Hydrophilic and Hydrophobic Nanostructured Copper Surfaces for Efficient Pool Boiling Heat Transfer with Water, Water/Butanol Mixtures and Novec 649

Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies.

Considerably improved water and oil washability of highly conductive stretchable fibers by chemical functionalization with fluorinated silane

The chemical functionalization with fluorinated silane provides considerably improved water and oil washability of highly conductive stretchable nanocomposite fibers.

Fabrication and Characterization of Superhydrophobic Film on Titanium Substrate

TiO2 nano film with different nano structure were prepared on the surfaces of titanium TA1 and titanium alloy TC4 via electrochemical anodization. The surface morphology was observed with SEM, and it is found that TiO2 films prepared on TA1 surface have porous structures, while the TiO2 films synthesized on TC4 surface look like nano rod. The contact angle(CA) test results show that the TiO2 nano film prepared on TA1 is hydrophilicity and the CA is about 19°,the film on the TC4 is super hydrophilicity and its CA is smaller than 2°. After combined with fluorinated silane the self-assembled film synthesized on the surfaces and they are superhy- drophobic and the Contact angle is 150° and 158° respectively. It is concluded that both nano- structures and fluoroalkysilanes with low surface energy have play an important role in the wettability with the contact angle is more than 150°. The surface only have nano roughness without the low energy does not show superhydrophobicity.

Effect of Silanization Film Thickness in Soft Lithography of Nanoscale Features

Soft lithography was used to replicate nanoscale features made using electron beam lithography on a polymethylmethacrylate (PMMA) master. The PMMA masters were exposed to fluorinated silane vapors to passivate its surfaces so that polydimethylsiloxane (PDMS) did not permanently bond to the master. From scanning electron microscopy, the silanization process was found to deposit a coating on the master that was a few hundreds of nanometers thick. These silane films partially concealed the nanoscale holes on the PMMA master, causing the soft lithography process to produce PDMS features with dimensions that were significantly reduced. The thickness of the silane films was directly measured on silicon or PMMA masters and was found to increase with exposure time to silane vapors. These findings indicate that the thickness of the silane coatings is a critical parameter when using soft lithography to replicate nanoscale features, and caution should be taken on how long a master is exposed to silane vapors.

Thermal Behavior with Mechanical Property of Fluorinated Silane Functionalized Superhydrophobic Pullulan/Poly(vinyl alcohol) Blends by Electrospinning Method

Fluorinated silane functionalized superhydrophobic pullulan/poly(vinyl alcohol) (PULL/PVA) blend membrane with water contact angle larger than 150° has been prepared by the electrospinning method. The morphology, thermal stability, and mechanical property of the membranes are characterized using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and ZWICK materials testing machine, respectively. Interactions between PULL and PVA and PULL/PVA blends with perfluorooctyltriethoxysilane (PFOTES) of the membranes are analyzed using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR). Contact angles and water drops on the surface of the membrane are measured by video microscopy. The study shows that the addition of minor quantity of PVA with PULL results in improvement in thermal stability and mechanical property (tensile strength) of the PULL membranes.