Double‐crosslinked reversible redox‐responsive hydrogels based on disulfide–thiol interchange

We present novel redox‐responsive hydrogels based on poly(N‐isopropylacrylamide) or poly(acrylamide), consisting of a reversible disulfide crosslinking agent N,N′‐bis(acryloyl)cystamine and a permanent crosslinking agent N,N′‐methylenebisacrylamide for microfluidic applications. The mechanism of swelling/deswelling behavior starts with the cleavage and reformation of disulfide bonds, leading to a change of crosslinking density and crosslinking points. Raman and ultraviolet‐visible spectroscopy confirm that conversion efficiency of thiol–disulfide interchange up to 99%. Rheological analysis reveals that the E modulus of hydrogel is dependent on the crosslinking density and can be repeatedly manipulated between high‐ and low‐stiffness states over at least 5 cycles without significant decrease. Kinetic studies showed that the mechanical strength of the gels changes as the redox reaction proceeds. This process is much faster than the autonomous diffusion in the hydrogel. Moreover, cooperative diffusion coefficient (D_coop) indicates that the swelling process of the hydrogel is affected by the reduction reaction. Finally, this reversibly switchable redox behavior of bulky hydrogel could be proven in microstructured hydrogel dots through short‐term photopatterning process. These hydrogel dots on glass substrates also showed the desired short response time on cyclic swelling and shrinking processes known from downsized hydrogel shapes. Such stimuli‐responsive hydrogels with redox‐sensitive crosslinkers open a new pathway in exchanging analytes for sensing and separating in microfluidics applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2590–2601     

误差合成空间固定系数模型的多阶段广义最小二乘估计

本文对时变系数的空间面板数据模型进行了研究,所研究的模型利用扰动项中的空间个体成分将不同时期的方程联系起来,同时,自变量系数和空间自回归系数是时变的,但不会随着观测个体的变动而变动。本文利用基于可行的广义最小二乘估计的多阶段方法对模型参数进行了估计,研究了估计量的大样本性质,并利用Monte Carlo方法模拟了其小样本性质。模拟结果表明,估计量的渐近性质随着样本容量的增加而改善。对中国省级地区间财税策略互动行为的实证案例也体现了本文理论模型的应用价值。     

基于迈克尔逊干涉原理测量金属热膨胀系数仪器的改进

在利用迈克尔逊干涉原理测量金属热膨胀系数的基础上,运用CCD成像系统取代原有观察屏,更利于观察并记录数据。     

Closed-form solutions of general second order linear recurrences and applications

Two closed-form solutions of a general second order linear recurrence with variable coefficients are established. The first form is combinatorial in nature and is derived through the use of a set which counts the number of elements which are two units apart. The second form is closely related to continued fractions and is derived through the use of continued fraction-like relation. Several applications including a number of verifications of conjectures produced from the Ramanujan Machine are worked out, which illustrate its versatility in this respect.     

Intermittency as a transition to turbulence in pipes: A long tradition from Reynolds to the 21st century

Intermittencies are commonly observed in fluid mechanics, and particularly, in pipe flows. Initially observed by Reynolds (1883), it took one century for reaching a rather full understanding of this phenomenon whose irregular dynamics (apparently stochastic) puzzled hydrodynamicists for decades. In this brief (non-exhaustive) review, mostly focused on the experimental characterization of this transition between laminar and turbulent regimes, we present some key contributions for evidencing the two concomittant and antagonist processes that are involved in this complex transition and were suggested by Reynolds. It is also shown that a clear explicative model was provided, based on the nonlinear dynamical systems theory, the experimental observations in fluid mechanics only providing an applied example, due to its obvious generic nature.     

Synthesis and Properties of Thermosensitive Poly(N-Isopropylacrylamide)/Waterborne Polyurethane Graded Concentration Hybrid Films

Temperature-sensitive hybrid films were synthesized with a concentration gradient by casting and UV curing of N-isopropylacrylamide (NIPAAm) monomers (0%–70%) on the free surface of waterborne polyurethane (WPU) films on a Teflon substrate. The surface hardness and contact angle of the free surface with a water drop increased asymptotically with the addition of NIPAAm, whereas those on the substrate side were virtually unchanged. The diffusion coefficient (D), rates of swelling at 20°C (below the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM)) and deswelling at 50°C (above the LCST) increased with increasing NIPAM content, showing favorable thermosensitivity. In addition, the glassy state modulus and glass transition temperature (Tg) of the film increased with increasing NIPAM content, whereas the rubbery modulus decreased due to the increased molecular weight between the crosslinks. In addition, as the NIPAM content increased, the film showed a positive yield with an increased yield and fracture stress and decreased ductility. Above 50% NIPAM, the film became brittle, showing a linear stress–strain relationship.     

Recent advances of ultrasonic testing of cement based materials at early ages

To summarize some of the most important findings in the field of ultrasonic (US) testing of early age hydration and formation of structure of different cement based materials (CBMs), a review of literature with focus on US P-wave transmission and S-wave reflection methods is presented in this paper. The review shows a great ability of both US techniques to observe setting phenomena and to determine different milestones during the early age formation of CBM’s microstructure. Clear physical basis, high accuracy, and non-destructive nature of the method indicate that US methods could become standardized in the near future.     

Nonlinear obstacle problems with double phase in the borderline case

In this paper we study a double phase problem with an irregular obstacle. The energy functional under consideration is characterized by the fact that both ellipticity and growth switch between a type of polynomial and a type of logarithm, which can be regarded as a borderline case of the double phase functional with $(p,q)$-growth. We obtain an optimal global Calderón–Zygmund type estimate for the obstacle problem with double phase in the borderline case.     

An experimental investigation on heat transfer enhancement in circular jet impingement on hot surfaces by using Al2O3/water nano-fluids and aqueous high-alcohol surfactant solution

The present article reports the heat transfer characteristics of a vertical stainless steel foil of 0.15 mm thickness (SS304) by circular impinging jets of various fluids such as pure water, nano-fluids (Al₂O₃-water, ф = 0.15%, 0.6%), and aqueous high-alcohol surfactant (HAS, i.e., 2-ethyl-hexanol, 100–400 ppm) studied using an infrared thermal imaging camera (A655sc, FLIR System). The enhancement in the heat transfer rates for Al₂O₃-water nano-fluids with ф = 0.15%, ф = 0.60%, and aqueous surfactant solution (150ppm) is found to be 140%, 207%, and 117% higher compared to pure water results, respectively. The surface characteristics of the foil after jet impingement by various fluids are also studied using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and surface wettability.