Assessment of Joule heating and its effects on electroosmotic flow and electrophoretic transport of solutes in microfluidic channels

Joule heating is inevitable when an electric field is applied across a conducting medium. It would impose limitations on the performance of electrokinetic microfluidic devices. This article presents a 3-D mathematical model for Joule heating and its effects on the EOF and electrophoretic transport of solutes in microfluidic channels. The governing equations were numerically solved using the finite-volume method. Experiments were carried out to investigate the Joule heating associated phenomena and to verify the numerical models. A rhodamine B-based thermometry technique was employed to measure the solution temperature distributions in microfluidic channels. The microparticle image velocimetry technique was used to measure the velocity profiles of EOF under the influence of Joule heating. The numerical solutions were compared with experimental results, and reasonable agreement was found. It is found that with the presence of Joule heating, the EOF velocity deviates from its normal "plug-like" profile. The numerical simulations show that Joule heating not only accelerates the sample transport but also distorts the shape of the sample band.     

基于FeWOx/SiO2载氧体的甲烷化学链部分氧化反应

甲烷具有价格低廉且储量丰富的优点,因此将甲烷转化为合成气(一种H2:CO为2的混合物),从而进一步合成有价值的化学品和液体燃料引起了人们的极大关注.化学链甲烷部分氧化(CLPOM)技术能避免甲烷与空气直接接触而引起爆炸的危险,可以降低后续对合成气与氮气分离操作所带来的费用,因此日益受到关注.CLOPM过程主要分为两步:第一步,CH4被载氧体所携带的氧部分氧化,载氧体被还原;第二步,利用氧化剂(例如空气)将被还原的载氧体再氧化恢复.因此,载氧体在CLOPM过程中起到至关重要的作用.载氧体的选择主要存在两个问题:(1)甲烷被活化所产生含碳产物的能力与晶格氧的给氧能力不匹配所带来的严重碳沉积;(2)金属离子间扩散速率不匹配而造成载氧体在氧化还原过程中结构的不可逆变化.基于上述两个问题,本文设计了FeWOx/SiO2载氧体用于CLPOM.与未改性的WO3/SiO2载氧体相比,甲烷的转化率和合成气的收率都有显著提高.FeWOx/SiO2在900℃、1 atm反应条件下表现出62%的甲烷转化率、93%的CO气相选择性、94%的H2选择性和2.4的H2/CO比值,同时在50个循环中表现出优异的催化活性和稳定性.本工作利用CH4脉冲反应研究了FeWOx/SiO2的甲烷表面反应过程;采用CH4-TPR和H2-TPR相结合探究了甲烷活化速率与晶格氧扩散速率之间的关系;通过XPS和XRD对FeWOx/SiO2在氧化还原过程中的结构稳定性进行了探讨.综合上述实验结果,对FeWOx/SiO2应用于CLPOM的反应机理进行了阐述.H2-TPR结果表明,在FeWOx/SiO2中,相较于Fe2O3/SiO2,Fe-O的活性受到抑制,使其更倾向于与甲烷发生部分氧化反应;相较于WO3/SiO2,W-O的活性得到明显提升,因此更多的晶格氧可以参与到部分氧化反应中来氧化积碳,从而使合成气收率大幅度提升.从CH4-TPR结果可以看出,对于FeWOx/SiO2,CO与H2的生成温度最接近,意味着晶格氧的传输速率较快并且能够与甲烷活化产生含碳中间物种的速率相匹配,将其及时氧化生成CO,避免由于积碳造成的催化剂失活.结合XPS和XRD结果可以得出,在甲烷还原过程中,FeWOx经历一步还原形成Fe-W合金,由于其间存在强相互作用,因而抑制了还原过程中催化剂相分离现象的发生.同时,根据铁钨离子在空气条件下扩散速率的公式计算可以得出,其相近的离子氧化速率也保证了在氧化过程中催化剂结构的稳定性.本工作为进一步构建用于甲烷化学链部分氧化制合成气的复合金属氧化物载氧体提供了研究思路.     

Experimental study on the smoke temperature evolution in a polyethylene (PE)-lined compartment on fire

Smoke temperature evolution in the upper layer of compartment fire, which is critical for the prediction of potential flashover, was experimentally investigated in a real building. Three-millimeter polyethylene (PE) slabs attached on the internal walls were employed as the lining material to address the effect of the melting and combustion of the lining material on the smoke temperature. A corner gasoline pool fire was utilized as the fire source. Two thermocouple trees, mounted vertically at the center and the open door, and a high-definition camera were utilized to record the smoke temperature history and experimental video. Meanwhile, some furniture was loaded to study its enhancement feature on fire intensity. Heat release rates (HRRs) at different stages were analyzed based on MQH method (McCaffrey, Quintiere and Harkleroad) and pool fire theory. Smoke temperature was estimated through an improved MQH correlation considering the melting of the PE slabs and an empirical model, BFD curve (Barnett in Fire Saf J 37: 437–463, 2002) combined. The results show that both the maximum HRR and smoke temperature, 925.91 kW and 491.7 °C, are lower than the critical values of flashover. The PE lining greatly intensifies the fire power and the resulting smoke temperature compared with the ones in noncombustible wall scenario. Combustion of the molten PE flowing down from the walls would lead to a secondary peak in smoke temperature curve, which is rarely considered in previous work.

     

An efficient method for recovery of target ssDNA based on amino-modified silica-coated magnetic nanoparticles

In this paper, an improved recovery method for target ssDNA using amino-modified silica-coated magnetic nanoparticles (ASMNPs) is reported. This method takes advantages of the amino-modified silica-coated magnetic nanoparticles prepared using water-in-oil microemulsion technique, which employs amino-modified silica as the shell and iron oxide as the core of the magnetic nanoparticles. The nanoparticles have a silica surface with amino groups and can be conjugated with any desired bio-molecules through many existing amino group chemistry. In this research, a linear DNA probe was immobilized onto nanoparticles through streptavidin conjugation using covalent bonds. A target ssDNA(I) (5′-TMR-CGCATAGGGCCTCGTGATAC-3′) has been successfully recovered from a crude sample under a magnet field through their special recognition and hybridization. A designed ssDNA fragment of severe acute respiratory syndrome (SARS) virus at a much lower concentration than the target ssDNA(I) was also recovered with high efficiency and good selectivity.     

Synthesis of monodisperse poly(chloromethylstyrene-co-divinylbenzene) beads and their application in separation of biopolymers

The monodisperse, porous poly(chloromethylstyrene-co-divinylbenzene) beads of 7.9 microm were prepared by a single-step swelling and polymerization method. The seed particles prepared by dispersion polymerization exhibited good absorption of the monomer phase. Based on this media, a weak cation-exchange (WCX) stationary phase for HPLC was synthesized by a new chemically modified method. The prepared resin has advantages of biopolymer separation, high column efficiency, low column backpressure, high protein mass recovery, and good resolution for proteins. The dynamic protein-loading capacity of the synthesized WCX packings was 18.2 mg/g. Five proteins were separated in 3.0 min using the synthesized WCX stationary phase. The experimental results show that the obtained WCX resin has very weak hydrophobicity. The WCX resin was also used for the rapid separation and purification of lysozyme from egg white in 5.0 min with only one step. The purity and specific bioactivity of the purified lysozyme were found to be more than 93% and 70 245 U/mg, respectively.     

Reactivity of anatase TiO(2) nanoparticles: the role of the minority (001) surface

Methanol adsorption on clean and hydrated anatase TiO(2)(001)-1 x 1 is studied using density functional theory calculations and first principles molecular dynamics simulations. It is found that (i) dissociative adsorption is favored on clean TiO(2)(001) at both low and high methanol coverages; (ii) on the partially hydrated surface, methanol dissociation is not affected by the coadsorbed water and can still occur very easily; (iii) the dissociative adsorption energy of methanol is always larger than that of water under similar conditions. This implies that water replacement by methanol is energetically favored, in agreement with recent experimental observations on colloidal anatase nanoparticles.     

Synthesis and characterizations of anion exchange organic-inorganic hybrid materials based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)

A series of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)-based organic-inorganic hybrid materials for anion exchange were prepared through sol-gel process of polymer precursors PPO-Si(OCH₃)₃. PPO-Si(OCH₃)₃ were obtained from the reaction of bromomethylated PPO with 3-aminopropyl-trimethoxysilane (A1110). These polymer precursors then underwent hydrolysis and condensation with additional A1110 to generate hybrid materials. The reaction to produce polymer precursors was identified by FTIR; while FTIR, TGA, XRD, SEM, as well as conventional ion exchange capacity (IEC) measurements were conducted for the structures and properties of the prepared hybrids. TGA results show that this series of hybrid materials possess high thermal stability; XRD and SEM indicate that the prepared hybrid materials are amorphous and the inorganic and organic contents show good compatibility if the ratio between them is proper. The IEC values of the hybrid materials due to the amine groups range from 1.13 mmol/gBPPO (material i) to 4.80 mmol/gBPPO (material iv).     

The Unusual Spectrum of the Mixture of Tin-octabutoxy Naphthalocyanine and SnCl2 in CH2Cl2

The Q-band position of tin-centered 5, 9, 14, 18, 23, 27, 32, 36-octabutoxy 2, 3-phthalocyanine(SnNc(OBu)8) exhibits dramatic red-shift as mixed with SnCl2 in CH2Cl2.     

DEHYDROGENATIVE COUPLING OF METHANE UNDER PULSE CORONA PLASMA OVER A MnO|?/|?-A12O3 CATALYST

Pulse corona plasma was used as an activation method fordehydrogenative coupling of methane at the normal temperature and under' the normal pressure. The exprimental results showed that a great enhancement in the selectivity to C2 was achieved by the combination of pulse corona plasma with a y-Mn2OO3/Y-A12O3 catalyst. The distribution of C2 hydrocarbons was mainly affected by the power input and the nature of the catalyst. In addition, the gas stream direction exerted an influence upon the C2 yield, especially at higher power input.