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.

Resistant energy analysis of self-pulling process during dropwise condensation on superhydrophobic surfaces with optimum surface roughness.

Superhydrophobic surfaces exhibit large contact angle (> 150°) and small hysteresis (< 5°) which facilitate liquid transport and are expected to enhance condensation heat transfer on the surfaces. By growing short carbon nanotubes (CNTs) on an array of microposts etched on a silicon wafer, we formed a two-tier multiscale texture mimicking the surface structure of lotus leaves. Compared to one-tier microtexture which energetically favors the Wenzel state, the two-tier texture with micro/nano-scale roughness favors the Cassie state, the desired superhydrophobic state. Using an environmental scanning electron microscope (ESEM), we investigated moisture condensation on the fluoropolymer-coated two-tier texture and we have observed continuous dropwise condensation on the engineered superhydrophobic surface. However, in a customer-designed vapor chamber our condensation measurements indicate that a film layer of condensate in Wenzel state was formed on the textured surface. In particular, due to the filmwise condensation, the condensation heat transfer coefficient of the lotus-leaf-like surface is lower than that of a smooth hydrophobic surface especially under high heat flux situations.

An analytical study on entropy generation considering viscous dissipation effect in the circular microchannel is reported. The fluid flow is steady, laminar, hydrodynamically fully developed and thermally developing. In the first law analysis, appropriate dimensionless variables are applied to solve the energy equation in the thermal entrance region of microchannel. Subsequently the obtained temperature field is used to derive an expression for entropy generation rate. The effect of Knudsen number and Brinkman number on the entropy generation rate and Bejan number in different axial location is presented.