Varied interfacial tension caused by the unsaturated adsorption of surfactants on dripping droplet surfaces is experimentally studied. The mass transfer and adsorption of surfactants, as well as the generation of fresh interfaces, are considered the main factors dominating the surfactant adsorption ratio on droplet surfaces. The diffusion and convective mass transfer of the surfactants are first distinguished by comparing the adsorption depth and the mass flux boundary layer thickness. A characterized mass transfer time is then calculated by introducing an effective diffusion coefficient. A time ratio is furthermore defined by dividing the droplet generation time by the characteristic mass transfer time, t/tm, in order to compare the rates of surfactant mass transfer and droplet generation. Different control mechanisms for different surfactants are analyzed based on the range of t/tm, and a criterion time ratio using a simplified characteristic mass transfer time, tm*, is finally proposed for predicting the appearance of dynamic interfacial tension.

In this work, we established a novel microstructured chemical system to intensify the synthesis process of poly(vinyl butyral) (PVB) based on the mixing enhancement principle of poly(vinyl alcohol) (PVA)–n-butanal aqueous solution with hydrochloric acid. Because of the high mixing performance, a higher temperature (30–60 °C) with much higher reaction rate was applied. Through a study of the parameters of flow rate, temperature, and feeding ratios of n-butanal and H+ to the hydroxyl groups in PVA, the apparent butyral group content in PVB reached more than 60% with a residence time of 1 min in a microreactor, and reached 75% with an additional hour of aging time, much shorter than the conventional 8–10 h in low temperature batch reactors. The size of the primary PVB particle was smaller than 10 μm, which might reduce the time and water consumption in the post washing process. This work provides a good example for operating reactions between macromolecules and small molecules in a microreactor system.

We herein propose a highly efficient method for the synthesis of polyvinyl butyral (PVB), an important resin material used for laminated glass in vehicles. The condensation reaction between polyvinyl alcohol (PVA) and n-butanal was successfully implemented in a reaction system containing a membrane dispersion microreactor and an aging vessel reactor, allowing the energy costs associated with cooling and mixing of the reactant solutions to be reduced. Upon changing the original reactant from a PVA–butanal emulsion to a PVA–HCl solution, the microreactor system allowed the development of a new recycling technology for the reuse of HCl, water, and excess n-butanal present in the product solution. In addition, with the aid of a two-step n-butanal feeding method, the developed recycling technology resulted in a 98.7% n-butanal utilization ratio, while the consumption of HCl and water during the reaction process was reduced by 85.6%. These results indicate that our novel process represents a more environmentally friendly approach to the PVB synthesis.

Surface tension is an important parameter that dominates gas–liquid dual-phase flow. In this study, the dynamic surface tension caused by changing substance concentrations at the gas–liquid interface was determined using a microflow interfacial tensiometer. The instantaneous surface tensions at the bubble rupturing moment in a CO2 chemical absorption process were determined from the bubble diameter. The mass transfer across the interface and the chemical reaction within the liquid caused up to a 7.2 mN m−1 reduction in the surface tension, as compared with that in a process without absorption, and the decrease in the surface tension can be expressed into a Langmuir–Szyszkowski equation of the CO2 interfacial concentration. This study clarifies the effect of chemical absorption on multiphase fluid dynamics parameters, which is less discussed in chemistry and chemical engineering studies.

•A simple phase separator was developed for microfluidic extraction studies.•The phase separator was easily assembled, cleaned and repaired in the laboratory.•The phase separator has superior phase separation performance.•The measured mass transfer coefficients are more accurate.•The phase separator has strong potential to be used in the kinetic study.A simple phase separator consisting of a hydrophobic T-connector and a hydrophilic needle was developed for microfluidic extraction studies. The phase separator based on the selective wettability of fluids has a very small volume and nearly zero hold-up of the continuous phase. It is easy to assemble, clean and repair in the laboratory. Four classical extraction systems with interfacial tensions ranging from 49.6 to 1.68 mN/m worked well in this phase separator over operating flow rates ranging from 150 and 1500 μL/min. Mass transfer experiments showed that the measured mass transfer coefficients from the new phase separator were more accurate than the data from the gravity phase separator. This phase separator has strong potential to be used in the kinetic study of mass transfer in tubular microfluidic device.Download high-res image (103KB)Download full-size image

Understanding the phenomenon of droplet coalescence in nanoparticle suspensions is extremely important for the preparation of Pickering emulsions. A microfluidic platform, which can provide compulsive droplet collisions, was developed to imitate the droplet coalescence process in the early stages of emulsification. Microscope videos showed the variations in the droplet coalescence percentage, droplet contact time, and liquid film drainage time in different working systems containing 158–306 nm polystyrene (PS) particles in the continuous oil phase. The intersections of the half and total droplet contact times as well as the liquid film drainage time indicated the transitions of coalescence percentage. The additional hydrodynamic resistance in the liquid film between the approaching interfaces caused by the embedded hydrophobic nanoparticles was understood to be the main reason for reduced droplet coalescence, whereas hydrophilic particles were found to promote coalescence. As a novel method, the microfluidic collision experiment provided accurate and quantitative data for analyzing the formation of Pickering emulsions.

•The equilibrium densities of [Emim][BF4] were used to determine interfacial tension.•The interfacial tension was mainly dependent on the CO2 solubility in [Emim][BF4].•A linear equation was proposed to correlate the interfacial tension and solubility.•The contact angles of [Emim][BF4] phase on solids in CO2 atmosphere were analyzed.In this study, the interfacial tension and wetting properties of [Emim][BF4] (1-ethyl-3-methylimidazolium tetrafluoroborate) in CO2 were measured at pressures from 0.1 to 15.0 MPa and temperatures between 308.15 and 343.15 K. Economical modifications of the commercial interfacial apparatus were implemented to pre-saturate the [Emim][BF4] phase by CO2 for the measurement of interfacial tension and production of small sessile droplets for contact angle measurements. The interfacial tension was found to be mainly dependent on CO2 solubility in the [Emim][BF4] phase, and a linear fitting equation was proposed. The contact angles of the [Emim][BF4] phase on quartz, 316L stainless steel, and polytetrafluoroethylene (PTFE) in CO2 atmosphere were analyzed to predict the interfacial tension differences between CO2/solid and IL/solid phases. The reduction of interfacial tension difference was caused by increased CO2 solubility in the [Emim][BF4] phase at low pressure for quartz and 316L stainless steel dense solids. The polymer PTFE material behaved very differently, being susceptible to swelling in compressed CO2.

A microstructured chemical system, mainly including a micro-sieve mixer, a delay loop and a stirring tank, was designed to implement the polycondensation of p-phenylenediamine (PPD) and terephthaloyl chloride (TPC) for the preparation of poly(p-phenylene terephthalamide) (PPTA). A two-step method was exhibited, which consisted of fast conversion of more than 90% of the reactive groups in the delay loop and further chain growth in the stirring tank. This new polycondensation method had several advantages, such as continuous operation at higher temperature and out of pyridine, which made the process more controllable and environmentally friendly. PPTA particles with a weight-average molecular weight (MW) from 4000 to 16000 were obtained at different synthesis conditions, and their structures were characterized by XRD, POM, IR, EA and SEM. The apparent reaction kinetics in the delay loop was also studied, which showed that the apparent activation energy was 12 kJ mol−1 with a pre-exponential factor of 5.4 × 103 L (mol−1 s−1).

A new microfluidic extraction method using a gas–liquid–liquid double emulsion with ultra-thin solvent film as the working system was developed, overcoming the defect of low extraction efficiency at extreme phase ratios (R > 10) by significantly reducing the mass transfer distance. Interesting bubble coalescence in the emulsion generation was reported and laws of bubble diameter (0.73–1.08 mm) and film thickness (0.35–4.06 μm) were proposed. Simulated by a Sudan IV extraction process, the gas–liquid–liquid extraction showed 16–28.3 times working efficiency compared to liquid–liquid extraction, appropriate for application requiring fast analysis and detection.

As an important green synthesis technology of caprolactam, the Beckmann rearrangement reaction of cyclohexanone oxime using high temperature and pressure water as the reaction media was carefully studied with a microreactor. An important byproduct – acetamide was confirmed with a selectivity higher than 40% in the subcritical water, which was more serious than the reported cyclohexanone. The effects of phase state and operating conditions, such as reaction temperature, pressure, flow rate ratio and reactant residence time, on the cyclohexanone oxime conversion and caprolactam selectivity were carefully analyzed, and the results showed that keeping the reaction close to the supercritical state was critical for the preparation of caprolactam.

The phase behavior of the 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and carbon dioxide (CO2) system from atmospheric to supercritical state was investigated at 0.1–15.0 MPa and 308.15–343.15 K. The solubility data of CO2 in [emim][BF4] and the volumetric expansion ratios of CO2-saturated [emim][BF4] were provided. Experimental results indicated that although CO2 dissolved significantly in [emim][BF4] (from 0 to 62 mol%), the ionic liquid phase only had an expansion ratio less than 17 vol%. The density difference between CO2-saturated [emim][BF4] and pure [emim][BF4] was obvious at high pressure and low temperature, with a maximum of 16.11% at 308.15 K and 15.0 MPa. Based on the experimental results, a fitting equation applicable to precise data interpolation was proposed for the CO2-saturated [emim][BF4] density in this study.

The dynamic interfacial tension caused by the lower surfactant adsorption rate than the droplet generation rate is an important issue in microfluidic study. A pressure drop measurement method is proposed to determine the dynamic interfacial tension during the droplet generation process at a T-junction microchannel. Experimental results show that in the transitional flow regime between squeezing and dripping, the maximum pressure drop appears at the end of filling stage defined by Glawdel et al. (Phys Rev E 85(1):016322, doi:10.1103/PhysRevE.85.016322, 2012a), as the liquid–liquid interface starts to leave the side channel. This pressure drop is composed of Laplace pressure and flow resistance. Using the pressure drop measurements, the contribution of fluctuated Laplace pressure is confirmed, and the instantaneous interfacial tension at the maximum pressure drop is calculated. Although the final droplet volume shows that almost no interfacial tension difference exists at the breakup moment, the pressure drop result unveils some discrepancies. Experimental results reflect the variation of dynamic interfacial tension is a quasi-surfactant diffusion-controlled process when the main channel is totally blocked by the growing droplet at the end of filling stage. However, this quasi-diffusion model no longer works as the droplet does not block the channel, due to the strong convection of two-phase flow. Surfactant sodium dodecyl sulfate and Tween 20 also show different micelle dissociation rates in the variation test of concentrations.

Microfluidic devices with micro-sieve plate as the dispersion medium have been widely used for the mass production of emulsions. While unfortunately, few studies have so far been made for the droplet generation rules in those devices. In this work, the droplet generation processes in micro-sieve dispersion devices are investigated with specially designed micro-sieve pore arrays. The effects of channel structure, pore arrangement, and feeding method of dispersed phase on the average size and distribution of droplets are studied carefully. It is found the dimensionless average droplet diameters (dav/de) in micro-sieve dispersion devices can be represented by a linear relation with Ca−1/4 of continuous phase, the same as the scaling law in T-junction microchannels. The flow distribution among pores and the steric hindrance between droplets affect the diameter distribution of generated droplet very much. Monodispersed droplets with polydispersity index less than 5% can be made at Ca number larger than 0.01 and phase ratio (QD/QC) less than 1/6 in the present investigation.