Dynamic interfacial tension of aqueous solutions of PVAAs and its role in liquid-liquid dispersion stabilization

In liquid-liquid dispersion systems, the dynamic change of the interfacial properties between the two immiscible liquids plays an important role in both the emulsification process and emulsion stabilization. In this paper, experimentally measured dynamic interfacial tensions of 1-chlorobutane in the aqueous solutions of various random copolymers of polyvinyl acetate and polyvinyl alcohol (PVAA) are presented. Theoretical analyses on these results suggest that the adsorption of the polymer molecules is controlled neither by the bulk diffusion process nor the activation energy barrier for the adsorption but the conformation of polymer molecules. Based on the concept of critical concentration of condensation for polymer adsorption, as well as the observation that the rate at which the dynamic interfacial tension changes does not correlate to the PVAA's ability to stabilize a single drop, it is postulated that the main stabilization mechanism for the PVAAs is by steric hindrance, not the Gibbs-Marangoni effect offered by the small molecule surfactants.

Phase equilibria and surface tension of pure fluids using a molecular layer structure theory (MLST) model

This paper presents a new model based on thermodynamic and molecular interaction between molecules to describe the vapour-liquid phase equilibria and surface tension of pure component. The model assumes that the bulk fluid can be characterised as set of parallel layers. Because of this molecular structure, we coin the model as the molecular layer structure theory (MLST). Each layer has two energetic components. One is the interaction energy of one molecule of that layer with all surrounding layers. The other component is the intra-layer Helmholtz free energy, which accounts for the internal energy and the entropy of that layer. The equilibrium between two separating phases is derived from the minimum of the grand potential, and the surface tension is calculated as the excess of the Helmholtz energy of the system. We test this model with a number of components, argon, krypton, ethane, n-butane, iso-butane, ethylene and sulphur hexafluoride, and the results are very satisfactory. (C) 2002 Elsevier Science B.V. All rights reserved.

Bioprocess-centered molecular design (BMD) for the efficient production of an interfacially active peptide

The efficient expression and purification of an interfacially active peptide (mLac21) was achieved by using bioprocess-centered molecular design (BMD), wherein key bioprocess considerations are addressed during the initial molecular biology work. The 21 amino acid mLac21 peptide sequence is derived from the lac repressor protein and is shown to have high affinity for the oil-water interface, causing a substantial reduction in interfacial tension following adsorption. The DNA coding for the peptide sequence was cloned into a modified pET-31(b) vector to permit the expression of mLac21 as a fusion to ketosteroid isomerase (KSI). Rational iterative molecular design, taking into account the need for a scaleable bioprocess flowsheet, led to a simple and efficient bioprocess yielding mLac21 at 86% purity following ion exchange chromatography (and >98% following chromatographic polishing). This case study demonstrates that it is possible to produce acceptably pure peptide for potential commodity applications using common scaleable bioprocess unit operations. Moreover, it is shown that BMD is a powerful strategy that can be deployed to reduce bioseparation complexity. (C) 2004 Wiley Periodicals, Inc.

A review of drainage and spontaneous rupture in free standing thin films with tangentially immobile interfaces

A review of spontaneous rupture in thin films with tangentially immobile interfaces is presented that emphasizes the theoretical developments of film drainage and corrugation growth through the linearization of lubrication theory in a cylindrical geometry. Spontaneous rupture occurs when corrugations from adjacent interfaces become unstable and grow to a critical thickness. A corrugated interface is composed of a number of waveforms and each waveform becomes unstable at a unique transition thickness. The onset of instability occurs at the maximum transition thickness, and it is shown that only upper and lower bounds of this thickness can be predicted from linear stability analysis. The upper bound is equivalent to the Freakel criterion and is obtained from the zeroth order approximation of the H-3 term in the evolution equation. This criterion is determined solely by the film radius, interfacial tension and Hamaker constant. The lower bound is obtained from the first order approximation of the H-3 term in the evolution equation and is dependent on the film thinning velocity A semi-empirical equation, referred to as the MTR equation, is obtained by combining the drainage theory of Manev et al. [J. Dispersion Sci. Technol., 18 (1997) 769] and the experimental measurements of Radoev et al. [J. Colloid Interface Sci. 95 (1983) 254] and is shown to provide accurate predictions of film thinning velocity near the critical thickness of rupture. The MTR equation permits the prediction of the lower bound of the maximum transition thickness based entirely on film radius, Plateau border radius, interfacial tension, temperature and Hamaker constant. The MTR equation extrapolates to Reynolds equation under conditions when the Plateau border pressure is small, which provides a lower bound for the maximum transition thickness that is equivalent to the criterion of Gumerman and Homsy [Chem. Eng. Commun. 2 (1975) 27]. The relative accuracy of either bound is thought to be dependent on the amplitude of the hydrodynamic corrugations, and a semiempirical correlation is also obtained that permits the amplitude to be calculated as a function of the upper and lower bound of the maximum transition thickness. The relationship between the evolving theoretical developments is demonstrated by three film thickness master curves, which reduce to simple analytical expressions under limiting conditions when the drainage pressure drop is controlled by either the Plateau border capillary pressure or the van der Waals disjoining pressure. The master curves simplify solution of the various theoretical predictions enormously over the entire range of the linear approximation. Finally, it is shown that when the Frenkel criterion is used to assess film stability, recent studies reach conclusions that are contrary to the relevance of spontaneous rupture as a cell-opening mechanism in foams. (C) 2003 Elsevier Science B.V. All rights reserved.

A parametric study of droplet deformation through a microfluidic contraction: Low viscosity Newtonian droplets

Numerical simulations are conducted to investigate how a droplet of Newtonian liquid. entrained in a higher viscosity Newtonian liquid, behaves when passing through an axisymmetric microfluidic contraction. Simulations are performed using a transient Volume of Fluid finite volume algorithm, and cover ranges of Reynolds and Weber numbers relevant to microfluidic flows. Results are presented for a droplet to surrounding fluid viscosity ratio of 0.001. In contrast to behaviour at higher viscosity ratios obtained previously by the authors, shear and interfacial tension driven instabilities often develop along the droplet Surface. leading to complex shape development, and in some instances, droplet breakup. (c) 2006 Elsevier Ltd. All rights reserved.

Controlling leucine zipper specificity with interfacial hydrophobic residues

Dimerisation of leucine zippers results from the parallel association of alpha-helices to form a coiled coil. Coiled coils comprise a heptad repeat, denoted as (abcdefg)(n), where residues at positions a and d are hydrophobic and constitute the core of the dimer interface. Charged amino acids at the e and g positions of the coiled coil are thought to be the major influence on dimerisation specificity through the formation of attractive and repulsive interhelical electrostatic interactions. However, the variability of a-position residues in leucine zipper transcription factors prompted us to investigate their influence on dimerisation specificity. We demonstrate that mutation of a single interfacial a-position Ala residue to either Val, Ile or Leu significantly alters the homo- and heterodimerisation specificities of the leucine zipper domain from the c-Jun transcription factor. These results illustrate the importance of a-position residues in controlling leucine zipper dimerisation specificity in addition to providing substantial contributions to dimer stability.

Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30 degrees of ankle displacement (75 degrees-105 degrees internal ankle angle) at lengthening and shortening velocities of 5, 15 and 30 degrees s(-1). To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60 degrees during all trials. Voluntary plantar flexor efforts were performed under two test conditions: (1) maximal voluntary activation (MVA) of the SOL, and (2) constant submaximal voluntary activation (SVA) of the SOL. SVA trials were performed with direct visual feedback of the SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90 degrees ankle angle) torque and EMG of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were recorded. In seven subjects from the initial group, the test protocol was repeated under submaximal percutaneous electrical activation (SEA) of SOL (to 30% isometric maximal effort). Lengthening torques were significantly greater than shortening torques in all test conditions. Lengthening torques in MVA and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at higher lengthening velocities. Shortening torques were lower than the isometric level for all conditions. However, whereas SVA and SEA torques decreased at higher velocities of shortening, MVA torques were independent of velocity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle.

Interfacial interactions and structure of organic-inorganic nanohybrids

Understanding the interfacial interactions and structure is important to better design and application of organic-inorganic nanohybrids. This paper presents our recent molecular dynamic studies on organoclays and polymer nanocomposites, including the layering behavior of organoclays, structural and dynamic properties of dioctadecyldimethyl ammoniums in organoclays, and interfacial interactions and structure of polyurethane nanocomposites. The results demonstrate that the layering behaviors of organoclays are closely related to the chain length of quaternary alkyl ammoniums and cation exchangeable capacity of clays. In addition to typical layered structures such as monolayer, bilayer and pseudo-trilayer, a pseudo-quadrilayer structure was also observed in organoclays modified with dioctadecyldimethyl ammoniums (DODDMA). In such a structure, alkyl chains do not lie flat within a single layer but interlace, and also jump to the next layer or even the next nearest layer. Moreover, the diffusion constants of nitrogen and methylene atoms increase with the temperature and methelene towards the tail groups. For polyurethane nanocomposite, the van der Waals interaction between apolar alkyl chains and soft segments of polyurethane predominates the interactions between organoclay and polyurethane. Different from most bulk polyurethane systems, there is no distinct phase-separated structure for the polyurethane.

Hall-Petch parameters for tension and compression in cast Mg

The flow stress in tensile and compressive deformation has been determined in cast Mg, for a wide range of grain sizes between 36 mum and 1.5 mm. The grain size was varied by alloying Mg with small amounts of Zr. It was found that the 0.2% offset flow stress, sigma(y) (MPa), varies with the grain size, d (m), as sigma(y) = 17.7 + 0.25 d(-1/2) in tension, while sigma(y) = -2.3 + 0.39 d(-1/2) in compression. Possible reasons for the difference between tension and compression are discussed.

Hall-Petch parameters in tension and compression in cast Mg-2Zn alloys

The flow stress in tension and compression has been measured as a function of plastic strain in Mg-2Zn castings with grain sizes ranging from 55 to 340 mum. Hall-Petch parameters have been calculated and are compared to those previously reported. In contrast to the behaviour of pure Mg grain refined with Zr and of hot-worked and recrystallised pure Mg and Mg-Zn alloys, the cast material shows little tension/compression asymmetry of the flow stress. The possible effects of texture and of twinning are noted. (C) 2004 Elsevier B.V. All rights reserved.

Morphological features of interfacial intermetallics and interfacial reaction rate in Al-11Si-2.5Cu-(0.15/0.60)Fe cast alloy/die steel couples

Soldering reactions are commonly observed during high pressure die casting of aluminium alloys, and involve the formation and growth of interfacial intermetallics between the die and the cast alloy. It is generally believed that close to 1% Fe is necessary in the aluminium alloy to reduce soldering. However, the role of iron in the interfacial reaction has not been studied in detail. In this investigation, reaction couples were formed between H13 tool steel substrates and an Al-11Si-2.5Cu melt containing either 0.15 or 0.60% Fe. Examination revealed distinctly different intermetallic layer morphology. The overall growth and chemistry of the reaction layer and the reaction rate measured by the consumption of the substrate were compared for the two alloy melts. It was demonstrated that a higher iron content reduces the rate of interfacial reaction, consistent with an observed thicker compact ( solid) intermetallic layer. Hence, the difference in reaction rate can be explained by a significant reduction in the diffusion flux due to a thicker compact layer. Finally, the mechanism of the growth of a thicker compact layer in the higher iron melt is proposed, based on the phase relations and diffusion both within and near the interfacial reaction zone. (C) 2004 Kluwer Academic Publishers.

A randomized, controlled trial to determine the efficacy of paper tape in preventing hypertrophic scar formation in surgical incisions that traverse Langer's skin tension lines

Background: How a scar is managed postoperatively influences Its cosmetic outcome. After Suture removal, scars are susceptible to skin tension, which may be the trigger for hypertrophic scarring. Paper tape to support the scar may reduce multidirectional forces and prevent hypertrophic scarring. Methods: Seventy patients who had under gone cesarean section at the Royal Brisbane and Women's Hospital were randomized to treatment and control groups. Patients in the control group received no postoperative intervention. Patients in the treatment group applied paper tape to their scars for 12 weeks. Scars were assessed at 6 weeks, 12 weeks, and 6 months after surgery using Ultrasound to measure intradermal scar volume. Scars were also assessed using the International Clinical Recommendations. Results: Paper tape significantly decreased scar volume by a mean of 0.16 cm(3), (95 percent confidence Interval, 0.00 to 0.29 cm(3)) At 12 weeks after surgery, 41 percent of the control group developed hypertrophic scars compared with none in the treatment group (exact test, p = 0.003). In the treatment group, one patient developed a hypertrophic scar and four developed stretched scars only after the tape was removed. The odds of developing a hypertrophic scar were 13.6 times greater in the control than in the treatment group (95 percent confidence interval, 3.6 to 66.9). Of the 70 patients randomized, 39 completed the study. Four patients in the treatment group developed a localized red rash beneath the tape. These reactions were minor and transient and resolved without medical intervention. Conclusions: The development of hypertrophic and stretched scars in the treatment group only after the tape was removed suggests that tension acting on a scar is die trigger for hypertrophic scarring. Paper tape is likely to be an effective modality for the prevention of hypertrophic scarring through its ability to eliminate scar tension.