A review of bubble break-up.

The coalescence and break-up of bubbles are important steps in many industrial processes. To date, most of the literature has been focussed on the coalescence process which has been studied using high speed cinematographic techniques. However, bubble break-up is equally important and requires further research. This review essentially details the break-up process and initially summarizes the different types of bubble deformation processes which lead to break-up. Break-up is considered in high and low turbulent (pseudo-static) conditions and the effect of fluctuations and shear forces on the break-up is reviewed. Different mechanisms of break-up are discussed including shearing-off, coalescence induced pitching and impact pinching following air entrapment. Also, the influence of bubble size, interfacial stability, and surfactant on break-up are reviewed and a summary of recent experimental techniques presented. Finally, the break-up process which occurs in micro-fluidics is summarized.

Transarterial chemoembolization in a patient with severe reactions to iodinated contrast: Successful treatment using gadolinium contrast with C-arm computed tomography.

Severe reactions to modern iodinated contrasts are uncommon. Breakthrough reactions in the setting of pretreatment with corticosteroids are even rarer. Patients with a history of these refractory reactions can create challenging situations in the diagnostic and therapeutic process. Here, we present a case of an 83-year-old male with hepatocellular carcinoma and a history of multiple severe reactions to iodinated contrast. The patient required a transarterial chemoembolization but the conventional technique was unavailable due to the allergy. Gadolinium-based contrast was substituted and used in conjunction with C-arm CT and a percutaneous ethanol injection to treat the tumor. After nearly 3 years, there is no evidence of residual or recurrent hepatocellular carcinoma.

Iliac bone biopsy complicated by hematoma: Hemostasis achieved with glue injection.

Serious hemorrhage after a bone biopsy is a rare complication of the procedure. Due to the infrequency of this complication, there is limited literature available regarding the treatment. Here, we present a case of a 70-year-old male who developed a symptomatic gluteal hematoma after an iliac bone lesion biopsy. Hemostasis was achieved by injecting an N-butyl cyanoacrylate glue solution into the bone cannulation site under CT-guidance.

Experimental Technique to Study the Interaction Between a Bubble and the Particle-Laden Interface.

An experimental apparatus was developed based on the Langmuir-Blodgett trough design to investigate the compression of monolayers of micron size spherical glass particles at the air-water interface and the interaction of an air bubble with the monolayers. The setup modifies the regular Langmuir-Blodgett trough by using a deep and clear glass cell. The cell allowed both the optical observation of the particle monolayer and the insertion of a capillary to produce a bubble under the layer of particles. Surface pressure-area (Π-A) isotherms were measured while the particles rearranged at the interface during compression and expansion for different pH values and particle wettability. We also analyzed the motion of particles in the monolayer by the surface pressure and packing factor to gain further insights into the behavior of particles during the coalescence process. The results suggested that the coalescence of a bubble was dependent on the formation of a defect in the particle layer and the defect size was both strongly influenced by particle hydrophobicity and the pH of the subphase.

Sequence-dependent base pair stepping dynamics in XPD helicase unwinding.

Helicases couple the chemical energy of ATP hydrolysis to directional translocation along nucleic acids and transient duplex separation. Understanding helicase mechanism requires that the basic physicochemical process of base pair separation be understood. This necessitates monitoring helicase activity directly, at high spatio-temporal resolution. Using optical tweezers with single base pair (bp) resolution, we analyzed DNA unwinding by XPD helicase, a Superfamily 2 (SF2) DNA helicase involved in DNA repair and transcription initiation. We show that monomeric XPD unwinds duplex DNA in 1-bp steps, yet exhibits frequent backsteps and undergoes conformational transitions manifested in 5-bp backward and forward steps. Quantifying the sequence dependence of XPD stepping dynamics with near base pair resolution, we provide the strongest and most direct evidence thus far that forward, single-base pair stepping of a helicase utilizes the spontaneous opening of the duplex. The proposed unwinding mechanism may be a universal feature of DNA helicases that move along DNA phosphodiester backbones. DOI:http://dx.doi.org/10.7554/eLife.00334.001.

Regulation of translocation polarity by helicase domain 1 in SF2B helicases.

Structurally similar superfamily I (SF1) and II (SF2) helicases translocate on single-stranded DNA (ssDNA) with defined polarity either in the 5'-3' or in the 3'-5' direction. Both 5'-3' and 3'-5' translocating helicases contain the same motor core comprising two RecA-like folds. SF1 helicases of opposite polarity bind ssDNA with the same orientation, and translocate in opposite directions by employing a reverse sequence of the conformational changes within the motor domains. Here, using proteolytic DNA and mutational analysis, we have determined that SF2B helicases bind ssDNA with the same orientation as their 3'-5' counterparts. Further, 5'-3' translocation polarity requires conserved residues in HD1 and the FeS cluster containing domain. Finally, we propose the FeS cluster-containing domain also provides a wedge-like feature that is the point of duplex separation during unwinding.

Ensemble and single-molecule fluorescence-based assays to monitor DNA binding, translocation, and unwinding by iron-sulfur cluster containing helicases.

Many quantitative approaches for analysis of helicase-nucleic acid interactions require a robust and specific signal, which reports on the presence of the helicase and its position on a nucleic acid lattice. Since 2006, iron-sulfur (FeS) clusters have been found in a number of helicases. They serve as endogenous quenchers of Cy3 and Cy5 fluorescence which can be exploited to characterize FeS cluster containing helicases both in ensemble-based assays and at the single-molecule level. Synthetic oligonucleotides site-specifically labeled with either Cy3 or Cy5 can be used to create a variety of DNA substrates that can be used to characterized DNA binding, as well as helicase translocation and unwinding. Equilibrium binding affinities for ssDNA, duplex and branched DNA substrates can be determined using bulk assays. Identification of preferred cognate substrates, and the orientation and position of the helicase when bound to DNA can also be determined by taking advantage of the intrinsic quencher in the helicase. At the single-molecule level, real-time observation of the helicase translocating along DNA either towards the dye or away from the dye can be used to determine the rate of translocation by the helicase on ssDNA and its orientation when bound to DNA. The use of duplex substrates can reveal the rate of unwinding and processivity of the helicase. Finally, the FeS cluster can be used to visualize protein-protein interactions, and to examine the interplay between helicases and other DNA binding proteins on the same DNA substrate.

Single-molecule analysis reveals differential effect of ssDNA-binding proteins on DNA translocation by XPD helicase.

An encounter between a DNA-translocating enzyme and a DNA-bound protein must occur frequently in the cell, but little is known about its outcome. Here we developed a multicolor single-molecule fluorescence approach to simultaneously monitor single-stranded DNA (ssDNA) translocation by a helicase and the fate of another protein bound to the same DNA. Distance-dependent fluorescence quenching by the iron-sulfur cluster of the archaeal XPD (Rad3) helicase was used as a calibrated proximity signal. Despite the similar equilibrium DNA-binding properties, the two cognate ssDNA-binding proteins RPA1 and RPA2 differentially affected XPD translocation. RPA1 competed with XPD for ssDNA access. In contrast, RPA2 did not interfere with XPD-ssDNA binding but markedly slowed down XPD translocation. Mechanistic models of bypassing DNA-bound proteins by the Rad3 family helicases and their biological implications are discussed.

Silica nanoparticle sols. Part 3: Monitoring the state of agglomeration at the air/water interface using the Langmuir-Blodgett technique.

Langmuir-Blodgett films were prepared at the air/water interface from dispersions of hydrophilic and partially, hydrophobically modified industrially manufactured silica nanoparticles. The hydrophilic particles featured expanded, fairly easily compressible, surface pressure (pi)-area (A) isotherms with well defined collapse pressures which appeared to be caused by the formation of loosely structured agglomerates which exhibited elastic behavior at low surface pressure and inelastic behavior at high surface pressure. Lateral electrostatic interparticle interactions seemingly played an important role in this hydrophilic system. This contrasted with the hydrophobically modified particles which were more difficult to disperse in the ethanol/chloroform spreading solvent and appeared to be in the semi-agglomerated state at low surface pressures and exhibited a more difficult to compress compacted film. Both types of particulate films were shown to be sensitive to the spreading environment and changes in pH were found to increase particle agglomeration which drastically reduced the particulate area for the hydrophilic sol but less so, in the case of the moderately hydrophobically modified sol. In general, the LB technique proved to be a useful method to monitor changes in the state of aggregation of nanosized silica particles at the air/water interface. These results also appear to give some support of our ideas, presented in earlier publications in which it was suggested that the major role of the hydrophobically modified hydrophilic particles in foaming was to produce an aggregated particulate film surrounding the air/water interface which provides a physical barrier preventing coalescence of bubbles.

Ferroplasma acidarmanus RPA2 facilitates efficient unwinding of forked DNA substrates by monomers of FacXPD helicase.

The strand-separation activity that is important for many cellular DNA processing machineries is provided by DNA helicases. In order to understand the physiological properties of a helicase acting in the context of its macromolecular machinery, it is imperative to identify the proteins that interact with the enzyme and to analyze how these proteins affect its helicase activities. The archaeal Rad3 helicase XPD (xeroderma pigmentosum group D protein) from Ferroplasma acidarmanus (FacXPD) is a superfamily II 5'-->3' DNA helicase. Similar to its mammalian homolog working as an integral part of the transcription factor IIH complex, FacXPD may play an important role in nucleotide excision repair (NER) and transcription initiation. Interaction between FacXPD and other archaeal NER proteins likely modulates their respective activities. Replication protein A (RPA), a single-stranded DNA (ssDNA)-binding protein, is one of the NER proteins that functionally interact with the human transcription factor IIH complex. There are two RPA proteins in F. acidarmanus: FacRPA1, a homodimer of two monomers consisting of two oligonucleotide/oligosaccharide binding folds, and FacRPA2, a monomer containing a single oligonucleotide/oligosaccharide binding fold. In this study, we analyzed the effect of these ssDNA-binding proteins on FacXPD helicase activity. We found that FacRPA2 stimulates DNA unwinding by FacXPD helicase through a novel mechanism by providing a helix-destabilizing function. In contrast, FacRPA1 fails to stimulate helicase activity to the same extent as FacRPA2 and competes with FacXPD for binding to the ssDNA-double-stranded DNA junction. We conclude that the FacRPA2-coated fork is a preferred and likely physiological substrate that a monomer of FacXPD can unwind with a processivity sufficient for expansion of the NER or transcription bubble. We also suggest that duplex melting by a cognate ssDNA-binding protein coordinated with translocation by a helicase may represent a common strategy for duplex unwinding by the Rad3 family of helicases.

The role of particles in stabilising foams and emulsions.

The use of particles as foam and emulsion stabilising species, with or without surfactants, has received great interest in recent years. The majority of work has studied the effects of particles as stabilisers in emulsion systems, but recent successes has widened consideration into foams, where industries such as flotation and food processing have encountered the effects of particle stabilisation for many years. This review seeks to clarify studies into emulsions, highlighting new research in this area, and relate similarities and differences to foam systems. Past research has focused on defining the interaction mechanisms of stability, such as principles of attachment energies, particle-particle forces at the interface and changes to the interfilm, with a view to ascertain conditions giving optimum stability. Studied conditions include effects of particle contact angle, aggregation formations, concentration, size and interactions of other species (i.e. surfactant). Mechanisms can be complex, but overall the principle of particles creating a steric barrier to coalescence, is a straitforward basis of interaction. Much research in emulsions can be applied to foam systems, however evidence would suggest foam systems are under a number of additional constraints, and the stability 'window' for particles is smaller, in terms of size and contact angle ranges. Also, because of increased density differences and interfilm perturbations in foam systems, retardation of drainage is often as important to stability as inhibiting coalescence.

The iron-containing domain is essential in Rad3 helicases for coupling of ATP hydrolysis to DNA translocation and for targeting the helicase to the single-stranded DNA-double-stranded DNA junction.

Helicases often achieve functional specificity through utilization of unique structural features incorporated into an otherwise conserved core. The archaeal Rad3 (xeroderma pigmentosum group D protein (XPD)) helicase is a prototypical member of the Rad3 family, distinct from other related (superfamily II) SF2 enzymes because of a unique insertion containing an iron-sulfur (FeS) cluster. This insertion may represent an auxiliary domain responsible for modifying helicase activity or for conferring specificity for selected DNA repair intermediates. The importance of the FeS cluster for the fine-tuning of Rad3-DNA interactions is illustrated by several clinically relevant point mutations in the FeS domain of human Bach1 (FancJ) and XPD helicases that result in distinct disease phenotypes. Here we analyzed the substrate specificity of the Rad3 (XPD) helicase from Ferroplasma acidarmanus (FacRad3) and probed the importance of the FeS cluster for Rad3-DNA interactions. We found that the FeS cluster stabilizes secondary structure of the auxiliary domain important for coupling of single-stranded (ss) DNA-dependent ATP hydrolysis to ssDNA translocation. Additionally, we observed specific quenching of the Cy5 fluorescent dye when the FeS cluster of a bound helicase is positioned in close proximity to a Cy5 fluorophore incorporated into the DNA molecule. Taking advantage of this Cy5 quenching, we developed an equilibrium assay for analysis of the Rad3 interactions with various DNA substrates. We determined that the FeS cluster-containing domain recognizes the ssDNA-double-stranded DNA junction and positions the helicase in an orientation consistent with duplex unwinding. Although it interacts specifically with the junction, the enzyme binds tightly to ssDNA, and the single-stranded regions of the substrate are the major contributors to the energetics of FacRad3-substrate interactions.

Forces and friction between hydrophilic and hydrophobic surfaces: influence of oleate species.

The atomic force microscope has been used to investigate normal surface forces and lateral friction forces at different concentrations of sodium oleate, a frequently used fatty acid in the deinking process. The measurements have been performed using the colloidal probe technique with bead materials consisting of cellulose and silica. Cellulose was used together with a printing ink alkyd resin and mica, whereas silica was used with a hydrophobized silica wafer. The cellulose-alkyd resin system showed stronger double layer repulsion and the friction was reduced with increasing surfactant concentration. The adhesive interaction disappeared immediately on addition of sodium oleate. The normal surface forces for cellulose-mica indicated no apparent adsorption of the sodium oleate however, the friction coefficient increased on addition of sodium oleate, which we ascribe to some limited adsorption increasing the effective surface roughness. The silica-hydrophobic silica system showed a completely different surface force behavior at the different concentrations. An attractive hydrophobic interaction was evident since the surfaces jumped into adhesive contact at a longer distance than the van der Waals forces would predict. The strong adhesion was reflected in the friction forces as a nonlinear relationship between load and friction and a large friction response at zero applied load. Indirect evidence of adsorption to the hydrophilic silica surface was also observed in this case, and QCM studies were performed to confirm the adsorption of material to both surfaces.

Silica nanoparticle sols 1. Surface chemical characterization and evaluation of the foam generation (foamability).

Surface characterization and foaming studies were carried out with nine industrially manufactured, colloidal silica dispersions with particles sizes from 5-40 nm. All the silica sols produced transient foams with short decay times and the dynamic foam generation (foamability) was found to vary according to the sol type with the greatest foamability occurring for the hydrophobically modified sol and the deionized hydrophilic sol. However, it was found that improved foamability of all the sols could be achieved by changing the pH to within the region of the pH(pzc) which corresponds to the region of lowest hydrophilicity. An increase in pH (and build-up of negative charge) enhances the surface hydrophilicity and caused a decrease in foamability. In addition, for selected hydrophilic sols, it was shown that the foamability (a) increased with decrease in particle size (within the 6-40 nm range) and (b) increased with particle concentration (within the range of 1-15 wt%). Overall, it was concluded that the foamability was primary controlled by hydrophobicity (and hence by pH) and also by the particle concentration, the particle size and the degree of agglomeration.

Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators.

A series of aluminum salen-type complexes [where salen is N,N'-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymerization of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymerization. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymerization rate, whereas large ortho substituents generally slowed down the polymerization. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.

Friction force measurements relevant to de-inking by means of atomic force microscope.

In the pulping step of the de-inking process, the ink detaches from the fibers due to shear and physical chemical interaction. In order to get a better understanding of the forces involved between cellulose and ink, the atomic force microscope and the colloidal probe technique have been used in the presence of a model chemical dispersant (hexa-ethyleneglycol mono n-dodecyl ether, C12E6). A cellulose bead was used as the colloidal probe and three different lower surfaces have been used, an alkyd resin, mica and a cellulose sphere. The normal and lateral forces have been measured at a range of nonionic concentrations. It was found that the lateral sliding friction forces deceased with increasing surfactant concentration for both the alkyd resin and mica while no differences were observed for the cellulose surface. In addition, only a very small change in normal force could be detected for the alkyd surface as the concentration changed.

Surface rheology and foaming properties of sodium oleate and C12(EO)6 aqueous solutions.

The dynamic surface tension (DST) and the surface viscoelastic modulus of sodium oleate aqueous solutions at different concentrations were measured using an image analysis tensiometer based on the oscillating bubble technique. The diffusion coefficient of oleate moieties was calculated from DST measurements and the surface viscoelastic modulus using the Langmuir-Szyszkowski and the diffusion-controlled adsorption models. The viscoelastic moduli obtained from model calculations were compared with the corresponding experimental values. The diffusion coefficient of C(12)(EO)(6) in water and the parameters of the Langmuir-Szyszkowski adsorption isotherm were taken from the literature and used to calculate the surface viscoelastic modulus of its aqueous solutions at different concentrations. The foaming properties of both C(12)(EO)(6) and sodium oleate solutions, viz., the foam conductance and the water volume fraction in the foam, were measured using a commercial Foamscan device. Foaming experiments with C(12)(EO)(6) and sodium oleate solutions were carried out either under static conditions; i.e., the foam conductance and the water volume fraction were measured as a function of time after the generation of a fixed volume of foam, or under dynamic conditions; i.e., the foam conductance and the water volume fraction were measured during foam formation. The variations in the foam permeability as a function of surfactant concentration were related to the viscoelastic properties of the air/water interface and to the presence of micelles in the foam films. With foams in which the water volume fraction was higher than 0.05, the foam electrical conduction could be described using a simple parallel resistor model and their conductance measurements were related to the foam water volume fraction. The results related to water drainage under static conditions were used to interpret water drainage under dynamic conditions. Preliminary conjectures on the influence of foam permeability and water volume fraction on the yield of the flotation deinking process were drawn from these results.

Synergism and foaming properties in mixed nonionic/fatty acid soap surfactant systems.

The synergism and foaming behavior of a mixed surfactant system consisting of a nonionic surfactant (polyethoxylated alkyl ether C(n)E(m)) and a fatty acid soap (sodium oleate) were studied. The micellar interaction parameter (the beta-parameter) was determined from the cmc following the approach of Rubingh's regular solution theory. For both the C(12)E(6)/sodium oleate and the C(14)E(6)/sodium oleate mixtures, the results indicate a fairly strong attractive interaction (negative beta-values), which were in agreement with previous data reported for other nonionic/anionic surfactant systems. The characteristics of the foam produced from the surfactants were evaluated using a glass column equipped with a series of electrodes measuring the conductance of the foam, which enabled the water content of the foam to be determined. From these measurements, since the total foam volume was almost the same for all concentrations and surfactants, we compared the amount of liquid in the foam produced under dynamic foaming and the ability of the foam to entrain the liquid after the airflow was switched-off (static foam stability). The amount of liquid in the foam 100 s after the air was switched-off followed the order NaOl > C(12)E(6) > C(14)E(6). Also, the mixtures had the same foam volumes as the pure surfactants at the same concentration. However, both mixtures had higher concentrations of liquid in the foam when the mole fraction of the nonionic surfactant in the mixed surfactant system was greater than about >0.3 in the solution.

The role of the surfactant head group in the emulsification process: Single surfactant systems.

To clarify the effect of the surfactant head group on the emulsification process, dilute dodecane in water emulsions were prepared in a small flow-through cell with three surfactants which had the same hydrocarbon tail length but different head groups. The different surfactants types were (a) a nonionic, hexa(ethyleneglycol) mono n-dodecyl ether (C12E6), (b) an anionic, sodium dodecyl sulfate (SDS), and (c) a cationic, n-dodecyl pyridinium chloride (DPC), and the emulsions were prepared under the same conditions. From dynamic light scattering measurements, it was shown that the mean steady state droplet size of the emulsions (obtained after 20 min dispersion) could be related to the interfacial tension at concentrations in the region of the cmc. This result was in agreement with laminar and turbulent viscous flow theory. However, the particle size versus surface tension data for the different surfactant systems did not fall on a single line. This behavior suggested that the surfactant played a secondary role in defining the droplet size (in addition to reducing the interfacial tension) possibly through diffusion and relaxation, during deformation of the interface. In addition, it was found that the values of the equilibrium "surfactant packing densities" of the different surfactants at the oil/water interface were almost equal near the cmc, but the mean droplet size and the interfacial tension at the cmc decreased following the order DPC>SDS>C12E6 .

Palladium catalysed copolymerisation of ethene with alkylacrylates: polar comonomer built into the linear polymer chain.

Copolymerisation of ethene and alkylacrylates is catalysed by palladium modified with di(2-methoxyphenyl)phosphinobenzene-2-sulfonic acid (DOPPBS); a linear polymer is produced in which acrylate units are incorporated into the polyethylene backbone.