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Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms. [ABSTRACT FROM AUTHOR]

We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7° wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are, up to respectively, 65% and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer. [ABSTRACT FROM AUTHOR]

• The bubble nucleation and growth in flat microchannels at localized ultra-high heat fluxes were studied. • The time duration of bubble growth was less than 200 μs. • A linear growth rate of about 20 m/s was evidenced at about the first half of bubble growth stage. • The short micro-pin fin arrays lowered the threshold heat flux and onset temperature of bubble nucleation. The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter "hot spots" characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized "hot spot" heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work. [ABSTRACT FROM AUTHOR]

• Wettability patterned surface can achieve higher HTC than homogeneous hydrophilic surfaces for subcooled flow boiling in a microchannel. • Patterns can control the nucleation sites and confine the bubble shapes in flow boiling. • Wettability patterned surface can improve the bubble mobility in comparison with homogeneous surface. • Mass flux affects the subcooled flow boiling heat transfer performance in a patterned microchannel. In this paper, we explore subcooled flow boiling in a microchannel with a chemically patterned surface. The effects of wettability patterns on bubble dynamics and heat transfer are reported and compared with those in a homogenous hydrophilic microchannel. The microchannel has a hydraulic diameter of 311.76 µm and a length of 20 mm. The mass flux of water ranges from 103 kg/m2 s to 203 kg/m2 s and the heat flux ranges from 2.56 W/cm2 to 78.61 W/cm2. It is found that the heat transfer coefficient (HTC) in the wettability-patterned microchannel is about 22% higher than that in the homogenous hydrophilic microchannel. The mechanism behind the heat transfer enhancement is that the nucleation easily occurs on the hydrophobic dots and the bubbles on the patterned surface show higher mobility and longer triple contact line. In addition, the mass flux effect on flow boiling in the wettability-patterned microchannel is investigated and it is found that the HTC improves as the mass flux increases. To understand the local forces required for bubble departure in a wettability-patterned microchannel, an analytical model is developed. The analytical results show that the local bubble detaching force will decrease on a chemically patterned surface compared to a homogenous hydrophobic surface, which also gives a useful insight to combine different wettability characteristics to increase bubble mobility and then improve flow boiling heat transfer in a microchannel. [ABSTRACT FROM AUTHOR]

• The effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop are characterized. • Bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. • HTC, CHF and pressure drop are found to significantly rely on the pitch distance of dots and the mass flux. • Bubble detachment diameters are predicted in hydrophilic, hydrophobic, and wettability-patterned microchannels, respectively. Flow boiling in microchannels offers a promising and attractive solution for thermal management of electronic devices and power systems. In this paper, microchannels composed of a hydrophilic surface with hydrophobic dots were studied to characterize the effects of non-wetting fraction and pitch distance of adjacent dots on flow boiling heat transfer and pressure drop. The pitch distances ranging from 122 µm to 172 µm were studied. Using deionized water as the working fluid, highly subcooled flow boiling experiments were conducted at different mass fluxes ranging from 41.1 to 246.6 kg/m2s over a heat flux up to 146.2 W/cm 2. Bubble dynamics and flow patterns were visualized using a high-speed camera. It was found that bubbles coalesced more easily, and flow patterns transited faster in the microchannel with smaller pitch distance. Heat transfer coefficient (HTC), critical heat flux (CHF) and pressure drop were found to significantly rely on the pitch distance of dots and the mass flux. Furthermore, based on a force-balance model, bubble detached diameters were predicted in hydrophilic, hydrophobic and wettability-patterned microchannels, respectively. This provides a useful insight to optimize the wettability pattern design and then improve flow boiling heat transfer in a microchannel. [ABSTRACT FROM AUTHOR]