Bubble Formation on Superhydrophobic Micropatterns

2011 ◽  
Author(s):  
Xinwei Wang ◽  
Siwei Zhao ◽  
Hao Wang ◽  
Tingrui Pan
Keyword(s):  
Superhydrophobic Surface ◽  
Bubble Formation ◽  
Copper Surface ◽  
Contact Angles ◽  
Controlled Experiments ◽  
Bare Copper

In this paper, boiling phenomena on a copper surface coated with superhydrophobic micropatterns have been investigated. The micropatterns consisted of chemically inert nanoparticles (PTFE) were in square patterns 180 μm on each side with contact angles above 150° for the superhydrophobic behavior. Boiling experiments were conducted on the patterned copper surface with or without degassing treatment prior to heating, from which bubbles were found to form only on the superhydrophobic sites. As controlled experiments, a uniformly-coated superhydrophobic surface as well as a bare copper surface was also tested in comparison.

Frosting Characteristics on Hydrophilic and Superhydrophobic Copper Surfaces

2016 ◽  
Vol 138 (2) ◽  
Author(s):  
Chan Ho Jeong ◽  
Jae Bin Lee ◽  
Seong Hyuk Lee ◽  
Jungho Lee ◽  
Seung Mun You ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Copper Substrate ◽  
Copper Surface ◽  
Contact Angles ◽  
Film Condensation ◽  
Time Interval ◽  
Copper Surfaces ◽  
Static Contact ◽  
Temperature Detector ◽  
Bare Copper

The main objective of this study is to examine the frosting characteristics affected by the surface wettability. Two different copper surfaces – bare and nano structured - were prepared for the experiments. Their static contact angles are 74° (bare: without surface treatment) and 154° (nano-structured), respectively. The temperature of the copper substrate was measured by using resistance temperature detector (RTD) sensors embedded inside small holes drilled at 1 mm underneath the surface. During the phase change, the temperature of the copper substrates remained -7.8±0.6°C and the ambient temperature was set as 24±0.5°C with the relative humidity of 45%. Images were captured by using the CMOS camera with the 5 second time interval. Film condensation occurred because of higher wettability of the bare copper surface. Film condensates were frozen at the early stage and frost crystal grew in the vertical direction. On the other hand, dropwise condensates formed on the nano-structured copper surface remained as the supercooled liquid phase for 44 minutes owing to its low wettability. After 4 minutes, frosting on the bare copper substrate was triggered and propagated until it covered the whole surface. The frosting was significantly delayed on the superhydrophobic copper surface due to the lower surface free energy. The different porous media composed of frost which directly influence the heat transfer characteristics was formed on each surfaces. Therefore, additional investigation for heat transfer phenomenon on superhydrophobic surface should be conducted.

Enhanced Heat Transfer Using Microporous Copper Inverse Opals

2018 ◽  
Vol 140 (2) ◽  
Author(s):  
Hyoungsoon Lee ◽  
Tanmoy Maitra ◽  
James Palko ◽  
Daeyoung Kong ◽  
Chi Zhang ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Thermal Management ◽  
Pore Diameter ◽  
Bubble Formation ◽  
Copper Surface ◽  
Porous Structures ◽  
Inverse Opals ◽  
Optical Images ◽  
Nucleation Sites ◽  
Enhanced Boiling

Enhanced boiling is one of the popular cooling schemes in thermal management due to its superior heat transfer characteristics. This study demonstrates the ability of copper inverse opal (CIO) porous structures to enhance pool boiling performance using a thin CIO film with a thickness of ∼10 μm and pore diameter of 5 μm. The microfabricated CIO film increases microscale surface roughness that in turn leads to more active nucleation sites thus improved boiling performance parameters such as heat transfer coefficient (HTC) and critical heat flux (CHF) compared to those of smooth Si surfaces. The experimental results for CIO film show a maximum CHF of 225 W/cm2 (at 16.2 °C superheat) or about three times higher than that of smooth Si surface (80 W/cm2 at 21.6 °C superheat). Optical images showing bubble formation on the microporous copper surface are captured to provide detailed information of bubble departure diameter and frequency.

Pool Boiling Heat Transfer of Water on Hydrophilic Surfaces With Different Wettability

2016 ◽  
Author(s):  
Adam R. Girard ◽  
Jinsub Kim ◽  
Seung M. You
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Pool Boiling ◽  
Copper Surface ◽  
Contact Angles ◽  
Copper Surfaces ◽  
Nanoscale Structures ◽  
Flat Surfaces ◽  
Hydrophilic Surfaces ◽  
Alkali Solutions

The effect of wettability on boiling heat transfer (BHT) coefficient and critical heat flux (CHF) in pool boiling of water on hydrophilic surfaces having different contact angles was investigated. Hot alkali solutions were utilized to promote cupric and cuprous oxide growth which exhibited micro and nanoscale structures on copper surfaces, with thicknesses on the order of a couple of micrometers. These structure and surface energy variations result in different levels of wettability and roughness while maintaining the effusivity of the bare copper surface. The study showed that the BHT coefficient has an inverse relationship to wettability; the BHT coefficient decreases as wettability increases. Furthermore, it was shown that this dependency between BHT coefficient and wettability is more significant than the relationship between BHT coefficient and surface roughness. The CHF was also found to increase with increases in wettability and roughness. For the most hydrophilic surface tested in this study, CHF values were recorded near the 2,000 kW/m2 mark. This value is compared with maximum values reported in literature for water on non-structured flat surfaces without area enhancements. Based on these results it is postulated that there exists a true hydrodynamic CHF limit for pool boiling with water on flat surfaces, very near 2,000 kW/m2, independent of heater material, representing an 80% increase in the limit suggested by Zuber [1].

Fabrication of titanium-based microstructured surfaces and study on their superhydrophobic stability

2008 ◽  
Vol 23 (9) ◽  
pp. 2491-2499 ◽  
Author(s):  
Baojia Li ◽  
Ming Zhou ◽  
Run Yuan ◽  
Lan Cai
Keyword(s):  
Superhydrophobic Surface ◽  
Contact Angles ◽  
Apparent Contact Angle ◽  
Sliding Angle ◽  
Pillar Array ◽  
Femtosecond Laser Micromachining ◽  
Microstructured Surfaces ◽  
Maximal Deviation ◽  
Four Square ◽  
Titanium Substrates

Based on the classical wetting theories, two theoretically predicted formulas of the apparent contact angles on square-pillar-array microstructured surfaces for Wenzel mode and Cassie mode have been derived, respectively. The theories of superhydrophobic stability on microstructured surfaces have been summarized. Four square-pillar-array samples were fabricated on titanium substrates by using the femtosecond laser micromachining technology, and wettability was analyzed by both experimental and analytical methods. The results showed that the titanium-based surfaces are superhydrophobic. The maximal apparent contact angle is up to 156.9°, while the corresponding sliding angle is 4.7°. Testing of the superhydrophobic stability of the surfaces showed that the maximal deviation of the apparent contact angles is only 0.6°. Analyses indicate that the stable superhydrophobicity of the supplied titanium-based surfaces is within a certain range and not perfect. To improve that, a practical controllable method is proposed herein for the design of a stable superhydrophobic surface.

Effects of Superhydrophobic and Superhydrophilic Surfaces on Heat Transfer and Oscillating Motion of an Oscillating Heat Pipe

2014 ◽  
Vol 136 (8) ◽  
Author(s):  
Tingting Hao ◽  
Xuehu Ma ◽  
Zhong Lan ◽  
Nan Li ◽  
Yuzhe Zhao
Keyword(s):  
Heat Transfer ◽  
Superhydrophobic Surface ◽  
Thin Liquid Film ◽  
Maximum Amplitude ◽  
Line Length ◽  
Copper Surface ◽  
Working Fluid ◽  
Liquid Slug ◽  
Vapor Interface ◽  
Liquid Vapor

The effects of superhydrophobic surface and superhydrophobic and superhydrophilic hybrid surface on the fluid flow and heat transfer of oscillating heat pipes (OHPs) were investigated in the paper. The inner surfaces of the OHPs were hydrophilic surface (copper), hybrid surface (superhydrophilic evaporation and superhydrophobic condensation section), and uniform superhydrophobic surface, respectively. Deionized water was used as the working fluid. Experimental results showed that superhydrophobic surface influenced the slug motion and thermal performance of OHPs. Visualization results showed that the liquid-vapor interface was concave in the OHP with copper surface. A thin liquid film existed between the vapor plug and the wall of the OHP. On the contrary, the liquid-vapor interface took a convex profile in the OHP with superhydrophobic surface and the liquid-vapor interface contact line length in the hybrid surface OHP was longer than that in the uniform superhydrophobic surface OHP. The liquid slug movements became stronger in the hybrid surface OHPs as opposed to the copper OHP, while the global heat transfer performance of the hybrid surface OHPs increased by 5–20%. Comparing with the copper OHPs, the maximum amplitude and velocity of the liquid slug movements in the hybrid surface OHPs increased by 0–127% and 0–185%, respectively. However, the maximum amplitude and velocity of the liquid slug movements in the uniform superhydrophobic OHPs was reduced by 0–100% and 0–100%, respectively. The partial dryout phenomenon took place in OHPs with uniform superhydrophobic surface. The liquid slug movements became weaker and the thermal resistance was increased by 10–35% in the superhydrophobic surface OHPs.

Small-chip Attachment on Copper Leadframe with Sintered Nanosilver Paste

2013 ◽  
Vol 2013 (1) ◽  
pp. 000072-000077
Author(s):  
Jesus N. Calata ◽  
Hanguang Zheng ◽  
Guo-Quan Lu ◽  
Khai Ngo ◽  
Luu Nguyen
Keyword(s):  
Shear Strength ◽  
Heat Dissipation ◽  
Applied Pressure ◽  
Copper Surface ◽  
Pure Nitrogen ◽  
Strong Bond ◽  
Thermomechanical Stress ◽  
Porous Microstructure ◽  
Chip Size ◽  
Bare Copper

Sintered nanoscale silver paste provides a low-temperature alternative to solder for die attachment. Unlike solder, the sintered attachment does not melt upon reaching the original attachment temperature and therefore may be used at higher temperatures. Higher electrical and thermal conductivities mean less Joule heating and better heat dissipation characteristics and the porous microstructure imparts low elastic modulus for lower thermomechanical stress and enhanced reliability. The state of the technology has reached a point where it is now possible to obtain die-shear strengths comparable to solder at sintering temperatures between 250°C and 280°C with little or no applied pressure, depending on the chip size. In addition to attachments on silver or gold-coated surfaces, it is possible to form bonds on bare copper if done under inert or slightly reducing atmosphere. Because attainment of a strong bond depends on the paste being in contact with a clean (oxide-free or untarnished) surface, a study was made to determine if the attachment process will work on copper leadframes with either an anti-tarnish or anti-EBO (epoxy bleed-out) coating. Small mechanical silicon chips less that 3 mm × 3 mm in size were attached without pressure at temperatures as low as 260°C. The sintering atmosphere in the chamber was varied from pure nitrogen, to nitrogen + 4% hydrogen and to nitrogen + 1% oxygen. Attachments sintered in pure nitrogen or nitrogen with hydrogen produced die-shear strengths of at least 30 MPa and were just as strong as those bonded on bare copper. Sintering in nitrogen + 1 % oxygen caused the die-shear strength to drop below 30 MPa but still above 20 MPa. In the presence of oxygen, the binder removal is due to oxidative combustion but the low level of oxygen caused incomplete binder burnout that interfered with the sintering while also causing some oxidation of the copper. On the other hand, the addition of hydrogen appeared to enhance the sintered microstructure accompanied by a slight increase in die shear strength. Sheared attachments that exposed the copper surface showed patches of silver still attached indicating formation of strong bond with the copper.

Effect of Wettability on Pool Boiling Incipience in Saturated Water

2016 ◽  
Vol 138 (8) ◽  
Author(s):  
Jinsub Kim ◽  
Seongchul Jun ◽  
Jungho Lee ◽  
Seong Hyuk Lee ◽  
Seung M. You
Keyword(s):  
Heat Flux ◽  
Life Cycle ◽  
Pool Boiling ◽  
Copper Surface ◽  
Contact Angles ◽  
Al2o3 Nanoparticles ◽  
Saturated Water ◽  
Boiling Incipience ◽  
Coated Surface ◽  
Bare Surface

Three different copper surfaces - bare, Al2O3 nano-coated, and Polytetrafluoroethylene (PTFE) coated - are prepared and tested to examine the effect of wettability on the pool boiling incipience in saturated water at 1 atm. A copper surface is coated with Al2O3 particles ranging 25~43 nm in diameter by immersing the surface in Al2O3/ethanol nanofluid (1g/l) and boiled for 3 min. SEM image in Fig. 1 shows the coated Al2O3 nanoparticles on the copper surface, together with the reference bare surface. PTFE coating is also applied to the copper surface using spin coating method with the mixture of Dupont AF 2400 particles and 3M FC-40 solvent. The final coating thickness of the PTFE coating is estimated to be 30 nm. The three surfaces exhibit different static contact angles, 78° (bare), 28° (nano-coated), and 120° (PTFE coated) in Fig. 2, respectively. Wettability affects the boiling incipience heat flux where initial bubble nucleation starts: 15 kW/m2 for the bare surface; 30 kW/m2 for the nano-coated surface; and 2.5 kW/m2 for the PTFE coated surface. Captured images from the high speed camera at 2,000 fps show significantly different bubble shapes and departure frequencies in Fig. 3. During the bubble growth, advancing contact angles are captured and shown qualitatively and found consistent with their static angle measurements for the sessile droplet observed at each surface. The larger bubble is generated on the nano-coated surface compared to that of the bare surface because improved wetting makes promising cavities flood and thus incipience is delayed, resulting in higher superheat. The single bubble life cycle appears to be much longer on the PTFE coated surface due to the increase of the contact angle which becomes hydrophobic (> 90°), resulting in lower bubble departure frequency. Successive tests at the same heat flux of 30 kW/m2 confirmed that life cycle on the PTFE coated surface (88.5 ms) is consistently longer than that on the bare surface (16.5 ms) and nano-coated surface (20 ms).

scholarly journals Application of Nano-Hydroxyapatite Derived from Oyster Shell in Fabricating Superhydrophobic Sponge for Efficient Oil/Water Separation

Molecules ◽  
2021 ◽  
Vol 26 (12) ◽  
pp. 3703
Author(s):  
Chao Liu ◽  
Su-Hua Chen ◽  
Chi-Hao Yang-Zhou ◽  
Qiu-Gen Zhang ◽  
Ruby N. Michael
Keyword(s):  
Oil Recovery ◽  
Superhydrophobic Surface ◽  
Oyster Shell ◽  
Contact Angles ◽  
Immersion Method ◽  
Water Separation ◽  
Promising Alternative ◽  
Recovery Capacity ◽  
Oil Water Separation ◽  
Oil Water

The exploration of nonhazardous nanoparticles to fabricate a template-driven superhydrophobic surface is of great ecological importance for oil/water separation in practice. In this work, nano-hydroxyapatite (nano-HAp) with good biocompatibility was easily developed from discarded oyster shells and well incorporated with polydimethylsiloxane (PDMS) to create a superhydrophobic surface on a polyurethane (PU) sponge using a facile solution–immersion method. The obtained nano-HAp coated PU (nano-HAp/PU) sponge exhibited both excellent oil/water selectivity with water contact angles of over 150° and higher absorption capacity for various organic solvents and oils than the original PU sponge, which can be assigned to the nano-HAp coating surface with rough microstructures. Moreover, the superhydrophobic nano-HAp/PU sponge was found to be mechanically stable with no obvious decrease of oil recovery capacity from water in 10 cycles. This work presented that the oyster shell could be a promising alternative to superhydrophobic coatings, which was not only beneficial to oil-containing wastewater treatment, but also favorable for sustainable aquaculture.

Sign in / Sign up

Export Citation Format

Share Document