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We present the design and experimental validation of source seeking control algorithms for a unicycle mobile robot that is equipped with novel 3D-printed flexible graphene-based piezoresistive airflow sensors. Based solely on a local gradient measurement from the airflow sensors, we propose and analyze a projected gradient ascent algorithm to solve the source seeking problem. In the case of partial sensor failure, we propose a combination of Extremum-Seeking Control with our projected gradient ascent algorithm. For both control laws, we prove the asymptotic convergence of the robot to the source. Numerical simulations were performed to validate the algorithms and experimental validations are presented to demonstrate the efficacy of the proposed methods., Published in IEEE Transactions on Robotics

A nitrogen plasma can be sustained in air using a high-power CO 2 laser. If nitriding of a Ti substrate is performed in the presence of such laser-sustained plasma (LSP) in open atmosphere, the following effects have been previously reported: increase in nitrogen intake, wider surface coverage, reduction in surface oxidation, and no attenuation of energy transfer to the substrate. This investigation expands on previous work by systematically studying the effects of varying process parameters on microstructure during LSP nitriding of CP-titanium. Microstructural features are reported as a function of off-focal distance, scan speed of the laser, and nitrogen:argon gas flow ratio. The top surfaces and transverse cross sections of these single trail nitriding runs were observed using optical and scanning electron microscopy. X-ray diffraction (XRD) and optical profilometry techniques were used to characterize the nitrided surface layer. The presence of LSP increased the solubility of nitrogen in liquid titanium leading to gas recombination and expulsion on the surface. This tendency decreased with increasing speed. Nitrogen dilution by argon was found to reduce crack formation. Further, nitrogen dilution reduced Marangoni convection in the melt pool. This changed the nitrogen transport mechanism in the melt pool from convection-dominated to diffusion-dominated, thus reducing nitrogen incorporation into the melt pool. Argon addition was also found to reduce surface roughness and surface oxidation. XRD and scanning electron microscopy revealed the presence of phases such as TiN, TiN 0.3 and acicular martensitic titanium (α′-Ti). Future work will focus on depositing multiple overlapping nitride trails to form wide-area titanium nitride surface layers.

Sensor designs found in nature are optimal due to their evolution over millions of years, making them well-suited for sensing applications. However, replicating these complex, three-dimensional (3D), biomimetic designs in artificial and flexible sensors using conventional techniques such as lithography is challenging. In this paper, we introduce a new processing paradigm for the simplified fabrication of flexible sensors featuring complex and bioinspired structures. The proposed fabrication workflow entailed 3D-printing a metallic mold with complex and intricate 3D features such as a micropillar and a microchannel, casting polydimethylsiloxane (PDMS) inside the mold to obtain the desired structure, and drop-casting piezoresistive graphene nanoplatelets into the predesigned microchannel to form a flexible strain gauge. The graphene-on-PDMS strain gauge showed a high gauge factor of 37 as measured via cyclical tension-compression tests. The processing workflow was used to fabricate a flow sensor inspired by hair-like &lsquo, cilia&rsquo, sensors found in nature, which comprised a cilia-inspired pillar and a cantilever with a microchannel that housed the graphene strain gauge. The sensor showed good sensitivity against both tactile and water flow stimuli, with detection thresholds as low as 12 &micro, m in the former and 58 mm/s in the latter, demonstrating the feasibility of our method in developing flexible flow sensors.

We present a study of the effect of strain on the thermal conductivity of the zinc oxide using ReaxFF reactive force field. Effect of water vapor on failure stress of zinc oxide has also been carried out. Force fields for zinc oxide and water have already been developed by Raymand et al. [1] , [2] , [3] . The thermal conductivity calculations were carried out using the Transient Non-Equilibrium Molecular Dynamics (T-NEMD) described by Bharathi and van Duin [4] . For a zinc oxide nanowire, the effect of different values of mechanical strain – 0%, 2%, 6% and 10% (in the absence of water) on thermal conductivity is studied. No significant change in thermal conductivity is observed as the nanowire is stretched up to 6% strain (∼5 W/m K) and is in good agreement with the results presented by Kulkarni and Zhou (Average value ∼5.5 W/m K) [6] . The conductivity drops by 30% when the wire is stretched to 10% of its length. However, in contrast to the observations by Kulkarni et al. [7] , no phase change is observed in our simulations. The effect of water vapor (system density – 1.04, 1.09, 1.18 and 1.24 kg/m3) on the failure stress of zinc oxide nanowire is evaluated. The studies on effect water vapor on failure indicate that the failure stress drops from 14.25 GPa in the absence of water vapor to 12.0 GPa (−15.8%) for water vapor density of 1.24 kg/m3. Significant hysteresis is observed with increasing water vapor density. As a practical application of T-NEMD, results from work on the sintering of zinc oxide nanoparticles in water vapor are reported.

Coal or biomass chars are complex carbonaceous materials that are important energy sources for electricity production. Reactive molecular dynamics simulations are a useful tool to examine the chemical reactions occurring in complex processes, providing that a realistic structural representation and an applicable reactive force field can be utilized. Combustion of coal (or biomass) char is one such area were additional insight would be helpful for utilization enhancements and pollution control. In this investigation a devolatilized Illinois No. 6 coal char atomistic representation was generated, using Fringe3D and additional Perl scripts, coupled with the ReaxFF reactive force field for hydrocarbon combustion. Fringe3D facilitates the char structure generation process by producing a distribution of aromatic structures based on HRTEM lattice fringe image analyses. Perl scripts were used for incorporating heteroatoms and aliphatic components to aid elimination of investigator bias, and facilitate a more rapid construction process. The char structure was constrained by a combination of elemental and NMR literature data. Chemical and physical parameters were found to be broadly consistent with the experimental data. The ReaxFF force field for hydrocarbon combustion was used to perform simulations to examine the structural transformations and chemical processes associated with char combustion. In this initial work, very high temperatures (3000–4000 K) were selected for ReaxFF simulation under stoichiometric, fuel lean and rich combustion conditions. These elevated temperatures were chosen to observe chemical reactions proceed to completion within a computationally practical simulation time. Analyses indicated that char oxidation process was primarily initialized by either thermal degradation of char structure to form small fragments, that were subsequently oxidized, or by hydrogen abstraction reactions by oxygen molecules and O and OH radicals. A more rapid oxidation and combustion of the polyaromatic structures occurred at fuel lean (oxygen rich) conditions compared with fuel rich combustion. Char transitions included 6-membered ring conversion into 5- and 7-membered rings that further decomposed or reacted with mostly O and OH radicals. This work further demonstrates the utility of ReaxFF force field integration with representative char structural models to investigate physical and chemical transformations of char structure during combustion at high-temperature conditions.

Laser-induced incandescence (LII) of soot has developed into a popular method for making in situ measurements of soot volume fraction and primary particle sizes. However, there is still a lack of understanding regarding the generation and interpretation of the cooling signals. To model heat transfer from the heated soot particles to the surrounding gas, knowledge of the collision-based cooling as well as reactive events, including oxidation (exothermic) and evaporation (endothermic) is essential. We have simulated LII of soot using the ReaxFF reactive force field for hydrocarbon combustion. Soot was modeled as a stack of four graphene sheets linked together using sp(3) hybridized carbon atoms. To calculate the thermal accommodation coefficient of various gases with soot, graphene sheets of diameter 40 Å were used to create a soot particle containing 2691 atoms, and these simulations were carried out using the ReaxFF version incorporated into the Amsterdam Density Functional program. The reactive force field enables us to simulate the effects of conduction, evaporation, and oxidation of the soot particle on the cooling signal. Simulations were carried out for both reactive and nonreactive gas species at various pressures, and the subsequent cooling signals of soot were compared and analyzed. To correctly model N(2)-soot interactions, optimization of N-N and N-C-H force field parameters against DFT and experimental values was performed and is described in this paper. Subsequently, simulations were performed in order to find the thermal accommodation coefficients of soot with various monatomic and polyatomic gas molecules like He, Ne, Ar, N(2), CO(2), and CH(4). For all these species we find good agreement between our ReaxFF results and previously published accommodation coefficients. We thus believe that Molecular Dynamics using the ReaxFF reactive force field is a promising approach to simulate the physical and chemical aspects of soot LII.

This paper reports the fabrication and characterization of a flexible piezoresistive flow sensor comprising a polydimethylsiloxane (PDMS) cantilever with a serpentine graphene nanoplatelets (GNP) strain gauge embedded at the cantilever base. A facile and cleanroom-free processing work flow involving a combination of high-resolution powder bed fusion and soft lithography was used to fabricate PDMS cantilevers (aspect ratio 20) with 150 μm × 150 μm microchannels on its surface. A high gauge factor of 55 (up to 5 times higher than reported in comparable piezoresistive flow sensors) was achieved using drop- casted GNP ink as the piezoresistive sensing element in the aforementioned microchannels. Finally, the use of the PDMS- graphene cantilever as an airflow sensor with enhanced sensitivity (20 times more than comparable piezoresistive cantilever sensors), low hysteresis, good repeatability, and bidirectional sensing capability was demonstrated.