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The electrochemical deposition of copper is carried out on graphene in a two-step process that involves the potentiostatic deposition of copper on a platinum disc electrode at -0.30 V vs saturated calomel electrode (SCE) in the first step, followed by a dip coating of graphene. The cyclic voltammetric and chronoamperometric data of copper ion in sodium sulfate as the supporting electrolyte shows enhanced currents that are due to the oxidation of the copper that is underlying graphene. The electrodeposited copper on graphene shows a porous bead like structure in scanning electron microscope (SEM). The electrochemically obtained graphene solution is dip coated on a copper chip and investigated for its pool boiling performance. An enhancement of 82% in heat transfer coefficient (HTC) was obtained compared to a plain uncoated copper chip.

The passage of a single bubble or a stream of bubbles through a liquid-liquid interface is a highly dynamic process that can result in a number of different outcomes. Previous studies focused primarily on a single bubble and single flow regime, and very few investigations have considered bubble streams. In the present work, six different liquid combinations made up of water, ethanol, a perfluorocarbon liquid, PP1, and one of three different viscosity silicone oils are tested with air bubbles from 2 to 6 mm in diameter rising between 5 and 55 cm/s. Both single bubbles and bubble streams varying in frequency from 5 to 40 bubbles/s are tested. High-speed imaging is used to capture and classify the flow regimes associated with each flow type. Four different flow regimes are identified for single-bubble passage, and six are found for bubble stream passage. On the basis of theoretical considerations, nondimensional numbers are developed for characterizing the flow regimes and maps are generated that distinguish them and define flow regime transitions.

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In addition to transient conduction, microconvection, and microlayer evaporation, contact line region heat transfer has been identified as an important mode of heat transfer during boiling. In this work, we demonstrate that generating additional contact line regions within the base of a nucleating bubble leads to critical heat flux (CHF) enhancement. The creation of a liquid meniscus adjacent to 10–20 μm deep microgrooves in the bubble base area was responsible for the generation of the additional contact line regions. The depth of the microgrooves was determined such that a sufficient reservoir of liquid is present in the meniscus to sustain evaporation in the contact line region throughout the bubble cycle. The effective contact line length at the base of the bubble was seen to be a good indicator of the CHF (wetted area) over the surface. The microgroove geometry played a significant role in influencing the bubble dynamics and bubble departure diameter during boiling. It was seen that the bubbles were able to bridge and grow over the shallow microgrooves of 10–20 μm depth and generate additional contact line regions but were pinned and constrained within the grooves in the case of the microgrooves deeper than 100 μm. For shallow grooves, narrow grooves resulted in smaller bubbles, which in turn reduced the contact line length. The findings of this work could be used to design heat transfer surfaces that significantly enhance the contact line region contribution and CHF by placing shallow enhancement features on the surface of the heater.