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33. Infinite Lifetime of Underwater Superhydrophobic States

Author

Xu, Muchen, Sun, Guangyi, and Kim, Chang-Jin

Subjects
Mathematical Sciences, Physical Sciences, Engineering, General Physics
Abstract

Submerged superhydrophobic (SHPo) surfaces are well known to transition from the dewetted to wetted state over time. Here, a theoretical model is applied to describe the depletion of trapped air in a simple trench and rearranged to prescribe the conditions for infinite lifetime. By fabricating a microscale trench in a transparent hydrophobic material, we directly observe the air depletion process and verify the model. The study leads to the demonstration of infinite lifetime (>50 days) of air pockets on engineered microstructured surfaces under water for the first time. Environmental fluctuations are identified as the main factor behind the lack of a long-term underwater SHPo state to date.

Published
2014

34. Superhydrophobic turbulent drag reduction as a function of surface grating parameters

Author

Park, Hyungmin, Sun, Guangyi, and Kim, Chang-Jin CJ

Subjects
drag reduction, MEMS/NEMS, turbulence control, Mathematical Sciences, Engineering, Fluids & Plasmas
Abstract

Despite the confirmation of slip flows and successful drag reduction (DR) in small-scaled laminar flows, the full impact of superhydrophobic (SHPo) DR remained questionable because of the sporadic and inconsistent experimental results in turbulent flows. Here we report a systematic set of bias-free reduction data obtained by measuring the skin-friction drags on a SHPo surface and a smooth surface at the same time and location in a turbulent boundary layer (TBL) flow. Each monolithic sample consists of a SHPo surface and a smooth surface suspended by flexure springs, all carved out from a 2.7 × 2.7 mm2 silicon chip by photolithographic microfabrication. The flow tests allow continuous monitoring of the plastron on the SHPo surfaces, so that the DR data are genuine and consistent. A family of SHPo samples with precise profiles reveals the effects of grating parameters on turbulent DR, which was measured to be as much as ∼75.

Published
2014

37. Electrostatic Side-Drive Rotary Stage on Liquid-Ring Bearing

Author

Sun, Guangyi, Liu, Tingyi, Sen, Prosenjit, Shen, Wenjiang, Gudeman, Chris, and Kim, Chang-Jin

Subjects
Bioengineering, Affordable and Clean Energy, Liquid bearing, liquid ring bearing, rotary stage, Electrostatic actuation, ionic liquid, superhydrophobic, through-silicon-vias, Electrical and Electronic Engineering, Manufacturing Engineering, Mechanical Engineering, Nanoscience & Nanotechnology
Abstract

We present an electrostatically actuated rotary stage featuring liquid rings, which serve as both mechanical bearings and electric connections between the rotor and the substrate. The liquid rings are formed by confining a liquid inside hydrophilic grooves and repelling it from the superhydrophobic surfaces outside the grooves. Made of a fluid, the liquid-ring bearing avoids the dry friction of the solid bearings, significantly improving the reliability. Formed as rings, it avoids the resistance of contactangle hysteresis sliding over droplets, and hence dramatically reducing the static friction. Furthermore, surface tension facilitates the self-alignment of the rotor to the substrate and stator during the assembly and provides the stability against drift and shock during operation. Electrically, each liquid ring passes an independent electric signal, allowing a direct electrical path between the substrate and potential components on the rotor. A three-phase electrostatic rotary stage has been design, fabricated, and tested. The minimum torque to initiate the rotation is ∼2.5nṄm - hundreds of times smaller than droplet-based counterparts. The device has operated successfully by applying sequential voltages of 50V DC between the rotor and the stators. The electric transmission has been verified by powering an LED on a rotating rotor. This is the first report of an electrostatically actuated rotating microdevice with a liquid bearing and a direct power transmission. © 2013 IEEE.

Published
2014

39. Fabrication of Very-High-Aspect-Ratio Microstructures in Complex Patterns by Photoelectrochemical Etching

Author

Sun, Guangyi, Zhao, Xin, and Kim, Chang-Jin CJ

Subjects
Electroplating, high aspect ratio, photoelectrochemical (PEC) etching, surfactant-added tetramethylammonium hydroxide, vacuum degassing, very high aspect ratio, Electrical and Electronic Engineering, Manufacturing Engineering, Mechanical Engineering, Nanoscience & Nanotechnology
Abstract

We have fabricated very-high-aspect-ratio (VHAR) silicon and metal microstructures in complex geometric patterns. The recently developed surfactant-added tetramethylammonium hydroxide etching allows the formation of V-grooves in any pattern, i.e., not limited by the crystal direction, on a silicon surface. As the resulting sharp pits allow very deep photoelectrochemical etching, VHAR silicon microstructures (4-μm-wide and over-300-μm-deep trenches) are successfully fabricated in complex patterns (spiral and zigzag demonstrated), overcoming the prevailing limitations of simple pores and straight trenches. Furthermore, by filling the VHAR silicon mold with nickel and removing the silicon, high-aspect-ratio metal microstructures of complex patterns are also obtained. These VHAR microstructures in complex patterns, which are structurally much stronger than the simple posts and straight plates, overcome the stiction problem even when densely populated. © 2012 IEEE.

Published
2012

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