Dropwise condensation can enhance heat transfer by an order of magnitude compared to film condensation. Superhydrophobicity appears ideal to promote continued dropwise condensation which requires rapid removal of condensate drops; however, such promotion has not been reported on engineered surfaces. This letter reports continuous dropwise condensation on a superhydrophobic surface with short carbon nanotubes deposited on micromachined posts, a two-tier texture mimicking lotus leaves. On such micro-/nanostructured surfaces, the condensate drops prefer the Cassie state which is thermodynamically more stable than the Wenzel state. With a hexadecanethiol coating, superhydrophobicity is retained during and after condensation and rapid drop removal is enabled.

We investigated water vapor condensation on a two-tier superhydrophobic surface in an environmental scanning electron microscope (ESEM) and in a customer-designed vapor chamber. We have observed continuous dropwise condensation (DWC) on the textured surface in ESEM. However, a film layer of condensate was formed on the multiscale texture in the vapor chamber. Due to the filmwise condensation, the condensation heat transfer coefficient of the superhydrophobic surface is lower than that of a flat hydrophobic surface especially under high heat flux situations. Our studies indicate that adaptive and prompt condensate droplet purging is the dominant factor for sustaining long-term DWC.

No abstract

With the increase in power consumption in compact electronic devices, passive heat transfer cooling technologies with high-heat-flux characteristics are highly desired in microelectronic industries. Carbon nanotube (CNT) clusters have high thermal conductivity, nanopore size, and large porosity and can be used as wick structure in a heat pipe heatspreader to provide high capillary force for high-heat-flux thermal management. This paper reports investigations of high-heat-flux cooling of the CNT biwick structure, associated with the development of a reliable thermometer and high performance heater. The thermometer/heater is a 100-nm-thick and 600 μm wide Z-shaped platinum wire resistor, fabricated on a thermally oxidized silicon substrate of a CNT sample to heat a 2×2 mm2 wick area. As a heater, it provides a direct heating effect without a thermal interface and is capable of high-temperature operation over 800°C. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation versus temperature after the annealing process is applied. The thermally oxidized layer on the silicon substrate is around 1-μm-thick and pinhole-free, which ensures the platinum thermometer/heater from the severe CNT growth environments without any electrical leakage. For high-heat-flux cooling, the CNT biwick structure is composed of 250 μm tall and 100 μm wide stripelike CNT clusters with 50 μm stripe-spacers. Using 1×1 cm2 CNT biwick samples, experiments are completed in both open and saturated environments. Experimental results demonstrate 600 W/cm2 heat transfer capacity and good thermal and mass transport characteristics in the nanolevel porous media.

We report an electrowetting-controlled optofluidic system for adaptive beam tracking and agile steering. With two immiscible fluids in a transparent cell, we can actively control the contact angle along the fluid-fluid-solid tri-junction line and hence the orientation of the fluid-fluid interface via electrowetting. The naturally formed meniscus between the two liquids can function as an optical prism. We have fabricated a liquid prism module with an aperture size of 10 mm × 10mm. With 1 wt. % KCl and 1 wt. % Sodium Dodecyl Sulfate added into deionized water, the orientation of the water-silicone oil interface has been modulated between −26° and 26° that can deflect and steer beam within the incidence angle of 0°–15°. The wide-range beam tracking and steering enables the liquid prism work as an electrowetting solar cell.