Influence of Back Electrostatic Field on the Collection Efficiency of an Electrostatic Lunar Dust Collector

Protecting sensitive surfaces from dust deposition in the limiting condition of the lunar atmosphere is imperative for space exploration. In this study, how back electrostatic field due to charge build-up on collection plates may affect the performance of an electrostatic lunar dust collector (ELDC) was investigated. The relationships between ELDC dimensions, collection efficiency and electrical properties of lunar dust particles were derived to develop a model, appropriate for any size of the ELDC. A Lagrangian-based discrete element method (DEM) was applied to track particle trajectories, and sensitivity analyses were conducted for the concentration of the incoming particles, the number of pre-collected particles and the applied voltages. The results revealed that the collection efficiency reduced over time due to the back electrostatic field of the collected particles, which ultimately led to a suspended regime, rather than just collected and penetrated fractions considered in conventional models. The generated back electrostatic field and the cloud of suspended particles were strong enough to disrupt the performances of both the ELDC and the protected device. The maximum time ELDC can run without significant loss in collection efficiency was estimated to be 10 terrestrial days for the studied ELDC size and applied voltage. Because the electrical power was negligible compared to the provided power by the solar panels, increasing the applied voltage was found to be the best option to counteract back electrostatic growth

Efficiency determination of an electrostatic lunar dust collector by discrete element method

Lunar grains become charged by the sun\u27s radiation in the tenuous atmosphere of the moon. This leads to lunar dust levitation and particle deposition which often create serious problems in the costly system deployed in lunar exploration. In this study, an electrostatic lunar dust collector (ELDC) is proposed to address the issue and the discrete element method (DEM) is used to investigate the effects of electrical particle-particle interactions, non-uniformity of the electrostatic field, and characteristics of the ELDC. The simulations on 20-mu m-sized lunar particles reveal the electrical particle-particle interactions of the dust particles within the ELDC plates require 29% higher electrostatic field strength than that without the interactions for 100% collection efficiency. For the given ELDC geometry, consideration of non-uniformity of the electrostatic field along with electrical interactions between particles on the same ELDC geometry leads to a higher requirement of similar to 3.5 kV/m to ensure 100% particle collection. Notably, such an electrostatic field is about 10 3 times less than required for electrodynamic self-cleaning methods. Finally, it is shown for a half-size system that the DEM model predicts greater collection efficiency than the Eulerian-based model at all voltages less than required for 100% efficiency. Halving the ELDC dimensions boosts the particle concentration inside the ELDC, as well as the resulting field strength for a given voltage. Though a lunar photovoltaic system was the subject, the results of this study are useful for evaluation of any system for collecting charged particles in other high vacuum environment using an electrostatic field

Characterization of Emissions from a Desktop 3D Printer

3D printers are currently widely available and very popular among the general public. However, the use of these devices may pose health risks to users, attributable to air-quality issues arising from gaseous and particulate emissions in particular. We characterized emissions from a low-end 3D printer based on material extrusion, using the most common polymers: acrylonitrile-butadiene-styrene (ABS) and polylactic acid (PLA). Measurements were carried out in an emission chamber and a conventional room. Particle emission rates were obtained by direct measurement and modeling, whereas the influence of extrusion temperature was also evaluated. ABS was the material with the highest aerosol emission rate. The nanoparticle emission ranged from 3.7.10(8) to 1.4.10(9) particles per second (# s(-1)) in chamber measurements and from 2.0.10(9) to 4.0.10(9) # s(-1)in room measurements, when the recommended extruder temperature was used. Printing with PLA emitted nanoparticles at the rate of 1.0.10(7) # s(-1) inside the chamber and negligible emissions in room experiments. Emission rates were observed to depend strongly on extruder temperature. The particles' mean size ranged from 7.8 to 10.5 nanometers (nm). We also detected a significant emission rate of particles of 1 to 3nm in size during all printing events. The amounts of volatile organic and other gaseous compounds were only traceable and are not expected to pose health risks. Our study suggests that measures preventing human exposure to high nanoparticle concentrations should be adopted when using low-end 3D printers.Peer reviewe

In-Situ Tribology of Engineered Surfaces with 3D-Printed Micro/Nano-Textures

Surface texturing is an effective way to reduce the adhesion and friction between two contacting surfaces by reducing the real area of contact between them, providing a potential solution for adhesion and friction-induced failures, for example, failures in micro-/nano-electromechanical systems (MEMS/NEMS). Surface texturing is also essential to the realization of functional surfaces such as superhydrophobic/philic and icephobic surfaces. However, textures are prone to deformation due to being subjected to higher contact pressures. The advancement of 3D nanoprinting by two-photon lithography (TPL), combined with the ability to make an in-situ observation of the interplay between the forces and the sliding surfaces at a single micro/nano structure level, provides opportunities for gaining a better understanding of textured surfaces, enabling better engineering of novel 3D micro-/nano-textures. Using TPL, textures were fabricated with precise shape, dimension, and position control for a systematic investigation of the effects of texture three-dimensionality by comparing 3D textures (truncated cones) to 2.5D textures (cylinders and rods). Moreover, macro- and micro-scale tribological testing and in-situ monitoring of the experiments using a digital microscope at the macro-scale and a scanning electron microscope (SEM) at the micro-scale provided unique insights into the multi-scale tribological properties of the textured surfaces by real-time monitoring of the interplay between the forces and the sliding surfaces down to a single micro-scale structure level. Macro-scale tests showed that cones not only had a lower coefficient of friction due to their reduced area of contact but also slide more smoothly and are more durable. Micro-scale tests shed new light on the relationship between friction and microstructure deformation by in-situ SEM monitoring of texture-counterface interactions. Micro/nano-hierarchical textures play essential roles in realizing the functionalities of surfaces. Their friction and deformation behavior at the nanoscale are relatively unknown. Targeted friction testing of individual micro/nano-hierarchical structures (micropillars covered with nanohairs) inside an SEM, helped show the coupling between micropillar deformation and nanohair height. It was also found that the bending of long nanohairs can provide assistive sliding forces and that buckling of the long nanohairs resulted in the development of lateral forces under only normal loading and before sliding started. Varying the tapering angle of the micropillars and the length of the nanohairs enabled control over the effective stiffness of the micro/nano-hierarchical structures. It was revealed that changes in the structure stiffness by varying the tapering angle affected the onset of sliding motion, friction force, and coupling between the deformation of the nanohair and the micropillar. Additionally, it was shown the nanohair buckling was delayed as a result of increasing the stiffness of the micropillar underneath the nanohair. Finally, atomic layer deposition (ALD) of ZnO on micro/nano-hierarchical structures was done and in-situ SEM mechanical and tribological tests were carried out to shed light on the effect of different ZnO thicknesses and polydopamine (PDA) underlayer. PDA showed improvements in the performance of structures at the lowest ZnO thickness (10 nm), while as the thickness increased to 30 nm, whether or not PDA underlayers existed, coatings exhibited increased failure and cracking

In-Situ Tribology of Engineered Surfaces with 3D-Printed Micro/Nano-Textures

Surface texturing is an effective way to reduce the adhesion and friction between two contacting surfaces by reducing the real area of contact between them, providing a potential solution for adhesion and friction-induced failures, for example, failures in micro-/nano-electromechanical systems (MEMS/NEMS). Surface texturing is also essential to the realization of functional surfaces such as superhydrophobic/philic and icephobic surfaces. However, textures are prone to deformation due to being subjected to higher contact pressures. The advancement of 3D nanoprinting by two-photon lithography (TPL), combined with the ability to make an in-situ observation of the interplay between the forces and the sliding surfaces at a single micro/nano structure level, provides opportunities for gaining a better understanding of textured surfaces, enabling better engineering of novel 3D micro-/nano-textures. Using TPL, textures were fabricated with precise shape, dimension, and position control for a systematic investigation of the effects of texture three-dimensionality by comparing 3D textures (truncated cones) to 2.5D textures (cylinders and rods). Moreover, macro- and micro-scale tribological testing and in-situ monitoring of the experiments using a digital microscope at the macro-scale and a scanning electron microscope (SEM) at the micro-scale provided unique insights into the multi-scale tribological properties of the textured surfaces by real-time monitoring of the interplay between the forces and the sliding surfaces down to a single micro-scale structure level. Macro-scale tests showed that cones not only had a lower coefficient of friction due to their reduced area of contact but also slide more smoothly and are more durable. Micro-scale tests shed new light on the relationship between friction and microstructure deformation by in-situ SEM monitoring of texture-counterface interactions. Micro/nano-hierarchical textures play essential roles in realizing the functionalities of surfaces. Their friction and deformation behavior at the nanoscale are relatively unknown. Targeted friction testing of individual micro/nano-hierarchical structures (micropillars covered with nanohairs) inside an SEM, helped show the coupling between micropillar deformation and nanohair height. It was also found that the bending of long nanohairs can provide assistive sliding forces and that buckling of the long nanohairs resulted in the development of lateral forces under only normal loading and before sliding started. Varying the tapering angle of the micropillars and the length of the nanohairs enabled control over the effective stiffness of the micro/nano-hierarchical structures. It was revealed that changes in the structure stiffness by varying the tapering angle affected the onset of sliding motion, friction force, and coupling between the deformation of the nanohair and the micropillar. Additionally, it was shown the nanohair buckling was delayed as a result of increasing the stiffness of the micropillar underneath the nanohair. Finally, atomic layer deposition (ALD) of ZnO on micro/nano-hierarchical structures was done and in-situ SEM mechanical and tribological tests were carried out to shed light on the effect of different ZnO thicknesses and polydopamine (PDA) underlayer. PDA showed improvements in the performance of structures at the lowest ZnO thickness (10 nm), while as the thickness increased to 30 nm, whether or not PDA underlayers existed, coatings exhibited increased failure and cracking

Within-day and between-day reliability of thickness measurements of abdominal muscles using ultrasound during abdominal hollowing and bracing maneuvers

Ultrasonography imaging has been used as a non-invasive method to estimate the thickness and relative activities of the abdominal muscles in patients with lower back pain (LBP). However, the statistical reliability of US thickness measurements of abdominal muscles, including transversus abdominis (TrA), internal oblique (IO) and external oblique (EO) muscles during abdominal hollowing (AH) and abdominal bracing (AB) maneuvers has not been well-investigated. This study was performed on a total of 20 female subjects (10 with LBP and 10 without LBP) in the age range of 25�55 years to assess within-day and between-day reliability of the measurements. US measurements on maneuvers were repeated after two hours for the within-day reliability and after five days for the between-day reliability assessment. High intra-class correlation coefficient (ICC) values (>0.75) for within-day and between-day reliability assessments during AH maneuver were concluded. The ICC values were moderate for reliability assessment during AB. The ICC values for AH were greater than AB both for within- and between-day reliabilities. The small standard error of measurement and minimal detectable change values (0.16�0.78 and 0.44 to 2.15, respectively) were found for both AH and AB. We recommend real-time US imaging as a reliable way of determining the thicknesses of the TrA and IO muscle (and to some extent, EO muscle) for both healthy and LBP patients. © 2017 Elsevier Lt

Within-day and between-day reliability of thickness measurements of abdominal muscles using ultrasound during abdominal hollowing and bracing maneuvers

Ultrasonography imaging has been used as a non-invasive method to estimate the thickness and relative activities of the abdominal muscles in patients with lower back pain (LBP). However, the statistical reliability of US thickness measurements of abdominal muscles, including transversus abdominis (TrA), internal oblique (IO) and external oblique (EO) muscles during abdominal hollowing (AH) and abdominal bracing (AB) maneuvers has not been well-investigated. This study was performed on a total of 20 female subjects (10 with LBP and 10 without LBP) in the age range of 25�55 years to assess within-day and between-day reliability of the measurements. US measurements on maneuvers were repeated after two hours for the within-day reliability and after five days for the between-day reliability assessment. High intra-class correlation coefficient (ICC) values (>0.75) for within-day and between-day reliability assessments during AH maneuver were concluded. The ICC values were moderate for reliability assessment during AB. The ICC values for AH were greater than AB both for within- and between-day reliabilities. The small standard error of measurement and minimal detectable change values (0.16�0.78 and 0.44 to 2.15, respectively) were found for both AH and AB. We recommend real-time US imaging as a reliable way of determining the thicknesses of the TrA and IO muscle (and to some extent, EO muscle) for both healthy and LBP patients. © 2017 Elsevier Lt

Multi-Scale In-Situ Tribological Studies of Surfaces with 3D Textures Fabricated via Two-Photon Lithography and Replica Molding

Surface texturing not only decreases friction by reducing the real area of contact but also is crucial for achieving multifunctionality. However, texturing a surface might undermine its deformation resistance due to increased sliding contact pressure. High resolution, 3D control over texture shapes can potentially address this issue. Utilizing a micro/nano‐scale additive manufacturing method based on two‐photon polymerization, textures are fabricated with precise shape, dimension, and position control. This allows for a systematic investigation of the effects of texture three‐dimensionality by comparing 3D textures (truncated cones) with 2.5D textures (cylinders and rods). Moreover, macro‐ and micro‐scale tribological testing and in situ monitoring of the experiments using a digital microscope at macro‐scale and a scanning electron microscope (SEM) at micro‐scale provides unique insights into the multi‐scale tribological properties of the textured surfaces by real‐time monitoring of the interplay between the forces and the sliding surfaces down to a single micro‐scale structure level. Macro‐scale tests show that cones not only have a lower coefficient of friction due to their reduced area of contact but also slide more smoothly and are more durable. Micro‐scale tests shed new light on the relationship between friction and the microstructure deformation by in situ SEM monitoring of texture‐counterface interactions

Digitization, replication, and modification of physical surfaces using two-photon lithography

Many surfaces in nature have interesting topographies that lead to properties that may be desirable for engineering applications, such as superhydrophobicity (lotus leaf), high adhesion (frog toes), and drag reduction (shark skin). Although simple replication processes via polydimethylsiloxane molding have been used to replicate these topographies, they cannot be used to modify or combine these topographies for added functionality. This paper presents a novel method to digitize an arbitrary surface, either from nature or manmade, replicate it, modify it, and print it. A banana skin, an eastern wahoo leaf, and a coin were digitized using a 3D laser scanning confocal microscope and modified with arbitrary additional textures. The digitized surfaces were then printed using a UV-sensitive polymer based on two-photon polymerization. The printed surfaces replicate the original surfaces with submicron accuracy; and additional textures were added to illustrate the ability to modify the surface topography. This method enables novel surface topographies to be created that cannot be found in nature