Browsing by Author "Yang, Fengchang"

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    Dynamics of Micro-Particles in Complex Environment
    Yang, Fengchang (Virginia Tech, 2017-07-21)
    Micro-particles are ubiquitous in microsystems. The effective manipulation of micro-particles is often crucial for achieving the desired functionality of microsystems and requires a fundamental understanding of the particle dynamics. In this dissertation, the dynamics of two types of micro-particles, Janus catalytic micromotors (JCMs) and magnetic clusters, in complex environment are studied using numerical simulations. The self-diffusiophoresis of JCMs in a confined environment is studied first. Overall, the translocation of a JCM through a short pore is slowed down by pore walls, although the slowdown is far weaker than the transport of particles through similar pores driven by other mechanisms. A JCM entering a pore with its axis not aligned with the pore axis can execute a self-alignment process and similar phenomenon is found for JCMs already inside the pore. Both hydrodynamic effect and 'chemical effect', i.e., the modification of the concentration of chemical species around JCMs by walls and other JCMs, play a key role in the observed dynamics of JCMs in confined and crowded environment. The dynamics of bubbles and JCMs in liquid films covering solid substrates are studied next. A simple criterion for the formation of bubbles on isolated JCMs is developed and validated. The anomalous bubble growth law (r~t^0.7) is rationalized by considering the relative motion of growing bubbles and their surrounding JCMs. The experimentally observed ultra-fast collapse of bubbles is attributed to the coalescence of the bubble with the liquid film-air interface. It is shown that the collective motion of JCMs toward a bubble growing on a solid substrate is caused by the evaporation-induced Marangoni flow near the bubble. The actuation of magnetic clusters using non-uniform alternating magnetic fields is studied next. It is discovered that the clusters' clockwise, out-of-plane rotation is a synergistic effect of the magnetophoresis force, the externally imposed magnetic torque and the hydrodynamic interactions between the cluster and the substrate. Such a rotation enables the cluster to move as a surface walker and leads to unique dynamics, e.g., the cluster moves away from the magnetic source and its trajectory exhibits a periodic fluctuation with a frequency twice of the field frequency.
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    A physical catalyst for the electrolysis of nitrogen to ammonia
    Song, Yang; Johnson, Daniel; Peng, Rui; Hensley, Dale K.; Bonnesen, Peter V.; Liang, Liangbo; Huang, Jingsong; Yang, Fengchang; Zhang, Fei; Qiao, Rui; Baddorf, Arthur P.; Tschaplinski, Timothy J.; Engle, Nancy L.; Hatzell, Marta C.; Wu, Zili; Cullen, David A.; Meyer, Harry M.; Sumpter, Bobby G.; Rondinone, Adam J. (AAAS, 2018-04-01)
    Ammonia synthesis consumes 3 to 5% of the world’s natural gas, making it a significant contributor to greenhouse gas emissions. Strategies for synthesizing ammonia that are not dependent on the energy-intensive and methane-based Haber-Bosch process are critically important for reducing global energy consumption and minimizing climate change. Motivated by a need to investigate novel nitrogen fixation mechanisms, we herein describe a highly textured physical catalyst, composed of N-doped carbon nanospikes, that electrochemically reduces dissolved N2 gas to ammonia in an aqueous electrolyte under ambient conditions. The Faradaic efficiency (FE) achieves 11.56 ± 0.85% at −1.19 V versus the reversible hydrogen electrode, and the maximum production rate is 97.18 ± 7.13 mg hour−1 cm−2. The catalyst contains no noble or rare metals but rather has a surface composed of sharp spikes, which concentrates the electric field at the tips, thereby promoting the electroreduction of dissolved N2 molecules near the electrode. The choice of electrolyte is also critically important because the reaction rate is dependent on the counterion type, suggesting a role in enhancing the electric field at the sharp spikes and increasing N2 concentration within the Stern layer. The energy efficiency of the reaction is estimated to be 5.25% at the current FE of 11.56%.