Combination of two physical phenomena,
capillary pressure gradient
and wettability gradient, allows a simple two-step fabrication process
that yields a reliable hydrophobic self-cleaning condenser surface.
The surface is fabricated with specific microscopic topography and
further treatment with a chemically inert low-surface-energy material.
This process does not require growth of nanofeatures (nanotubes) or
hydrophilic–hydrophobic patterning of the surface. Trapezoidal
geometry of the microfeatures facilitates droplet transfer from the
Wenzel to the Cassie state and reduces droplet critical diameter.
The geometry of the micropatterns enhances local coalescence and directional
movement for droplets with diameter much smaller than the radial length
of the micropatterns. The hydrophobic self-cleaning micropatterned
condenser surface prevents liquid film formation and promotes continuous
dropwise condensation cycle. Upon dropwise condensation, droplets
follow a designed wettability gradient created with micropatterns
from the most hydrophobic to the least hydrophobic end of the surface.
The surface has higher condensation efficiency, due to its directional
self-cleaning property, than a plain hydrophobic surface. We explain
the self-actuated droplet collection mechanism on the condenser surface
and demonstrate experimentally the creation of an effective wettability
gradient over a 6 mm radial distance. In spite of its fabrication
simplicity, the fabricated surface demonstrates self-cleaning property,
enhanced condensation performance, and reliability over time. Our
work enables creation of a hydrophobic condenser surface with the
directional self-cleaning property that can be used for collection
of biological (chemical, environmental) aerosol samples or for condensation
enhancement.
Undesired oscillations commonly encountered in engineering practice can be harmful to structures and machinery. Vibration isolation systems are used to attenuate undesired oscillations. Recently, there has been growing interest in nonlinear approaches towards vibration isolation systems design. This work is focused on investigating the effect of nonlinear cubic viscous damping in a vibration isolation system consisting of a magnetic spring with a positive nonlinear stiffness, and a mechanical oblique spring with geometric nonlinear negative stiffness. Dynamic model of the vibration isolation system is obtained and the harmonic balance method (HBM) is used to solve the governing dynamic equation. Additionally, fourth order Runge–Kutta numerical simulation is used to obtain displacement transmissibility of the system under investigation. Results obtained from numerical simulation are in good agreement with those obtained using HBM. Results show that introducing nonlinear damping improves the performance of the vibration isolation system. Nonlinear damping purposefully introduced into the described vibration isolation system appears to eliminate undesired frequency jump phenomena traditionally encountered in quasi-zero-stiffness vibration isolation systems. Compared to its rival linear vibration isolation system, the described nonlinear system transmits less vibrations around resonant peak. At lower frequencies, both nonlinear and linear isolation systems show comparable transmissibility characteristics.
The operation of a MEMS-based micro heat engine at resonant and off-resonant conditions is presented. Both model and experiment are used to investigate resonant and off-resonant operation of the engine. In this work, we look at the pressure-volume (PV) diagrams of an engine operated at resonance and off-resonance. Model predictions of the PV diagram are in favorable agreement with measured data. The results show that resonant operation is beneficial. At resonance, the pressure and volume in the engine cavity are decoupled and more mechanical work is observed. The PV diagram describes an elliptical shape. However, for an off-resonant operation the pressure and volume become more coupled and less mechanical work is observed. The PV diagram is described by a sigmoidal shape.
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