Electrostatic collection of tribocharged lunar dust simulants

Advanced Powder Technology 25 (2014) 1800–1807

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Original Research Paper

Electrostatic collection of tribocharged lunar dust simulants Nima Afshar-Mohajer a, Chang-Yu Wu a,⇑, Nicoleta Sorloaica-Hickman b a b

Department of Environmental Engineering Sciences, Engineering School of Sustainable Infrastructure and Environment, University of Florida, Gainesville, FL 32611, USA Florida Solar Energy Center, University of Central Florida, Cocoa, FL 32922, USA

a r t i c l e

i n f o

Article history: Received 24 May 2014 Received in revised form 9 July 2014 Accepted 12 July 2014 Available online 27 July 2014 Keywords: Lunar dust Particle collection Electrostatic field Tribocharging Vacuum Particle charger

a b s t r a c t Levitation and consecutive deposition of naturally charged particles on lunar surface were troublesome in previous NASA explorations. Protecting sensitive surfaces from dust deposition in the limiting condition of the lunar atmosphere is imperative for future space exploration. This study reports experimental investigation of the collection efficiency of an electrostatic lunar dust collector (ELDC). A dual-functional remotely controlled particle charger/dropper was designed for tribocharging 20 lm lunar dust simulants, and a system of Faraday cup connected to an electrometer working in the nC range was used to measure the particle charges. Tribochargeability of two lunar dust simulants was studied, and the process was found to be the most effective with the JSC-1A samples. Aluminum was verified to be a more effective plate material than stainless steel. For the tested range of electrostatic field strength (0.66–2.6 kV/m), the mass-based and charge-based collection efficiencies were in the range of 0.25–1% and 0.45–1.45% for the low vacuum (101 Torr), and 8–35% and 12–54% for the high vacuum (105 Torr) conditions. The linear relationship between the applied voltage and ELDC collection efficiency predicted by the theoretical model was confirmed, and the collection pattern of the collected particles over the collection plate was consistent with the previously computed charge distribution on the collection plate. Aside from validating the predictability of the theoretical model, this study offers a novel method of particle charging inside a vacuum chamber with a variety of applications for studying chargeability of particles at different temperatures and pressures. Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Protection of solar panels and optical systems installed on lunar surface from depositing charged lunar dust is a great concern to the National Aeronautics and Space Administration (NASA) for future lunar, a steroidal and Martian explorations. Charging of lunar dust is due to exposure to intense high energetic solar radiation on the dayside of the moon, as well as exposure to low energetic electrons impingement on the nightside of the moon [1,2]. Astronauts in Apollo 17 mission reported a layer of micron-sized and submicron-sized lunar dust levitated from the surface because of repelling forces between the like-charged particles [3]. This was imaged by the lunar orbiter as a horizon glow over the lunar terminator [4]. Strong adhesion of charged lunar dust to all surfaces, rise and fall cycles of the particles and consequent dust deposition on ⇑ Corresponding author. Address: Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, FL 32611-6450, USA. Tel.: +1 (352) 392 0845; fax: +1 (352) 392 3076. E-mail address: cywu@ufl.edu (C.-Y. Wu).

mission-critical equipment have been studied within the past two decades leading to development of different control technologies. However, only electrostatic- or electrodynamic-based methods seem practical which have gained the most acceptance in the literature [5,6]. Electrodynamic dust shield (EDS) technology introduced by Calle et al. [7] and Kawamoto et al. [8,9] is based on electric curtain concepts developed by Tatom et al. at NASA [10] and Masuda et al. at University of Tokyo [11]. The EDS consists of embedded electrodes inside a transparent insulator film installed on the solar cell surfaces, and it utilizes standing or traveling waves to lift and transport both charged and uncharged particles deposited on the surface. However, according to a study by Qian et al. [12] the rate of electric power required for the EDS operation may be significantly higher than that provided by a solar panel, although the cleaning operation is infrequent. Inspired from industrial electrostatic precipitators (ESP) [13], an electrostatic lunar dust collector (ELDC) was suggested by AfsharMohajer et al. [14]. The ELDC provided electric potential between a parallel set of conducting and transparent thin plates normal to the protected surface to attract incoming like-charged particles to the oppositely charged collection plate. The proficiency of the ELDC

http://dx.doi.org/10.1016/j.apt.2014.07.010 0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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running at relatively low applied voltage has been investigated numerically (170 V needed for 100% collection efficiency of 20 lm lunar dust for plate dimensions of L = W = 10 cm and D = 5 cm) [15,16]. Although the application of ELDC is limited to the sufficiently charged falling lunar dust, ELDC has the advantages of negligible power consumption for efficient dust collection before the deposition [17]. Alternatively, an electrostatic lunar dust repeller (ELDR) was proposed to protect surfaces by applying needle-shape electrodes that are connected to the same terminal of the power supply to repel the incoming like-charged lunar dust. Although the ELDR came with three advantages of less sunlight blockage, less weight and no need of electrode cleaning compared to the ELDC, power consumption of the ELDR is significantly higher, making its application limited to the surface areas smaller than 900 cm2 [18]. While the EDS has been well studied experimentally, all previous studies on the ELDC offering two major advantages of low material cost and low electric power consumption have been theoretical with simplified assumptions of spherical particles, identical particles size and density, and uniformly distributed charges on the particles. Moreover, the perfect vacuum (1014 Torr) assumed in the theoretical model ignores the effect of vacuum level on the ELDC performance and particle chargeability required for ELDC applications in other environments (e.g., Martian atmosphere). The approximated electrostatic field and charge distribution on the ELDC plates using numerical models also need verification. Therefore, the current study was embarked to address the above-mentioned modeling limitations, to investigate tribochargeability of most common lunar dust simulants (JSC-1A and Chenobi), and to evaluate the collection efficiency of an ELDC experimentally using a novel remotely controlled tribocharging device. The reasons for selecting tribocharging as the charging method of lunar dust simulants were its productivity and ease of use for application inside the vacuum chambers [19]. For instance, application of UV source inside vacuum chamber requires an insulating coating layer on the chamber inner walls as well as conducting surfaces of other equipment to prevent photoemission [20], which interferes with the charge measurement and electrostatic field generation of this study. Tribocharging of particles is a common technique for separating insulator particles in various industrial applications (e.g., treatment of ash from coal in power plants) [21]. In this charging mechanism (also known as triboelectricity or contact electrification), electrical charges are generated and exchanged due to the particles’ contacts to the container wall as well as particles’ slides on other particles. Such an exchange is due to the difference in energy levels (the energy required to release the outmost electron respect to the core of an atom) of the contacting materials until establishment of an equilibrium between the energy levels. Electrons flow from the material with a lower work function (U1) to the material with a higher work function (U2) until equilibrium is reached [22]. In contrast to conducting materials, there is no unified model for charge transfer between insulating materials. However, results of a study by Castle [23] showed that the total transferred charge is linearly proportional to the absolute difference between work functions of the contacting materials. To choose the appropriate material for tribocharging, work function of lunar dust simulants should be available. The work function of Chenobi lunar dust simulants is still unknown, and there are only a handful of studies regarding the work function of JSC-1A lunar dust simulations. Sternovsky et al. [22] conducted tribocharging experiments inside a vacuum chamber on fine particles to infer the work function of JSC-1A by comparing the total measured charge of the JSC-1A sample compared to other particles made from known materials. Their result suggested 5.9 eV as the work function of JSC-1A lunar dust simulants [24]. Trigwell et al.

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[25] took a similar approach for determining the JSC-1A work function by adding inclined planes made of select materials between the dust dropper and Faraday Cup for strengthening the electrification. Their work identified the JSC-1A work function to be 5.4 eV. In the first stage of this study, tribocharging properties of two selected lunar dust simulants (JSC-1A and Chenobi) were investigated. Then, the importance of plate conductivity in particle collection was investigated by comparing stainless steel and aluminum as the material for the ELDC plates. Finally, the collection efficiency of a customized ELDC as a function of applied voltage at low vacuum (101 Torr) and high vacuum (105 Torr) levels was determined. 2. Experimental As shown in Fig. 1, the experimental set-up consisted of a chamber enclosing four key elements: particle tribocharger, particle dropper, particle collector (ELDC), and particle charge measurer (Faraday Cup and electrometer). First, the particle tribocharger rotating around its longitudinal axis charged particles for a preset period of time. Then, these charged particles were released from the tribocharger. The exact elevation of the tribocharger inside the chamber was set to attain the same particle velocity at the ELDC entrance determined in the previous modeling efforts [14–16,18]. Since the first set of experiments aimed to obtain charging properties of the lunar dust simulants as a function of time, falling particles directly entered the Faraday Cup (MONROE, diameter = 10 cm, depth = 15 cm) with the ELDC turned-off. The total charge of the ensemble particles inside the Faraday Cup was read using an electrometer (KEITHLEY, Model 6514 with 0.1 pC reading precision). To study the effect of air pressure on tribocharging, experiments were conducted at three pressure levels: atmospheric, low vacuum and high vacuum. The low vacuum experiments were conducted at the vacuum level of about 101 Torr using a 2-stage rotary vane vacuum pump (Oerlikon Leybold TriVac D16B) connected to a transparent cylindrical chamber made of 1-in-thick PVC (see Fig. 2a). The high vacuum experimental set-up was designed using a turbo-molecular vacuum pump (Oerlikon Leybold TurboVac 361) added between the mentioned 2-stage rotary vacuum pump and a stainless steel vacuum chamber (HVB-100-N088171, RIBER Inc., Bezons, France) to achieve a vacuum level of about 105 Torr (see Fig. 2b). Evaluating the influence of tribocharging duration on the created charge on particles was the main goal of the first set of experiments. In the second set of experiments, ELDC was turned on, and its

Fig. 1. Schematic of the experimental set-up.

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Fig. 2. Vacuum systems used in the study: (a) low vacuum, (b) high vacuum and (c) particle tribocharger/dropper, ELDC and Faraday Cup inside the high vacuum chamber.

collection efficiencies at different applied voltages, different plate materials and different vacuum levels were determined. 2.1. Particle properties A variety of lunar dust simulants made for different purposes are commercially available. Two types of most commonly used lunar dust simulants, JSC-1A and Chenobi, were tested in this study. Developed by NASA–Johnson Space Center, JSC-1A lunar dust simulant is produced from a basaltic ash and sheet deposit located in San Francisco Volcanic Field of Arizona. JSC-1A has been shown to have similar mechanical properties and characteristics to the lunar grains [26]. On the other hand, Chenobi is a chemically enhanced version of OB-1 dust simulant. It is produced from a mixture of anorthosite from northern Ontario of Canada and smelter glass made of anorthosite itself [27]. The average particle density of JSC-1A lunar simulant is 2.91 g/cm3 [28], while the density of Chenobi lunar dust simulant is 2.76 g/cm3 [29]. The most conservative particle size for the mentioned cycle of lunar dust levitation– deposition was estimated to be dp  20 lm in previous studies [15,16,18]. For consistency, lunar dust simulants with the size ranging from 20 to 25 lm were used in the experiments. Particle velocity at the entrance of the ELDC was controlled by changing the distance between the tribocharger exit and the ELDC entrance. Based on the dynamic fountain model for lunar dust levitation developed by Stubbs et al. [3], the particle velocity (m0p) at the ELDC entrance can be estimated conservatively as in Eq. (1):

m0p ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi / 2g l Z max ¼ 2 s dp

sffiffiffiffiffiffiffiffi 6e0

qp

ð1Þ

where /s is the surface potential of the lunar grains, e0 is the permittivity of the space (8.854  1012 F/m), qp is the particle density, Zmax is the distance between the ELDC entrance and the height particles are released from, and gl is the lunar gravitational acceleration (1.62 m/s2). To attain the same initial particle velocity at the ELDC entrance (m0p) as in the previous studies (1.31 m/s for dp = 20 lm) [15,16,18], the distance between the tribocharger exit and the ELDC entrance was set according to the kinematic equation of motion (m0p2/2 g ’ 9 cm). 2.2. Sample preparation The standard test sieves (Gilson Inc.) conforming with ASTM E-11 specifications were used to classify lunar dust simulants with

the size of interest. The sieves were mounted on a vibratory sieve shaker (Retsch AS200 control) running at an amplitude of 1 mm for 1 h. Preliminary sieving tests showed that continuing the particle sieving longer than 1 h at the 1 mm amplitude does not affect the mass distribution of the accumulated particles on the sieves. For both types of lunar dust simulants, particles that passed through sieve No. 500 (corresponding to the mesh size of 25 lm) and got collected on sieve No. 635 (corresponding to the mesh size of 20 lm) were selected for the experiments of this study. Due to the specific condition of the lunar environment, lunar grains are fully dry. To keep the particles away from the high relative humidity of the laboratory (75% in average), particles were stored inside sealed glass containers with silica gel packets installed on the internal walls. Before starting each experiment, the weighed sample was kept inside an oven (Fisher Isotemp Oven, 100 Series, Model 106G) at 100 °C for an hour. A 5-gram simulant sample weighed by a high precision microbalance (Sartorius MC210S, readability = 10 lg) was used for each experiment. Each experiment was replicated 5 times. 2.3. Remotely controlled particle tribocharger Considering UJSC-1A = 5.4–5.9 eV, in this study, the selected material to be in contact with the lunar dust simulants was aluminum (UAl = 4.28 eV) with two advantageous of having relatively greater |U2–U1| compared to other common materials, as well as being adequately light to be held horizontally while it is rotating to tribocharge. Different tribocharging devices such as fluidized beds, vibrating feeders, static tribochargers, tribo-cyclones, tribo-fans, and rotating tubes are commonly used. However, the choice of particle tribocharger in this study was limited to the ones that would not disturb the air pressure inside the chamber since the experimental set-up had to run at low pressures. Both the inclined plane tribocharger proposed by Captain et al. [30] and the rotary milling tube proposed by Sharma et al. [31] are known as effective tribocharging methods for the lunar dust simulants inside vacuum. In this study, a hybrid tribocharger synergistically integrating both the rotary tube and the inclined plane was designed to obtain the highest possible surface charges. As shown in Fig. 3, the aluminum connector attached the aluminum tube to the armature, which was connected to a low voltage power supply. Due to the centrifugal force acting on fine particles with negligible weights, these particles may stick to the tube wall preventing them from the repeatedly effective contacts with the

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Fig. 3. Components of the designed particle tribocharger/dropper.

aluminum surfaces. This acting centrifugal force depends on particle size, tube diameter and rotational speed of the tube. Yang et al. [32] conducted sensitivity analysis on the rotational speed of a drum partially full with 3 mm particles through numerical simulations. That study identified six regimes for the particle dynamics inside the rotating drum: slumping, slumping-rolling transition, rolling, cascading, cataracting and centrifuging. Effective particle contacts occur only at slumping and rolling regimes. Tribocharging of the particles requires repeated contacts between the particles and the aluminum wall surface. Thus, the rotational speed of the armature was controlled to be 100 rpm by limiting the applied voltage of the armature to 1 V in all experiments. Moreover, chains of 2 mm aluminum beads were glued to the aluminum connector to enhance particles friction with aluminum surfaces while they were sliding inside the tube (see Fig. 3). The dual-functionality of the tribocharger (tribocharging and dropping) was achieved using a small lever connected to the remotely controlled armature to switch the particle tribocharger/dropper orientation from horizontal (to tribocharge the particles) to vertical (to drop the tribocharged particles down toward the Faraday Cup through the aluminum funnel) (see Fig. 2c). Part of the fallen particles slid on the funnel for further tribocharging improvement of the system. Geometry and rotational speed of the tribocharger determine the appropriate amount of sampled micron-sized particles to ensure effective tribocharging. Zelmet et al. [21] performed a comparative experimental study on different types of tribocharging devices to charge 1 mm PVC particles in atmospheric pressure. Their study revealed that charging efficiency of the static tribocharger utilizing inclined surfaces to direct particles falling under gravity and rotating tubes is insensitive to the mass of particles inside the tribocharger. However, not all of the tribocharged particles reach the ELDC entrance after the release, because a fraction of particles either sticks to the tribocharger wall, stays on the funnel, or puffs out from the tribocharger while rotating. In order to get adequate amount of released particles inside the Faraday Cup and possessing enough charge to be detected by the electrometer, the mass of all sampled particles initially put inside the tribocharger was set to be 5 g ± 10 lg.

streamlines originating from the positively charged plate toward the negatively charged plate. Dimensions of the plates and separation distance were 10 cm  10 cm and 5 cm, respectively, following the values used in previous studies [15,16]. Since aluminum-made tribocharger produced negatively charged particles, the positively charged plate of the ELDC was the collection plate for the entire experiments. Two plate materials of stainless steel and aluminum were tested in this study. 2.5. ELDC collection efficiency Both mass- and charge-based ratios (g1 and g2) were considered for determining the ELDC collection efficiencies as defined in Eqs. (2) and (3):

gm ð%Þ ¼

mc  100 mc þ m1 

gq ð%Þ ¼ 1 

   q1 =m1 q m2  100 ¼ 1  1  100 q2 =m2 q2 m1

ð2Þ

ð3Þ

where mc is the mass of particles collected on the collection plate, m1 and q1 are the total mass and total charge of the particles inside the Faraday Cup respectively when ELDC is turned on at a certain applied voltage, and m2 and q2 are the total mass and total charge of the particles inside the Faraday Cup when ELDC is turned off. Eq. (2) is solely mass-based, requiring one run per data-point, but Eq. (3) requires two experimental runs per data-point: one in the presence and one in the absence of the ELDC. Although ideally the particles are monodisperse with identical properties, the created surface charges on particles are different from one another due to the randomness of their tribocharging contacts. Consequently, the probability of collection depends not only on its initial displacement with respect to the collection plate, but also is a function of the acquired surface charge on the target particle. This explains the importance of presenting ELDC collection efficiency following the charged-based Eq. (3). 3. Results and discussion 3.1. Tribocharging properties of JSC-1A and Chenobi simulants

2.4. Charge measurement and electrostatic particle collection A standard Faraday Cup, connected to the nano-Coulomb electrometer was implemented for the charge measurements. The ELDC was made of two square-shape conducting parallel plates mounted on a wooden frame. Using alligator clips, the ELDC plates were connected to a low voltage DC power supply with capability of voltage provision up to 150 V. Due to the insulating property of the vacuum medium existing between the plates, ELDC acted practically as a parallel-plate capacitor with electrostatic field

Using the low vacuum experimental set-up, the first set of experiments obtained insight into tribochargeability of the lunar dust simulants. Tribocharging duration was increased in 5 min increments to investigate how total charge of the sample changes with time. Fig. 4 displays the results on the time of tribocharing (tt) at atmospheric and low vacuum pressures. In the presence of air molecules (atmospheric pressure), the maximum total charge occurred at tt = 5 min as the interactions with air molecules hindered achievement of any greater values of

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the total charge. The tribo-chargeability improved significantly (2–3 times) at the low vacuum condition due to the reduced interactions with the air molecules while charging. Hence, charge build-up on the particles continued at the provided 101 Torr vacuum level. The tribochargeability of the Chenobi particles was lower than the JSC-1A particles. Apparently, Chenobi particles had higher cohesivity and agglomerated even inside the storing container which may be the main reason for obtaining lower surface charge on the Chenobi particles. Although cohesivity of the JSC-1A particles have been studied using a DEM model [33] and through shear stress test following ASTM-6773 standard [34], unfortunately, no studies have been conducted on Chenobi particles to compare the interparticle cohesion forces. One should notice that the tribocharging in Fig. 4a and b increased with time, and continuing the experiments for longer than 20 min would result in a higher amount of surface charge on the sample. However, the rate of tribocharging is expected to decrease with time and extrapolating the plotted graph to obtain the charging time corresponding to the saturation charge on particles is not practical. Moreover, since the required tribocharging time to achieve saturation charge may be excessively long, the tribocharging time of 20 min was maintained for the remaining set of experiments. Due to higher tribochargeability of the JSC-1A particles, experiments on ELDC collection efficiency at both low and high vacuum conditions were only conducted on 5 replicates of the JSC-1A particles running each for 20 min. Under the high vacuum condition, which is four orders of magnitude greater, almost 250 times greater charge per mass (|Q/m|) on the JSC-1A samples (629 ± 58 nC/g) was achieved. The effective tribocharging method proposed in this study offered about 21 times higher |Q/m| compared to a previous study by Captain et al. [30] where tribocharging property of the JSC-1A was evaluated against aluminum inclined planes. Although their experiments were run under a higher vacuum level of 8  106 Torr, the more efficient tribocharging method introduced in the present study and the fact that 2–3 times larger JSC-1A particles (50– 75 lm) tested in their study presents 2–3 times smaller surfaceto-volume ratio, contributed to the higher surface charge on the particles in the present study. Since the average size range of the sieved particles was 20 lm < dp < 25 lm, the average charge per particle (qp) can be approximated. Assuming spherical particles with an average diameter of dp = 22.5 lm, qp for JSC-1A particles at low and high vacuum

conditions were 4.34  1017 C (about 271 electrons) and 1.10  1014 C (about 68,700 electrons), respectively. According to the model by Goertz [14,35], the theoretical charge under ultra-high vacuum can be estimated by:

qp ¼ 2pe0 dp /s

ð4Þ

At the electric surface potential (/s) of 100 V for the same size of particles which was considered as the average electric surface potential in the previous models, the surface charge on a particle is about 1.11  1013 C, which is significantly larger than that obtained experimentally at the low vacuum but only 10 times greater than that obtained experimentally at the high vacuum. The surface potential of the lunar grains is a function of environmental factors such as temperature, density of the incoming solar particles and intensity of the solar radiations. Therefore, the different methods of charging (tribocharging versus photoemissive solar radiation) and environment (105 Torr inside vacuum chamber compared to 1013 Torr on the moon) are at least partially responsible in observing 10 times smaller corresponding value for the surface potential appeared in Eq. (4) for the tested lunar dust simulants. 3.2. Effect of plate material on the ELDC collection efficiency Except having the ELDC turned on, the same experimental procedure as for the first set of experiments was followed for this set of experiments. The tribocharging time of tt = 20 min corresponding to the highest obtained surface charge on particles in the first set of experiments was chosen for the entire set of experiments. The effect of electrical conductivity (r) of the ELDC plates on ELDC collection efficiency was examined by replacing stainless steel (r = 1.45  106 S/m) with aluminum (r = 3.5  107 S/m) at low vacuum condition. This is equivalent to 24 times increase in electron mobility of the collection plate. As shown in Fig. 5, collection efficiency of the ELDC at low vacuum level was improved using aluminum electrode for the same set of conditions. Collection efficiency improvement due to replacement of aluminum increased at the higher applied voltages. In both cases, the charge-based collection efficiencies were higher than the mass-based collection efficiencies for the same applied voltage. If surface charge on each particle and initial displacement of falling particles at ELDC entrance were uniform, the values for the collection efficiencies obtained from Eqs. (2) and (3) should be the same for the tested lunar dust simulants. Obtaining higher

Fig. 4. Total charge/mass (|Q/m|) of the sample at different tribocharging elapsed times and air pressures: (a) JSC-1A and (b) Chenobi.

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Fig. 5. Collection efficiencies of steel-made and aluminum-made ELDC plates for JSC-1A simulants as a function of applied voltage at the low vacuum: (a) mass-based collection efficiency and (b) charge-based collection efficiency.

charge-based collection efficiency means the fraction of particles collected over the collection plate had acquired a higher surface charge compared to the particles that penetrated through the ELDC and settled inside the Faraday Cup. This leads to the conclusion of having a deviation for the distributed charges over the tribocharged sample. Due to higher effectiveness of aluminum ELDC plate (40–95% greater collection efficiency value at the same applied voltage), all remaining experiments of this study were conducted with aluminum plates. 3.3. Collection efficiency at low and high vacuums Fig. 6 displays collection efficiency at law and high vacuum conditions. Collection efficiency of the ELDC improved with an increase in electric potential difference between the plates. This relationship is consistent with the derived 2-D equation (Eq. (5)) and the numerically obtained graphs reported in previous studies [14–16].

2vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffi32 u 6DV e0 /s 4u 24e0 /2s /s 6e0 5 t g¼ þ 2g l L  2 dp qp qp g 2l D2 d2p qp d2p

ð5Þ

where DV is the applied voltage (electric potential difference between the ELDC plates), L is the length of ELDC plates and D is the spacing between the plates. Although the range of the selected potential difference was the same as in the previously developed numerical models [15,16], at low vacuum condition, experimentally obtained collection efficiencies were lower, 0.25–1% for the massbased and 0.45–1.45% for the charge-based definition, than what were expected from the analytical and numerical models. Owing to the higher created electric surface potential on the particles at high vacuum, the collection efficiencies boosted up to 8–35% for mass-based and 12–54% for the charge-based definition. The provided vacuum levels inside the chambers were about 101 and 105 Torr, which are far from the reported range of 1013–1014 Torr lunar environment. Interactions between the air molecules and charged particles shorten the time particles maintain the created surface charge [36]. Considering the obtained results of this study at two different vacuum levels and previous studies by Trigwell et al. [25] Captain et al. [30] and Sharma et al. [31], the vacuum level can be concluded to be the most important factor to achieve high surface charge on the particles, and consequently high collection efficiencies as predicted from the models. Ultimately, insufficient charges on the particles were

Fig. 6. ELDC collection efficiency for JSC-1A simulants as a function of applied voltage: (a) at low vacuum level and (b) at high vacuum level.

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the main reason for obtaining 1.1–3.1 times lower ELDC collection efficiencies compared to the previously developed theoretical methods for the same applied voltage. The second reason for observing lower measured ELDC collection efficiency values through the experimental approach is the difference between the lunar gravitational acceleration (gl = 1.62 m/s2) in the models and the terrestrial gravitational acceleration that governed the experiments (g = 9.81 m/s2). Thus, the falling lunar dust accelerated faster than simulated while penetrating through the ELDC. Considering the kinematic equations of motion for the falling particles, the experimental ELDC has about 15% shorter residence time (0.062 s) for lunar dust simulants collection compared to the case occurring on the moon (0.073 s). The ELDC collection efficiency at terrestrial gravitational acceleration (g = 9.81 m/s2) can be estimated using Eq. (5) for the four tested applied voltages of the experiments. This obtains 1.39 times lower collection efficiency values compared to the case using lunar gravitational acceleration: 21.7%, 42.8%, 63.9% and 85.6% for the applied voltages of 33, 65, 97 and 130 V, respectively. The estimated values using Eq. (5) are still higher than the experimentally calculated ELDC collection efficiencies of this study (7.35 ± 1.11%, 18.0 ± 1.7%, 28.4 ± 1.9% and 35.4 ± 1.6% for the mass-based approach, and 12.0 ± 1.5%, 32.0 ± 1.1%, 42.4 ± 1.5% and 54.2 ± 1.5% for the charge-based approach). Ideal ultra-vacuum level and other simplifying assumptions of the analytical model are responsible for the observed lower experimental results. 3.4. Pattern of particle collection over the ELDC collection plate Fig. 7 illustrates the pattern of the collected particles for applied voltage of 65 V at low vacuum condition. The result demonstrates that the areal density of the collected particles was higher near the corners of the collection plate. This observation is consistent with the non-uniform electrostatic field predicted in the previous study [15] which is also known as fringe effect [37]. It should be noted that most particles were collected particularly on the upper one third of the plate. The reason is that falling particles which entered the ELDC closer to the collection plate were attracted with a relatively stronger electrostatic field toward the upper one third of the plate. In comparison, particles which entered the ELDC relatively further from the collection plate needed to move a longer path to reach the plate, where the exertion of a relatively weaker e-field at the center and a relatively stronger e-field from the lower one third was insufficient to collect these particles before they exited the ELDC [15]. Moreover, an increase in particle velocity while particles were passing through the ELDC, which is significant at the terrestrial gravitational acceleration with approximately 60%

higher velocity at the ELDC exit compared to the ELDC entrance, was another reason for observing a lower number of collected particles at the lower third of the collection plate. 4. Conclusions Tribocharging properties of two noted lunar dust simulants (JSC-1A and Chenobi) and proficiency of a previously proposed electrostatic lunar dust collector (ELDC) were studied experimentally. Tribochargeability of both simulants strongly depended on the air pressure as the total amount of created charge on the sample at low vacuum condition of 101 Torr was 2–3 times higher than the case at the ambient pressure. The accumulated surface charge at the standard pressure increased with time initially but decreased later on due to the contacts of identical particles with the same work function and the interactions with existing air molecules. At low vacuum, the total charge increased with time within the 20-min time frame of the experiment. Application of aluminum plates, which have 24 times higher conductivity than stainless steel plates, improved ELDC collection efficiency especially at the higher applied voltages. Tribochargeability of the JSC-1A particles was better than the Chenobi particles. The maximum surface charge on the JSC-1A particles after 20 min of tribocharging was about 2.5 nC/g. At high vacuum, lowering the air pressure to the higher vacuum level of 105 Torr increased the total charge/mass of the JSC-1A sample about 250 times (total charge per mass value of 629 ± 58 nC/g). The charge-based and mass-based ELDC collection efficiencies confirmed the predicted linear relationship between the efficiency and the applied voltage. The observed pattern of particle collection over the collection plate was consistent with the theoretical prediction in that the majority of the collected particles were located close to the plate corners (especially in the upper third). The resultant range of collection efficiencies (0.25–1.0% for the mass-based and 0.45–1.4% for the charge-based definition at the low vacuum; 8–35% for the mass-based and 12–54% for the charge-based definition at the high vacuum) were averagely 1.1– 3.1 times lower than the values based on theoretical models. The differences were stemmed from the ultra-high vacuum level on the moon and existence of the six times smaller gravitational acceleration of the lunar environment. Acknowledgements The authors appreciate the financial support of the Space Research Initiative by the State of Florida (Grant No. 20040028). Acknowledgements are extended to Particle Engineering Research Center at University of Florida and Drs. Kevin Powers and Jennifer Curtis in particular for all supports in preparation of lunar dust simulants and particle charge measurements. Nima Afshar-Mohajer is thankful to Sally & William Glick Graduate Research Award for financial supports in setting up the system. All insights from Dr. Brian Damit, Dr. Brent Gila and Dr. Poom Bunchatheeravate, and supports from Scott Reinhart in building the high vacuum system are also gratefully appreciated. References

Fig. 7. Collected JSC-1A particles on the positively charged collection plate at DV = 65 V. Dark spots are deposited particles.

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