Dynamics of bubble formation in highly viscous liquids

There has recently been considerable interest in the development of devices for the preparation of monodisperse microbubble suspensions for use as ultrasound contrast agents and drug delivery vehicles. These applications require not only a high degree of bubble uniformity but also a maximum bubble size of 8 mum, and this provides a strong motivation for developing an improved understanding of the process of bubble formation in a given device. The aim of this work was to investigate bubble formation in a T-junction device and determine the influence of the different processing parameters upon bubble size, in particular, liquid viscosity. Images of air bubble formation in a specially designed T-junction were recorded using a high-speed camera for different ratios of liquid to gas flow rate (Ql/Qg) and different liquid viscosities (microl). It was found that theoretical predictions of the flow profile in the focal region based on analysis of axisymmetric Stokes flow were accurate to within 6% when compared with the experimental data, indicating that this provided a suitable means of describing the bubble formation process. Both the theoretical and experimental results showed that Ql/Qg and mul had a significant influence upon bubble formation and eventual size, with higher flow rates and higher viscosities producing smaller bubbles. There were, however, found to be limiting values of Ql/Qg and mul beyond which no further reduction in bubble size was achieved.     

Hydrophilic strips for preventing air bubble formation in a microfluidic chamber

In a microfluidic chamber, unwanted formation of air bubbles is a critical problem. Here, we present a hydrophilic strip array that prevents air bubble formation in a microfluidic chamber. The array is located on the top surface of the chamber, which has a large variation in width, and consists of a repeated arrangement of super‐ and moderately hydrophilic strips. This repeated arrangement allows a flat meniscus (i.e. liquid front) to form when various solutions consisting of a single stream or two parallel streams with different hydrophilicities move through the chamber. The flat meniscus produced by the array completely prevents the formation of bubbles. Without the array in the chamber, the meniscus shape is highly convex, and bubbles frequently form in the chamber. This hydrophilic strip array will facilitate the use of a microfluidic chamber with a large variation in width for various microfluidic applications.     

Forming process and stability of bubble domains in dielectrically positive cholesteric liquid crystals

The nucleation of bubble domains in homeotropic samples of a dielectrically positive cholesteric liquid crystal is described. These domains are found to be more stable in an electric field than the rectilinear double-twisted fingers. The electric field-induced transformation of a looped finger into a bubble domain is described in detail: it is discontinuous and irreversible, and operates only at a large enough confinement ratio C = d/p, where d is the sample thickness and p the quiescent cholesteric pitch. Finally, in contrast with Stieb's model [4], we propose that there are two point defects along the bubble axis and not a disclination line.     

Kinetics of hydrate formation using gas bubble suspended in water

An innovative experimental technique, which was devised to study the effects of temperature and pressure on the rate of hydrate formation at the surface of a gas bubble suspended in a stagnant water phase, was adapted in this work. Under such conditions, the hydrate-growth process is free from dynamic mass transfer factors. The rate of hydrate formation of methane and carbon dioxide has been systematically studied. The measured hydrate-growth data were correlated by using the molar Gibbs free energy as driving force. In the course of the experiments, some interesting surface phenomena were observed.     

Experimental investigation of bubble and drop formation at submerged orifices

The aim of this study was to investigate bubble/drop formation at a single submerged orifice in stagnant Newtonian fluids and to gain qualitative understanding of the formation mechanism. The effects of various governing parameters were studied. Formation behavior of bubbles and drops in Newtonian aqueous solutions were investigated experimentally under different operating conditions with various orifices. The results show that the volume of the detached dispersed phase (bubble or drop) increases with the viscosity of the continuous phase (or dispersion medium), surface tension, orifice diameter, and dispersed phase flow rate. A PIV system was employed to measure the velocity flow field quantitatively during the bubble/drop formation, giving interesting information useful for the elucidation of the fundamental formation process at the orifice. It was revealed that the orifice shape strongly influences the size of the bubble formed. Furthermore, based on a simple mass balance, a general correlation successfully predicting both bubble and drop sizes has been proposed.     

Observation of bubble domains at the cholesteric to homeotropic-nematic transition in a confined chiral nematic liquid crystal

We report observations on a chiral nematic liquid crystal confined in a plane cell treated for homeotropic alignment. The characteristics of the specimen investigated are such that at room temperature a uniform homeotropic alignment is observed and at higher temperatures the material displays the helical structure. It is observed that the changeover between homeotropic and helical textures does not occur at one single threshold. There are two thresholds, between which there is an intermediate texture where small circular domains are observed. These domains may be regarded as circular analogues of the plane solitons predicted by previous authors.     

吡喃木糖和O-乙酰基吡喃木糖热解形成羟基乙醛的机理研究

以半纤维素主要成分木聚糖的两种单体--吡喃木糖和O-乙酰基吡喃木糖为模型化合物,运用密度泛函理论(DFT),采用B3LYP方法和6-31+G(d,p)基组进行计算,研究了吡喃木糖热解形成HAA的6条可能的反应路径和O-乙酰基吡喃木糖热解形成HAA的3条可能的反应路径。由此确定了吡喃木糖热解形成HAA的最优路径为:吡喃木糖首先开环得到链式木糖,然后C3羟基和C2氢脱水,随后经重排和逆醇醛缩合反应生成包含C4/C5的HAA;该路径的决速步骤为脱水反应,能垒为253.3 kJ/mol。O-乙酰基吡喃木糖热解形成HAA的最优路径为:O-乙酰基吡喃木糖首先支链断裂脱出乙酸(AA),开环后的链式中间体经氢转移反应得到包含C4/C5的HAA;该路径的决速步骤为最后的氢转移反应,能垒为317.6 kJ/mol。     

Mechanism of levulinic acid formation

Based on the intermediates revealed by ¹³C-NMR spectroscopy, the mechanism of levulinic acid ($\text{6}$) formation in acid catalysed hydrolysis of 2-hydroxymethylfurane ($\text{1}$) and 5-hydroxymethylfuran-2-carbaldehyde ($\text{14}$) is proposed.     

Mechanism of surfactant micelle formation

The mechanism of micelle formation of surfactants sodium dodecyl sulfate (SDS), n-hexyldecyltrimethylammonium bromide (CTAB) and Triton X-100 (TX-100) in heavy water solutions was studied by 1H NMR (chemical shift and line shape) and NMR self-diffusion experiments. 1H NMR and self-diffusion experiments of these three surfactants show that their chemical shifts (delta) begin to change and resonance peaks begins to broaden with the increase in concentration significantly below their critical micelle concentrations (cmc's). At the same time, self-diffusion coefficients ( D) of the surfactant molecules decrease simultaneously as their concentrations increase. These indicate that when the concentrations are near and lower than their cmc's, there are oligomers (premicelles) formed in these three surfactant systems. Carefully examining the dependence of chemical shift and self-diffusion coefficient on concentration in the region just slightly above their cmc's, one finds that the pseudophase transition model is not applicable to the variation of physical properties (chemical shift and self-diffusion coefficient) with concentration of these systems. This indicates that premicelles still exist in this concentration region along with the formation of micelles. The curved dependence of chemical shift and self-diffusion coefficient on the increase in concentration suggests that the premicelles grow as the concentration increases until a definite value when the size of the premicelle reaches that of the micelle, i.e., the system is likely dominated by the monomers and micelles. Additionally, the approximate values of premicelle coming forth concentration (pmc) and cmc were obtained by again fitting chemical shifts to reciprocals of concentrations at a different perspective, and are in good accordant with experimental results and literature values and prove the former conclusion.     

Mechanism of cyclic acetal formation

The mechanism of formation of 1,3-dioxolanes is considered in the light of Baldwin's rule for ring-closure.     

Mechanism of the formation of porous silicate mesophases

The formation of mesoporous mesophase systems prepared by precipitation of soluble forms of SiO₂ at the surface of micelles of cetyltrimethylammonium cations was examined. A molecular mechanism of the formation of a silicate coating and a macroscopic mechanism of the formation of a mesophase were found and discussed. A combination of these mechanisms describes the processes proceeding during the synthesis. It can also explain the observed changes in the structure and texture characteristics of the mesophase.     

Mechanism of nanocapsules formation by the emulsion-diffusion process

A detailed investigation into the mechanisms of nanocapsule formation by means of the two stages "emulsion-diffusion" process is reported. Such widely used process is still poorly understood. An emulsion of oil, polymer and ethyl acetate is fabricated as a first step; dilution with pure water allows ethyl acetate to diffuse out from the droplets, leaving a suspension of nanocapsules at the end. It has been shown that the size of nanocapsules was related to the chemical composition of the organic phase and the size of primary emulsion through a simple geometrical relationship. As a consequence, most of the properties of the nanocapsules were decided at the emulsification step. The influence of several formulation and processing parameters of the primary emulsion was studied accordingly. The thin polymer membrane of nanocapsules was observed by means of cryo-fracture electron microscopy. Finally two experiments were designed for a mechanistic investigation of the diffusion step. A step-by-step diffusion of the organic solvent takes place by successive partition equilibria of ethyl acetate between the droplets and aqueous phase. A time-resolved experiment shows the fast diffusion (less than 20 ms) related to the small droplet size of the emulsion.     

A theoretical model for bubble formation during horizontal gas injection into liquid flow in vertical tubes

Bubble formation in flowing liquid is an important process for wastewater treatment, processing of molten metals, and biological processes. Based on a global balance of force on the bubble, this report describes a new theoretical model for bubble formation during horizontal gas injection into turbulent liquid flow in a vertical tube. This work highlights the importance of choosing the correct drag law in accordance with the bubble size. Five models for drag coefficient are compared, and of these, model III is recommended. Modified detachment criteria are applicable, depending on the liquid velocity. The new analytical model yields good predictions compared with experimental data. Based on the theoretical model, this study investigates the effects of the direction of liquid flow, liquid velocity, gas velocity, and orifice diameter on the bubble formation behavior.     

Mechanism of formation of γ-lactones

A new mechanism is proposed for the formation of -lactones via a six-membered intermediate state of the peroxy radical occurring with subsequent cyclization and formation of a tetrahydrofuran derivative, the oxidation of which gives the -lactone. A thermodynamic substantiation is given for the suitability of the proposed mechanism. An analysis of the literature data confirms the occurrence of the cyclization mechanism of the peroxy radical.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 370–374, May–June, 1989.     

Oxygen bubble mould effect: serrated nanopore formation and porous alumina growth

We discovered a unique formation of serrated nanopore in porous anodic alumina (PAA). A new growth model is proposed for the formation mechanism of PAA. The model emphasizes the close relationship between pore generation and oxygen evolution. The initial pore formation is ascribed to oxygen bubble mould effect. Our model provides a satisfactory explanation for the growth process of PAA, alleviating the difficulties encountered in existing theories. These findings represent a decisive new step towards the full understanding of the nature of PAA films. The serrated nanopore arrays in PAA could also be used in a wide range of future nanostructure fabrications. Correspondence: Ye Song, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.