the baseline vapor
chamber [10].
Thin-film evaporation, occurring at the contact line of liquid, accounts for more than 50% total heat transfer incapillary wick of a two-phase heat spreader [11, 12]. Increasing the area of thin-film evaporation by elongating thelength of the contact line probably enhances the thermal performance of UTVC. In our previous research, wettability
patterned surface can enhance the evaporation rate of a droplet through prolonging the length of the droplet contact line
[13]. Besides, boiling which might also happened at the high heat flux in the UTVC can be also improved by the surface[14, 15]. Therefore, integrating a wettability patterned surface on the evaporator of a vapor chamber may enhance thethermal performance of the vapor chamber. In this study, we investigated the effect of an evaporator with a wettability
patterned surface on the vapor chamber, founding it enhances the thermal performance both in horizontal and verticaldirection. Besides, a primary investigation on the characteristic of the wettability patterned surface is also conducted.
The experimental results show that the pitch distance of nearby hydrophobic islands greatly affect the extent of the
improvement for thermal performance.

2. Fabrication of the Ultrathin Vapor Chamber and Experimental Setup
2.1. Design and Fabrication of Ultrathin Vapor Chamber
The ultrathin vapor chambers consist of three main parts, condenser with support micropillar array, multiscale
micro/nanostructured mesh wick and chemically wettability patterned evaporator as shown in Figure 1(a). Copper(C1100P)
plates with 30 μm and 40 μm thickness are selected as the bottom casing material and top casing material, respectively.
Support micropillar array is fabricated on the top casing material using the method in Ref. [16]. The thickness of it defines
the height of vapor core. The #450 phosphor bronze mesh (C5191, wire diameter: 28 μm, wire spacing: 28 μm) is adopted
as the wick. As shown in Figure 2(a), a needle-like nanostructure is fabricated on the surface of wick to improve capillarypressure and enlarge the evaporation rate. It is fabricated by immersing the mesh into an aqueous solution with 0.065 M
K2S2O8 and 2.5 M KOH for 30 minutes at 70 ℃, followed by rinsed with deionized water and dried in a 180 ℃ oven for 2
hours [17].

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Figure 1 (a) Schematic of vapor chamber with wettability patterned evaporator (b) Fabrication procedures of wettability patterned
evaporator (c) Schematic of wettability patterned evaporator (d) Micro-view of a wettability patterned surface

To investigate the effect of chemically wettability patterned evaporators on the ultrathin vapor chambers, chemically
wettability patterned surfaces with different geometry are fabricated on the inner surface of the bottom casing material,
forming the evaporator of ultrathin vapor chamber. It consists of hydrophilic region and hydrophobic island, as shown in
Figure 1(b). The dimension of hydrophobic island is 45 μm × 45 μm with a square array distributed on the surface of thebottom casing material while the pitch distance between the centre of two nearby hydrophobic islands is 65 μm or 90 μm.
The procedures for fabricating the chemically wettability patterned evaporator is shown in Figure 1(c). First, copper plate iscleaned by acetone with 15 minutes ultrasonic, followed by rinsed with IPA and deionized water. After that, dilute sulfuric
acid is used to remove native oxide layer on the copper surface and the copper is rinsed with deionized water again. To
protect the hydrophilic region, cleaned copper with HPR506 photoresist pattern is formed by photolithography. Then it isimmersed is immersed into a fluoroalkylsilane (FAS) solution and baked in a 115 ℃ oven to form hydrophobic islands. The
FAS solution has a mass ratio of FAS to hexane of 1.6%. Finally, the HPR506 photoresist is removed by acetone to finish
the fabrication of wettability patterned surface which is shown in Figure 1(d).

Figure 2 (a) SEM picture of nanostructured #450 phosphor bronze mesh; (b) Amplified view of the mesh; (c) Wettability check forpattern with 65 μm pitch distance; (d) Wettability check for pattern with 90 μm pitch distance; (e) Water contact angle of thehydrophilic surface; (f) Water contact angle of the hydrophobic island

The wettability of the surface is checked before assembly of the ultrathin vapor chamber. The evaporator is fixed on the
10 ℃ stage of microscopy and monitored when a 40 ℃ vapor flows pass the surface of evaporator, which is shown in Figure

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2(c) and (d), vapor condenses on hydrophilic region, proving the successful fabrication of the evaporator. As shown in Figure2(e) and (f), the static water contact angle of the hydrophilic region and hydrophobic island are measured, which are 78° and
110°, respectively. Then the mesh layer is sandwiched between the condenser and the evaporator, followed by sealed with
Sn/3Ag/0.5Cu solder. A copper tube is inserted into the ultrathin vapor chamber to do the evacuation and filling process.
The dimension of the fabricated ultrathin vapor chamber is 70 × 70 × 0.246 mm3 with a 60 × 60 mm2 working area.
After the vapor chamber is assembled, a certain amount of water needs to be filled into the vapor chamber before
experimental testing. Vapor chamber firstly is evacuated to 1 Pa, followed by charged with degassed deionized water. As
shown in Figure 3, the thickness of vapor chamber after evacuation and charging process is around 0.228 mm. To evaluate
the amount of water inside the vapor chamber, charge ratio is defined as following:(1)𝜂 = 𝑉𝑤𝑎𝑡𝑒𝑟 ⁄𝑉𝑡𝑜𝑡𝑎𝑙 × 100%
where 𝑉𝑤𝑎𝑡𝑒𝑟 is the volume of water inside the vapor chamber and 𝑉𝑡𝑜𝑡𝑎𝑙 is the total volume of the vapor chamber which is0.4545 ml. Charge ratio greatly affects the thermal performance of vapor chambers. High charge ratio increases the flow
resistance of vapor by narrowing the vapor core, while small charge ratio may result in early dry out due to a lack of water.
Thus, three different charge ratio, 57.2%, 61.6% and 66%, are set to find the optimum charge ratio.

Figure 3 The thickness of an ultrathin vapor chamber after evacuation and charging process

2.2. Experimental Setup and Uncertainty Analysis
Figure 4 demonstrates the LW9510 (LongWin Co. Ltd.) vapor chamber thermal performance measurement apparatus
used in the experiment [2]. Heat is inputted from the heater with an area of 8 mm × 8 mm by an external power supply. An
adjustable cooling fan is used to cooling the condenser of vapor chamber. The flow rate of the cooling air is 6.875 cfm at 22℃ room temperature. Five T-type thermocouples with 0.1 ℃ resolution are mounted at the outside surface of the condenser
and another one is inserted into the heater for temperature measurement, which are shown in Figure 4(b) and (c).

Figure 4 Experimental setup: (a) Apparatus; (b) Heater temperature measurement; (c) Temperature measurement points on the top of
the vapor chamber

Thermal spreading resistance (Rspread) and vertical thermal resistance (Rvr) are defined to evaluate the thermal
performance of the vapor chamber:
𝑅𝑠𝑝𝑟𝑒𝑎𝑑 = (𝑇1 − 𝑇𝑎𝑣𝑔(2−5)) ⁄𝑄(2)
𝑅𝑣𝑟 = (𝑇𝑏 − 𝑇𝑎𝑣𝑔(1−5) )⁄𝑄(3)

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where Tavg(1-5) = (T1+T2+T3+T4+T5)/5 and Tavg(2-5) = (T2+T3+T4+T5)/4. 𝑇1 is the temperature at the centre of the condenser
outside surface, 𝑇2 , 𝑇3 , 𝑇4 and 𝑇5 are the temperatures at four side measurement points (Figure 4 (c)). 𝑄 is the input heatload. 𝑇𝑏 is the temperature at the centre of the evaporator outside surface, which can be calculated according to Fourier’slaw:(4)
𝑇𝑏 = 𝑇ℎ − 𝑞𝑙⁄𝑘𝐶𝑢
where 𝑇ℎ is the measured heater temperature. 𝑙 is the distance from the thermocouple point to the heater upper surface, which
is 0.001 m. kCu is the thermal conductivity of copper, which is 401 W/(m∙K). 𝑞 is the input heat flux, which is:(5)
𝑞 = 𝑄 ⁄𝐴ℎwhere 𝐴ℎ is the heater area, 8 × 8 mm2 in this study. As the heater is well insulated by insulation materials, the heat loss
from the heater to the ambient is small, only about 0.41 W in the experiments.
To characterize the temperature uniformity of the vapor chamber and compare with copper, in-plane effective thermal
conductivity (Keff) is defined to indicate its ability to conduct heat:
𝐾𝑒𝑓𝑓 = 𝑘𝐶𝑢 𝑅𝑆𝑝𝑟𝑒𝑎𝑑,𝐶𝑢 ⁄𝑅𝑆𝑝𝑟𝑒𝑎𝑑(6)
where 𝑅ℎ𝑟,𝐶𝑢 is the measured horizontal thermal resistance of copper.
For the testing apparatus LW9510, the relative uncertainty of the heat load is ±0.9%, the temperature measurement
uncertainty is ±0.2% and the length measurement uncertainty is ±0.4% [2].

3. Result and Discussion
3.1. Results of Thermal Spreading Resistance

Figure 5 Thermal performance of vapor chambers with various wettability patterned evaporator: Thermal spreading resistance of (a)D45P65, (b) D45P90 and (c) No pattern; Vertical thermal resistance of (d) D45P65, (e) D45P90 and (f) No patternFigure 5(a), (b) and (c) show thermal spreading resistance of ultrathin vapor chambers in this study. P65 and P90
represents an ultrathin vapor chamber with wettability patterned evaporator of 65 μm and 90 μm pitch distance between two

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adjacent hydrophobic islands, respectively. “No pattern” representing an ultrathin vapor chamber without wettability
patterned evaporator is also experimentally evaluated. Samples with three different charge ratio is assessed firstly to findoptimum charge ratio which is defined as the charge ratio where the minimum thermal resistance of sample occurs.
As shown in Figure 5(a), the thermal spreading resistance decreases with the increase of heat flux for all the charge
ratio of P65. When the charge ratio is 57.2%, thermal spreading resistance largely decreases from 0.515 ℃/W to 0.238℃/W when heat flux increases from 7.81 W/cm2 to 11.89 W/cm2. Then it gradually decreases from 0.238 ℃/W to theminimum thermal spreading resistance 0.087 ℃/W with the increase of heat flux from 11.89 W/cm 2 to 23.44 W/cm2.
P65 with 61.6% and 66.0% charge ratio show a similar trend with that of 57.2% charge ratio. The minimum thermal
spreading resistance of P65 occurred at the 23.44 W/cm2 of 61.6% charge ratio, which is 0.058 ℃/W.
Figure 5(b) illustrates the thermal spreading resistance of P90. The thermal spreading resistance for all testing
charge ratio also largely decreases with the heat flux increasing from 7.81 W/cm2 to 11.89 W/cm2, which are similar tothat of P65. Then it gradually decreases as heat flux increases to 19.38 W/cm2. The minimum thermal spreading
resistance of 57.2% and 61.6% charge ratio occur at the heat flux of 19.38 W/cm2, while for the 66.0% charge ratio, theminimum thermal spreading resistance happens at the heat flux of 23.44 W/cm2. However, the thermal spreading
resistance under same charge ratio at the heat flux of 19.38 W/cm2 and 23.44 W/cm2 are almost equal. P90 with 61.6%
shows the minimum thermal spreading resistance among all the testing conditions, which is 0.126 ℃/W at 19.38 W/cm2.
Figure 5(c) shows the thermal spreading resistance of “No pattern”. The optimum charge ratio referring to thermal
spreading resistance is 61.6% as lowest thermal spreading resistance exists at 61.6% charge ratio for all the testing heat
flux. The minimum thermal spreading resistance for “No pattern” is 0.152 ℃/W at 19.38 W/cm2. The thermal spreading
resistance of “No pattern” under optimum charge ratio also large decreases with the increase of heat flux from 7.81
W/cm2 to 11.89 W/cm2. However, it increases when the heat flux increases from 19.38 W/cm2 to 23.44 W/cm2, which
is different from ultrathin vapor chamber with wettability patterned evaporator.
2.2. Results of Vertical Thermal Resistance
Figure 5(d), (e) and (f) show the vertical thermal resistance of all the testing samples. As shown in Figure 5(d), thevertical thermal resistance of P65 largely decreases when heat flux increases from 7.81 W/cm2 to 15.63 W/cm2 for all the
charge ratios. A Minimum value of vertical thermal resistance for charge ratio of 57.2% and 66% exists at 15.63 W/cm2,
which is 0.911 ℃/W and 0.978 ℃/W, respectively. The minimum value for 61.6% happened at 19.38 W/cm2, which is
0.850 ℃/W. Then the vertical thermal resistance for 61.6% and 66.0% charge ratio increases with heat flux. The vertical
thermal resistance of P65 with 57.2% charge ratio shows a different trend. It increases when heat flux increases from 15.63
W/cm2 to 19.38 W/cm2 then decreases again when heat flux increases from 19.38 W/cm2 to 23.44 W/cm2. One possible
explanation for the decrease is that water vaporized much more quicker owing to the occurrence of nucleate boiling, greatly
lowering the vertical thermal resistance. P65 with 61.6% shows the lowest vertical thermal resistance among all the charge
ratios. Thus, 61.6% charge ratio is the optimum charge ratio referring to the vertical thermal resistance for P65.
Figure 5(e) illustrates the vertical thermal resistance of P90. A minimum value of vertical thermal resistance for all the
charge ratio exists at the 15.63 W/cm2, which is 0.947 ℃/W, 0.870 ℃/W and 0.973 ℃/W for 57.2%, 61.6% and 66.0%
charge ratio, respectively. Then the vertical thermal resistance increases with the increase of heat flux. The lowest vertical
thermal resistance occurs under 61.6% charge ratio. Thus, the optimum charge ratio for P90 is 61.6%.
Figure 5(f) shows the vertical thermal resistance of “No pattern”. The trends of it for all the charge ratios are the same.
It firstly decreases to the minimum value when heat flux increases from 7.81 W/cm2 to 11.89 W/cm2. Then it increases withheat flux. “No pattern” with 61.6% charge ratio shows the smallest vertical thermal resistance compared with those of othercharge ratio. The lowest vertical thermal resistance is 1.139 ℃/W, happened at 11.89 W/cm2 of 61.6% charge ratio.
All samples with 61.6% charge ratio show the best thermal performance among all charge ratios. Thus, the optimum
charge ratio for all samples is 61.6% In the following parts, the discussion will base on the thermal performance under the
optimum charge ratio.
3.3 Comparison of Vapor Chambers with Different Evaporators

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The best thermal performance of all samples is summarized in Figure 6. A 300 μm copper plate is assessed as a reference.
The in-plane effective thermal conductivity of all samples, which is shown in Figure 6(c), is calculated based on the resultsof thermal spreading resistance.

Figure 6 Comparison of ultrathin vapor chambers with various wettability patterned evaporators under optimum charge ratio: (a)Thermal spreading resistance (b) Vertical thermal resistance (c) In-plane effective thermal conductivity

The in-plane effective thermal conductivity of all samples at the heat flux of 7.81 W/cm2 is similar to each other, which
value is around 1500 W/(m·K). Vertical thermal resistance of “No pattern” is higher than that of P90 which is close to thatof P65 and P90. When the heat flux increases to 11.89 W/cm2, P65 shows a little higher in-plane effective thermal
conductivity with a value of 3229.2 W/(m·K) while P90 and “No pattern” give a value around 2600 W/(m·K). P65 and P90
show a further lower vertical thermal resistance than that of “No pattern”.
The difference of in-plane effective thermal conductivity between P65, P90 and “No pattern” increases when the heat
flux increases to 15.63 W/cm2. The in-plane effective thermal conductivity of P65 and P90 at 15.63 W/cm2 are 5255.2
W/(m·K) and 4487.6 W/(m·K), respectively. However, it of “No pattern” is around 3500 W/(m·K). Moreover, the differenceof vertical thermal resistance is also getting larger. The vertical thermal resistance difference between P90 and “No pattern”
at 11.89 W/cm2 is 0.155 ℃/W while it at 15.63 W/cm2 increases to 0.269 ℃/W.
The in-plane effective thermal conductivity of P65 quickly increases from 5255.2 W/(m·K) to 11499.5 W/(m·K) with
the increase of heat flux from 15.63 W/cm2 to 23.44 W/cm2 while it for other samples with wettability patterned evaporator
just gradually increases even slightly decreases. P90 shows 2nd highest in-plane effective thermal conductivity. However, thein-plane effective thermal conductivity only gradually increases within the heat flux range. “No pattern” shows the lowest
in-plane effective thermal conductivity. Besides, it decreases when the heat flux increases from 19.38 W/cm2 to 23.44 W/cm2.
As for the vertical thermal resistance, samples with wettability patterned evaporator with smaller pitch distance gives a lower
vertical thermal resistance. They all perform better in vertical direction than “No pattern”.
Generally, wettability patterned evaporator with smaller pitch distance gives more improvement on the thermal
performance of the ultrathin vapor chamber. P65 gives the largest and P90 also shows an obvious enhancement both inthermal spreading and vertical aspects. They all give a lower thermal spreading resistance and vertical thermal resistancewhen compared with “No pattern”.

4. Conclusion
A novel evaporator integrated with wettability pattern for enhancing the thermal performance of ultrathin vapor
chambers is proposed in this study. Needle like nanostructured #450 mesh is adopted as the wick and micropillar array isused to define the vapor core. Wettability patterns with various pitch distance of two adjacent hydrophobic islands arefabricated on the inner surface of the bottom casing material as an evaporator. The thermal spreading resistance and vertical
thermal resistance are experimentally evaluated and analyzed. The novel evaporator not just improve the vertical thermal
performance but also the horizontal thermal performance. Smaller pitch distance gives a larger enhancement of the thermal
performance. With a 65 µm pitch distance of the wettability patterned surface, the ultrathin vapor chamber has a 11499.5

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W/(m·K) in-plane effective thermal conductivity at 23.44 W/cm2 heat flux, giving a 212.7% improvement when comparedwith the vapor chamber without the novel evaporator.

Acknowledgements
This work was supported by the Innovation Technology Commission (ITC) of the Government of the Hong Kong
Special Administrative Region (HKSAR) with Project No. ITS/162/16FP, No. ITS/087/20 and the Research Grants
Council of the Government of Hong Kong Special Administrative Region (HKSAR) with RGC/GRF Project No.
16205018. We acknowledge the Nanosystem Fabrication Facility (NFF) of the HKUST for the vapor chamber
fabrication.

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