When a water–ethanol binary mixture condenses on a flat plate, one observes that the liquid film condensate rises locally and eventually forms many droplets on the film. Usually, filmwise condensation is expected because both substances are completely soluble in each other and they wet a copper plate well. This paper presents the droplet growth mechanism during so-called pseudo-dropwise condensation. Instability analysis is used to determine the transition from filmwise condensation to pseudo-dropwise condensation theoretically. In a stress balance at the vapor–liquid interface, the analysis considers not only the surface tension itself, but also the surface tension variation due to changes in temperature and concentration, assuming saturation conditions at the interface. Numerical results indicate that the Marangoni effect plays a more important role than the absolute value of the surface tension in pseudo-dropwise condensation. The change in surface tension with temperature is not always negative; it becomes positive for certain mixtures due to the dependence on concentration. Pseudo-dropwise condensation is only realized when surface tension increases with temperature. This analysis qualitatively predicts the critical Marangoni number experimentally observed during water–ethanol mixture condensation.
This paper studies the effects of gate voltage on heat generation and transport in a metal–semiconductor field effect transistor made of gallium arsenide (GaAs) with a gate length of 0.2 μm. Based on the interactions between electrons, optical phonons, and acoustic phonons in GaAs, a self-consistent model consisting of hydrodynamic equations for electrons and phonons is developed. Concurrent study of the electrical and thermal behavior of the device shows that under a source-to-drain bias at 3 V and zero gate bias, the maximum electron temperature rise in this device is higher than 1000 K whereas the lattice temperature rise is of the order of 10 K, thereby exhibiting nonequilibrium characteristics. As the gate voltage is decreased from 0 to −2 V the maximum electron temperature increases due to generation of higher electric fields whereas the maximum lattice temperature reduces due to lower power dissipation. The nonequilibrium hot-electron effect can reduce the drain current by 15% and must be included in the analysis. More importantly, it is found that the electron temperature rise is nearly independent of the thermal package conductance whereas the lattice temperature rise depends strongly on it. In addition, an increase of lattice temperature by 100 K can reduce the drain current by 25%.
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