Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Mechanical Engineering.

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Boosting photocatalytic hydrogen production from water by photothermally induced biphase systems
(Nature Communications, 2021-02-26) Guo, Shaohui; Li, Xuanhua; Li, Ju; Wei, Bingqing
Solar-driven hydrogen production from water using particulate photocatalysts is considered the most economical and effective approach to produce hydrogen fuel with little environmental concern. However, the efficiency of hydrogen production from water in particulate photocatalysis systems is still low. Here, we propose an efficient biphase photocatalytic system composed of integrated photothermal–photocatalytic materials that use charred wood substrates to convert liquid water to water steam, simultaneously splitting hydrogen under light illumination without additional energy. The photothermal–photocatalytic system exhibits biphase interfaces of photothermally-generated steam/photocatalyst/hydrogen, which significantly reduce the interface barrier and drastically lower the transport resistance of the hydrogen gas by nearly two orders of magnitude. In this work, an impressive hydrogen production rate up to 220.74 μmol h−1 cm−2 in the particulate photocatalytic systems has been achieved based on the wood/CoO system, demonstrating that the photothermal–photocatalytic biphase system is cost-effective and greatly advantageous for practical applications.
Electrochemical gas separation and inerting system
(Journal of Power Sources, 2021-05-15) Aryal, Utsav Raj; Chouhan, Ashish; Darling, Robert; Yang, Zhiwei; Perry, Mike L.; Prasad, Ajay K.
Following the TWA 800 flight disaster in 1996 which was attributed to an explosion in the fuel tank, inerting of the ullage (air volume above the fuel in the tank) has gained prominence. Fuel tank inerting is the process of reducing the flammability of the ullage by supplying it with an inert gas like nitrogen. Current inerting techniques are expensive, consume large amounts of energy, and fail prematurely. Here, we propose a novel in-flight electrochemical gas separation and inerting system (EGSIS) to produce and supply nitrogen-enriched air (NEA). EGSIS combines a polymer electrolyte membrane (PEM) fuel cell cathode with a PEM electrolyzer anode to generate humidified NEA as the cathode output which can be dehumidified and supplied directly to the fuel tank. The required rate of NEA varies during a typical flight and a major advantage of EGSIS is that the rate of NEA generation can be conveniently controlled by varying the voltage applied to the system. Here, we report on the performance of a single-cell EGSIS apparatus and evaluate its suitability for aircraft fuel tank inerting.
Blending poly(2-ethyl-2-oxazoline) with hydrophobic polymers as a hybrid adhesive with enhanced water-resistant properties
(Journal of Applied Polymer Science, 2021-07-16) Zhang, Yuanyuan; Li, Xuanhua; Guo, Shaohui; Wei, Bingqing
The use of poly(2-ethyl-2-oxazoline) (PEOX) in a wet environment is limited because of its high hydrophilicity. In this study, PEOX based blends were prepared via blending PEOX with hydrophobic polymers, such as poly(styrene-co-acrylonitrile) (SAN), poly(4-vinylphenol) (PVPh), and poly(vinylidene fluoride) (PVDF), in order to improve the water-resistance of PEOX. The blends' water resistance properties are evaluated by the contact angle, solubility, moisture absorption, and mechanical strength in a wet environment. The results show that the water resistance and the adhesion strength of PEOX in a wet environment are dramatically enhanced by polymer blending. The blend with 30 wt% PVPh demonstrates excellent performances in transparency and water-resistant abilities. It is found that the stable hydrogen bonding within the blend plays an important role in hydrophobic modification. The PVPh/PEOX blend can be applied as a new type of transparent coating or adhesive with enhanced water-resistant properties in a wet environment.
Resilient Supervisory Multiagent Systems
(IEEE Transactions on Robotics, 2021-09-28) Baxevani, Kleio; Zehfroosh, Ashkan; Tanner, Herbert G.
Accidental or deliberate disruption of the coordination function in a multiagent system has been discussed and referred to in the social sciences literature as leader decapitation; this article outlines a methodology for making multiagent networks resilient to this type of failure, enabling a timely restoration of operation normalcy by leveraging machine learning techniques. The approach involves endowing the agents with a cascade of independent learning modules that enable them to discover over time their role in the overall system coordinating strategy, so that they are able to autonomously implement it when central coordination seizes to function. Through these machine learning algorithms, the agents incrementally identify the overall system’s task specification and simultaneously optimize their strategy to serve the common goal.
Data-Driven Abstractions for Robots With Stochastic Dynamics
(IEEE Transactions on Robotics, 2021-11-22) Tanner, Herbert G.; Stager, Adam
This article describes the construction of stochastic, data-based discrete abstractions for uncertain random processes continuous in time and space. Motivated by the fact that modeling processes often introduce errors which interfere with the implementation of control strategies, here the abstraction process proceeds in reverse: the methodology does not abstract models; rather it models abstractions. Specifically, it first formalizes a template for a family of stochastic abstractions, and then fits the parameters of that template to match the dynamics of the underlying process and ground the abstraction. The article also shows how the parameter-fitting approach can be implemented based on a probabilistic model validation approach which draws from randomized algorithms, and results in a discrete abstract model which is approximately simulated by the actual process physics, at a desired confidence level. In this way, the models afford the implementation of symbolic control plans with probabilistic guarantees at a desired level of fidelity.
Development and Testing of an Aerial Radiation Detection System
(IEEE Sensors Journal, 2021-12-15) Yadav, Indrajeet; Tanner, Herbert G.
This paper reports on the design and implementation of an airborne radiation detection system together with its associated signal processing and decision-making algorithms. This system is envisioned as the building block of an aerial radiation sensor network and it is specifically designed to detect weak radiological signatures in transit. The whole system is developed based on low-cost commercial off the shelf (COTS) components, and through a series of detailed experiments and Monte-Carlo tests, the paper shows how it can be deployed in time-critical application scenarios, where the time allocated to detect the source is limited. Performance metrics for the detection algorithms utilized in the system indicate that the reported technology can offer a significant improvement on the detection speeds compared to alternative techniques utilizing the same hardware resources.
Navigation Functions with Moving Destinations and Obstacles
(Autonomous Robots, 2022) Wei, Cong; Chen, Chuchu; Tanner, Herbert G.
Dynamic environments challenge existing robot navigation methods, necessitating either stringent assumptions on workspace variation or sacrificing collision avoidance and convergence guarantees. This paper shows that the navigation function methodology can preserve such guarantees in a dynamic sphere-world with moving obstacles and a time-varying goal, without prior knowledge of environment variation. Assuming bounds on speeds of robot destination and obstacles, and sufficiently higher maximum robot speed, the navigation function gradient can be used produce robot feedback laws that guarantee obstacle avoidance, and theoretical guarantees of bounded tracking errors and eventual convergence to the target in the case where the latter seizes to move. The efficacy of the gradient-based feedback controller derived from the new navigation function construction is demonstrated both in numerical simulations as well as experimentally.
Decoupled Right Invariant Error States for Consistent Visual-Inertial Navigation
(IEEE Robotics and Automation Letters, 2022-01-04) Yang, Yulin; Chen, Chuchu; Lee, Woosik; Huang, Guoquan
The invariant extended Kalman filter (IEKF) is proven to preserve the observability property of visual-inertial navigation systems (VINS) and suitable for consistent estimator design. However, if features are maintained in the state vector, the propagation of IEKF will become more computationally expensive because these features are involved in the covariance propagation. To address this issue, we propose two novel algorithms which preserve the system consistency by leveraging the invariant state representation and ensure efficiency by decoupling features from covariance propagation. The first algorithm combines right invariant error states with first-estimates Jacobian (FEJ) technique, by decoupling the features from the Lie group representation and utilizing FEJ for consistent estimation. The second algorithm is designed specifically for sliding-window filter-based VINS as it associates the features to an active cloned pose, instead of the current IMU state, for Lie group representation. A new pseudo-anchor change algorithm is also proposed to maintain the features in the state vector longer than the window span. Both decoupled right- and left-invariant error based VINS methods are implemented for a complete comparison. Extensive Monte-Carlo simulations on three simulated trajectories and real world evaluations on the TUM-VI datasets are provided to verify our analysis and demonstrate that the proposed algorithms can achieve improved accuracy than a state-of-art filter-based VINS algorithm using FEJ.
Knee joint biomechanics during gait improve from 3 to 6 months after anterior cruciate ligament reconstruction
(Journal of Orthopaedic Research, 2022-01-06) Neal, Kelsey; Williams, Jack R.; Alfayyadh, Abdulmajeed; Capin, Jacob J.; Khandha, Ashutosh; Manal, Kurt; Snyder‐Mackler, Lynn; Buchanan, Thomas S.
Gait alterations after anterior cruciate ligament reconstruction (ACLR) are commonly reported and have been linked to posttraumatic osteoarthritis development. While knee gait alterations have been studied at several time points after ACLR, little is known about how these biomechanical variables change earlier than 6 months after surgery, nor is much known about how they differ over the entire stance phase of gait. The purpose of this study was to examine knee gait biomechanical variables over their entire movement pattern through stance at both 3 and 6 months after ACLR and to study the progression of interlimb asymmetry between the two postoperative time points. Thirty-five individuals underwent motion analysis during overground walking 3 (3.2 ± 0.5) and 6 (6.4 ± 0.7) months after ACLR. Knee biomechanical variables were compared between limbs and across time points through 100% of stance using statistical parametric mapping; this included a 2 × 2 (Limb × Time) repeated measures analysis of variance and two-tailed t-tests. Smaller knee joint angles, moments, extensor forces, and medial compartment forces were present in the involved versus uninvolved limb. Interlimb asymmetries were present at both time points but were less prevalent at 6 months. The uninvolved limb's biomechanical variables stayed relatively consistent over time, while the involved limb's trended toward that of the uninvolved limb. Statement of Clinical Significance: Interventions to correct asymmetrical gait patterns after ACLR may need to occur early after surgery and may need to focus on multiple parts of stance phase.
Assembling metal-polyphenol coordination interfaces for longstanding zinc metal anodes
(EcoMat, 2022-01-11) Huyan, Yu; Wang, Jian-Gan; Tian, Shan; Ren, Lingbo; Liu, Huanyan; Wei, Bingqing
Zn metals have gained the immense attention of researchers for their wide employment as the anode of high-performance aqueous batteries. Nonetheless, the Zn anodes suffer from uncontrollable dendrite growth and parasitic side reactions, which substantially shorten the battery lifespan. This study proposes an interfacial assembly of a metal-polyphenol coordination coating on Zn anodes to regulate Zn2+ deposition behavior. Bismush-coordinated polyphenolic ligands (i.e., tannic acid, TA) create a functional interface that could promote Zn's uniform nucleation and plating/striping kinetics. Moreover, the artificial coating acts as a physical barrier to inhibit surface corrosion. As a consequence, the TA-Bi-modified Zn anodes display a small voltage hysteresis of ~38 mV at 1 mA cm−2 over 2600 h and an ultra-long lifespan for 3100 h (~4.3 months) even at a high-current density of 10 mA cm−2. When assembled with a vanadium-based cathode, the full Zn-ion batteries achieve improved electrochemical performance.
Effect of the Coastline Geometry on the Boundary Currents Intruding through the Gap
(Fluids, 2022-02-08) Kuehl, Joseph; Sheremet, Vitalii A.
The problem of a geophysical western boundary current negotiating a gap in its supporting boundary is considered. For traditional straight, parallel gaps, such systems are known to exhibit two dominant states, gap penetrating and leaping, with the transitional dynamics between states displaying hysteresis. However, for more complex geometries, such as angled or offset gap configurations, the question of multiple states and hysteresis is unresolved. In such cases, the inertia of the western boundary current is oriented into the gap, hence the assumption that increased inertia promotes gap penetrating loop current states. Here we address the problem numerically in an idealized setting. It is found that despite the inertia of the current being directed into the gap, for large western boundary current transport values, leaping states will be present. That is, we show here that the presence of multiple states with hysteresis for gap-leaping western boundary current systems is robust to both angled and offset gap geometries.
Catalytic Boosting Bidirectional Polysulfide Redox using Co0.85Se/C Hollow Structure for High-Performance Lithium-Sulfur Batteries
(ChemElectroChem, 2022-02-17) Zhang, Xingyuan; Gu, Honghui; Shen, Chao; Wei, Bingqing; Wang, Jian-Gan
Achieving effective adsorption and fast conversion of soluble polysulfides confined in the sulfur cathode is critical yet challenging for building high-performance lithium-sulfur batteries. Herein, we construct a unique hollow-structured Co0.85Se/C as a separator modifier (CSPP) to effectively suppress the polysulfide shuttle effect. The Co0.85Se/C demonstrates strong anchoring with polysulfide species and smooth bidirectional electrocatalysis. The unique mesoporous hollow architecture affords sufficient catalytic sites and Li+ diffusion channels for promoting the reaction kinetics. Benefiting from the merits, the CSPP-cell could yield a superior electrochemical utilization of active sulfur, excellent rate capability (679 mAh g−1 at 5 C), and stable cycling performance with an ultralow fading rate of 0.056 % per cycle over 500 cycles. The work highlights great promise of developing cobalt-based materials as kinetic regulators for highly stable lithium-sulfur batteries.
Maize brace root mechanics vary by whorl, genotype, and reproductive stage
(Annals of Botany, 2022-03-03) Hostetler, Ashley N.; Erndwein, Lindsay; Ganji, Elahe; Reneau, Jonathan W.; Killian, Megan L.; Sparks, Erin E.
Background and Aims: Root lodging is responsible for significant crop losses world-wide. During root lodging, roots fail by breaking, buckling, or pulling out of the ground. In maize, above-ground roots, called brace roots, have been shown to reduce root lodging susceptibility. However, the underlying structural-functional properties of brace roots that prevent root lodging are poorly defined. In this study, we quantified structural mechanical properties, geometry, and bending moduli for brace roots from different whorls, genotypes, and reproductive stages. Methods: Using 3-point bend tests, we show that brace root mechanics are variable by whorl, genotype, and reproductive stage. Key Results: Generally, we find that within each genotype and reproductive stage, the brace roots from the first whorl (closest to the ground) had higher structural mechanical properties and a lower bending modulus than brace roots from the second whorl. There was additional variation between genotypes and reproductive stages. Specifically, genotypes with higher structural mechanical properties also had a higher bending modulus, and senesced brace roots had lower structural mechanical properties than hydrated brace roots. Conclusions: Collectively these results highlight the importance of considering whorl-of-origin, genotype, and reproductive stage for quantification of brace root mechanics, which is important for mitigating crop loss due to root mechanical failure.
A Hybrid PAC Reinforcement Learning Algorithm for Human-Robot Interaction
(Frontiers in Robotics and AI, 2022-03-09) Zehfroosh, Ashkan; Tanner, Herbert G.
This paper offers a new hybrid probably approximately correct (PAC) reinforcement learning (RL) algorithm for Markov decision processes (MDPs) that intelligently maintains favorable features of both model-based and model-free methodologies. The designed algorithm, referred to as the Dyna-Delayed Q-learning (DDQ) algorithm, combines model-free Delayed Q-learning and model-based R-max algorithms while outperforming both in most cases. The paper includes a PAC analysis of the DDQ algorithm and a derivation of its sample complexity. Numerical results are provided to support the claim regarding the new algorithm’s sample efficiency compared to its parents as well as the best known PAC model-free and model-based algorithms in application. A real-world experimental implementation of DDQ in the context of pediatric motor rehabilitation facilitated by infant-robot interaction highlights the potential benefits of the reported method.
Analytical Representation and Efficient Computation of the Effective Conductivity of Two-Phase Composite Materials
(International Journal for Numerical Methods in Engineering, 2022-04-04) Roy, R. Valéry
Many engineered materials display ordered or disordered microstructures. Such materials exhibit transport properties which are unmatched by their single-phase homogeneous counterparts. These properties are obtained by the mixture of two or more phases typically characterized by a large contrast in their properties. For the development of these materials, it is critical to develop a robust computational framework in order to provide a fundamental understanding of how microstructure affects performance. This hinges on predicting their macroscopic properties, given the constitutive laws and spatial distribution of their constituents. To this end, this work presents a computational framework based on formulating periodic conduction transport problems in terms of boundary integral equations whose kernel is expressed in terms of Weierstrass zeta-function. The components of the effective conductivity tensor are then sought in the form of power series expansions of a conductivity contrast parameter. To accelerate their convergence, these expansions are transformed into Padé approximants. Presently restricted to the case of two-dimensional, two-phase microstructures, this framework is shown to yield accurate results over the entire range of the contrast parameter. Representation of the kernel as a lattice sum allows the use the fast multipole method, thereby making computations significantly more efficient.
Non-Smooth Control Barrier Navigation Functions for STL Motion Planning
(Frontiers in Robotics and AI, 2022-04-13) Zehfroosh, Ashkan; Tanner, Herbert G.
This paper reports on a new approach to Signal Temporal Logic (STL) control synthesis, that 1) utilizes a navigation function as the basis to construct a Control Barrier Function (CBF), and 2) composes navigation function-based barrier functions using nonsmooth mappings to encode Boolean operations between the predicates that those barrier functions encode. Because of these two key features, the reported approach 1) covers a larger fragment of STL compared to existing approaches, 2) alleviates the computational cost associated with evaluation of the control law for the system in existing STL control barrier function methodologies, and 3) simultaneously relaxes some of the conservativeness of smooth combinations of barrier functions as a means of implementing Boolean operators. The paper demonstrates the efficacy of this new approach with three simulation case studies, one aiming at illustrating how complex STL motion planning specification can be realized, the second highlights the less-conservativeness of the approach in comparison to the existing methods, and another that shows how this technology can be brought to bear to push the envelope in the context of human-robot social interaction.
3D Computational Model for an Electrochemical Gas Separation and Inerting System
(Journal of The Electrochemical Society, 2022-04-25) Aryal, Utsav Raj; Aziz, Majid; Prasad, Ajay K.
Aircraft fuel tank inerting is employed to reduce the flammability of the fuel vapor in the ullage (air volume above the fuel) by restricting its oxygen concentration to a safe value—12% for commercial aircraft and 9% for military aircraft. Inerting is typically accomplished by displacing oxygen in the ullage with an inert gas like nitrogen. Electrochemical gas separation and inerting system (EGSIS) is an on-board method to generate and supply nitrogen-enriched air (NEA) to the fuel tank. EGSIS combines a polymer electrolyte membrane (PEM) electrolyzer anode which dissociates water to evolve oxygen, and a PEM fuel cell cathode which reduces oxygen from atmospheric air to produce NEA at its outlet. This paper represents the first attempt to model and simulate EGSIS using a three-dimensional, steady state, isothermal model. Various EGSIS performance indicators such as current density, reactant concentration distribution, and polarization curves are studied as a function of operating conditions and design parameters. The results from the computational model are validated against our previous experimental results for various operating conditions. The simulation results reveal the effects of temperature, reactant flowrates, and material property optimization on EGSIS performance. Different operating strategies are explored with the goal of improving system performance.
Plasmon-induced super-semiconductor at room temperature in nanostructured bimetallic arrays
(Applied Physics Reviews, 2022-05-17) Li, Zhigang; Cui, Xiangke; Wang, Xiaowei; Wang, Zongpeng; Fang, Minghu; Feng, Shangshen; Liu, Yanping; Chen, Jigen; Wang, Tianle; Liu, Hengji; Xia, Zhenhai; Wei, Bingqing
Solid-state electrical conducting materials can be roughly categorized as superconductors, conductors, and semiconductors, depending on their conducting carriers, resistance, and band structures. This research reports the discovery of super-semiconductors, whose resistivity is 3–10 orders of magnitude lower than conventional semiconductors at room temperature. In addition, there is a transition from a metal state to a super-semiconducting state at near room temperatures, which is accompanied by an increase in hole carrier density and the mobility increase in electrons. For the first time, a hole-dominated carrier metal is observed in nanostructured bimetallic arrays near room temperature, and no other special conditions are required. Such a behavior is due to the generation of hot electrons and holes induced by metal plasmon resonance in the infrared range in the nanostructured bimetallic arrays. Our research empowers metals with semiconductor features and paves the way to realize ultra-low-power metal-based semiconductor devices.
A three-dimensional numerical model for the motion of liquid drops by the particle finite element method
(Physics of Fluids, 2022-05-20) Mahrous, Elaf; Roy, R. Valéry; Jarauta, Alex; Secanell, Marc
Analysis of drop spreading and sliding on solid substrates is critical for many industrial applications, such as microfluidic devices, cooling towers, and fuel cells. A new three-dimensional model is proposed for droplet dynamics. Its numerical solution is obtained by the particle finite element method, based on an updated Lagrangian framework to accurately track the deformation of the droplet. The model hinges on boundary conditions at the solid–liquid interface to account for viscous dissipation and retention forces. These conditions are essential to obtain mesh-independent solutions and a realistic spatiotemporal evolution of the droplet deformation. Several numerical simulations are performed to assess the performance of the model for spreading and sliding drops, and results are compared to experimental data found in the literature. Good agreement is obtained with the available data. Simulations performed in two dimensions show striking discrepancies with the experimental data, thus demonstrating the need for three-dimensional simulations.
Plasma-Wind-Assisted In2S3 Preparation with an Amorphous Surface Structure for Enhanced Photocatalytic Hydrogen Production
(Nanomaterials, 2022-05-21) Guo, Shaohui; Luo, Hui; Duan, Xiaochuan; Wei, Bingqing; Zhang, Xianming
Photocatalytic production from water is considered an effective solution to fossil fuel-related environmental concerns, and photocatalyst surface science holds a significant interest in balancing photocatalysts’ stability and activity. We propose a plasma-wind method to tune the surface properties of a photocatalyst with an amorphous structure. Theoretical calculation shows that the amorphous surface structure can cause an unsaturated coordination environment to adjust the electron distribution, forming more adsorption sites. Thus, the photocatalyst with a crystal–amorphous (C–A) interface can strengthen light absorption, harvest photo-induced electrons, and enrich the active sites, which help improve hydrogen yield. As a proof of concept, with indium sulfide (In2S3) nanosheets used as the catalyst, an impressive hydrogen production rate up to 457.35 μmol cm−2 h−1 has been achieved. Moreover, after plasma-assisted treatment, In2S3 with a C–A interface can produce hydrogen from water under natural outdoor conditions. Following a six-hour test, the rate of photocatalytic hydrogen evolution is found to be 400.50 μmol cm−2 g−1, which demonstrates that a catalyst prepared through plasma treatment is both effective and highly practical.

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