Rui Chen

Amorphous topological states, which are independent of the specific spatial distribution of microscopic constructions, have gained much attention. Recently, higher-order topological insulators, which are a new class of topological phases of matter, have been proposed in amorphous systems. Here, we propose a density-driven higher-order topological phase transition in a two-dimensional amorphous system. We demonstrate that the amorphous system hosts a topological trivial phase at low density. With an increase in the density of lattice sites, the topological trivial phase converts to a higher-order topological phase characterized by a quantized quadrupole moment and the existence of topological corner states. Furthermore, we confirm that the density-driven higher-order topological phase transition is size dependent. In addition, our results should be general and equally applicable to three-dimensional amorphous systems. Our findings may greatly enrich the study of higher-order topological states in amorphous systems.

The deflagration-to-detonation transition remains one of the most interesting and mysterious physical phenomena in the combustion of energetic materials, which contains substantial complicated and nonlinear characteristics. In the present work, the effect of the chemical reactivity of different fuels and diluent gases on the end-gas autoignition and detonation development in a confined space was investigated. Five fuels (hydrogen, methane, iso-octane, n-heptane, and PRF50) and three diluent gases (argon, nitrogen, and carbon dioxide) were used to change the chemical reactivity. The results showed that both the chemical reactivity and shock wave had a significant influence on the end-gas autoignition and detonation development. For mixtures with different diluent gases, it was observed that the transition thresholds (denoted by critical oxygen fraction) increased in the order of argon, nitrogen, and carbon dioxide. Different detonation modes with varying shock compressions were observed under different diluents for n-heptane. Although the flame propagation of different fuels differs at 21% oxygen fraction, end-gas autoignition and detonation development processes can still be observed in all kinds of fuels when the oxygen fraction was elevated to a certain value. The transition thresholds increased in the order of hydrogen, n-heptane, PRF50, iso-octane, and methane. Further analysis revealed that the fuel with a shorter ignition delay usually required a lower flame tip velocity, accomplished with a delayed occurrence of detonation. In addition, the transition threshold was determined by the chemical reactivity and flame speed.

As one of the most fascinating confined water/ice phenomena, two-dimensional square ice has been extensively studied and experimentally confirmed in recent years. Apart from the unidirectional homogeneous square icing patterns considered in previous studies, the multidirectional partitioned square icing patterns are discovered in this study and characterized by molecular dynamics (MD) simulations. Square icing parameters are proposed to quantitatively distinguish the partitioned patterns from the homogeneous patterns and the liquid water. The number of graphene monolayers n is varied in this study, and the results show that it is more energetically favorable to form partitioned square icing patterns when the water molecules are confined between graphite sheets (n ≥ 2) compared to graphene (n = 1). This phenomenon is insensitive to n as long as n ≥ 2 because of the short-range nature of the interaction between water molecules and the carbon substrate. Moreover, it is energetically unfavorable to form partitioned square icing patterns for a single layer of water molecules even for n ≥ 2, verifying that the interaction between layers of water molecules is another dominant factor in the formation of partitioned structures. The conversion from partitioned structure to homogeneous square patterns is investigated by changing the pressure and the temperature. Based on the comprehensive MD simulations, this study unveils the formation mechanism of the partitioned square icing patterns.

Imaging photo-plethysmography (IPPG) technique has been proven to detect heart rate (HR) with a camera. Currently it might be used in a rigid scenario, such as the static subject, the short measurement distance and good illumination. In this paper, we try to relax these constraints by extending the measurement distance and applying it to the non-static subject with IPPG technique using the off-the-shelf camera. To achieve it, the landmark detection method is used to locate the facial region of interest (ROI) for the subject during movements. On this ROI, R/G/B channel signals are extracted with the spatial average of pixels. Then we use a skin appearance model (CHROME) to remove the noise, and HR is estimated from the output of the CHROME through the power spectrum. Experimental results showed that our method works effectively in both static and non-static scenarios. In static scenario, the measurement distance can extend to 4 meters, while the accuracy can reach 86% compared with the results from a finger detection equipment. In non-static scenario, our method achieve better results than previous studies.

Proton exchange membrane fuel cells (PEMFCs) using porous metallic foam flow-field plates have been demonstrated as an alternative to conventional rib and channel designs, showing high performance at high currents. However, the transport of liquid product water through metal foam flow-field plates in PEMFC conditions is not well understood, especially at the individual pore level. In this work, ex-situ experiments are conducted to visualise liquid water movement within a metal foam flow-field plate, considering hydrophobicity, foam pore size and air flow rate. A two-phase numerical model is then developed to further investigate the fundamental water transport behaviour in porous metal foam flow-field plates. Both the experimental and numerical work demonstrate that unlike conventional PEMFC channels, air flow rate does not have a strong influence on water removal due to the high surface tensions between the water and foam pore ligaments. A hydrophobic foam was seen to transport liquid water away from the initial injection point faster than a hydrophilic foam. In ex-situ tests, liquid water forms and maintains a random preferential pathway until the flow-field edge is reached. These results suggest that controlled foam hydrophobicity and pore size is the best way of managing water distribution in PEMFCs with porous flow-field plates.