Construction and Observation of Flexibly Controllable High-Dimensional Non-Hermitian Skin Effects.

Non-Hermitian skin effect (NHSE) is one of the most fundamental phenomena in non-Hermitian physics. It is established that 1D NHSE originates from the nontrivial spectral winding topology. However, the topological origin behind the higher-dimensional NHSE remains unclear, which poses a substantial challenge in constructing and manipulating high-dimensional NHSEs. Here, an intuitive bottom-to-top scheme to construct high-dimensional NHSEs is proposed, through assembling multiple independent 1D NHSEs. Not only the elusive high-dimensional NHSEs can be effectively predicted from the well-defined 1D spectral winding topologies, but also the high-dimensional generalized Brillouin zones can be directly synthesized from the 1D counterparts. As examples, two 2D nonreciprocal acoustic metamaterials are experimentally implemented to demonstrate highly controllable multi-polar NHSEs and hybrid skin-topological effects, where the sound fields can be frequency-selectively localized at any desired corners and boundaries. These results offer a practicable strategy for engineering high-dimensional NHSEs, which can boost advanced applications such as selective filters and directional amplifiers.

Lipase-catalyzed transesterification of soybean oil and phytosterol in supercritical CO2.

The transesterification of phytosterol and soybean oil was performed using Novozym 435 in supercritical carbon dioxide (SC-CO2). The transesterification reaction was conducted in soybean oil containing 5-25% phytosterol at 55-95 °C and free-water solvent. The effects of temperature, reaction time, phytosterol concentration, lipase dosage and reaction pressure on the conversion rate of transesterification were investigated. The optimal reaction conditions were the reaction temperature (85 °C), reaction time (1 h), phytosterol concentration (5%), reaction pressure (8 Mpa) and lipase dosage (1%). The highest conversion rate of 92% could be achieved under the optimum conditions. Compared with the method of lipase-catalyzed transesterification of phytosterol and soybean oil at normal pressure, the transesterification in SC-CO2 reduced significantly the reaction temperature and reaction time.