Fluoro- vs hydrocarbon surfactants: Why do they differ in wetting performance?

AbstractFluorosurfactants are the most effective compounds to lower the surface tension of aqueous solutions, but their wetting properties as related to low energy hydrocarbon solids are inferior to hydrocarbon trisiloxane surfactants, although the latter demonstrate higher surface tension in aqueous solutions. To explain this inconsistency available data on the adsorption of fluorosurfactants on liquid/vapour, solid/liquid and solid/vapour interfaces are discussed in comparison to those of hydrocarbon surfactants. The low free energy of adsorption of fluorosurfactants on hydrocarbon solid/water interface should be of a substantial importance for their wetting properties

Foams built up by non-Newtonian polymeric solutions: Free drainage

A mathematical model of free drainage of foam built up by a power-law non-Newtonian liquid is developed. The theory predictions are compared with the experimental data on the drainage of foams formed using commercially available Aculyn™22 and Aculyn™33 polymeric solutions. The rheological parameters of the polymeric solutions were independently measured and used in the calculations. The deduced dimensionless equations were solved using finite element method with appropriate boundary conditions. The numerical simulations show that the decrease in the foam height and liquid content is very fast in the very beginning of the drainage; however, it reaches a steady state at longer time. The predicted values of the time evolution of the foam height and liquid content are in good agreement with the measured experimental data

Effect of surfactant on emulsification in microchannels

Drop formation in a microfluidic flow-focusing device (cross-junction) was studied in absence and presence of one of two ionic surfactants. Four different flow regimes: squeezing, dripping, jetting, and threading were observed in line with existing literature. The effect of surfactant on the transition between flow regimes was shown to depend upon the value of critical micelle concentration and correlates with dynamic surface tension. Drop length in the channel increased as the ratio of flow rate of dispersed to continuous phase, φ increased. For drops smaller than the channel width, the increase was slow, proportional to φ 0.1 , yet was much faster, proportional to φ for larger drops. In contradiction to the expected stabilisation of drops by surfactant, surfactant-laden drops larger than the channel height coalesced inside the channel at a higher rate than surfactant-free drops. It is proposed that the coalescence is caused by the electrostatic attraction due to surfactant redistribution under the high shear stresses near the wall of the channel

Experimental studies on droplet formation in a flow-focusing microchannel in the presence of surfactants

The formation of an aqueous droplet in an organic continuous phase was studied experimentally inside a flow-focusing microchannel (190 μm × 195 μm: depth × width) in the presence of surfactants. A low viscosity silicone oil (0.0046 Pa s) was used as the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. Two ionic surfactants, C₁₂TAB (50 mM) and C₁₆TAB (5 mM) were added in the aqueous phase, at concentrations above the CMC values. Four regimes of drop formation were identified, namely squeezing, dripping, jetting and threading, whose boundaries changed when the surfactants were present. The drop formation process and the velocity profiles in both phases in the squeezing and dripping regimes were studied in more detail using a two-colour Particle Image Velocimetry technique. For all solutions studied, three distinct drop formation stages were identified, expansion, necking and pinch-off. The surfactant-laden solutions produced smaller drops. Considering the dynamic interfacial tension, rather than the equilibrium one, it was possible to explain differences in the drop formation between the two surfactant systems in the expansion stage. The forces acting on the forming drops were estimated and showed that the drag force overcomes the interfacial tension force at the transition between the expansion and necking stages. During this transition, the curvature of the neck changed while its thinning rate was increased. The transition from the necking to the pinch-off stage was signified by a flow reversal at the bottom part of the drop

Genomic and Epigenomic Responses to Chronic Stress Involve miRNA-Mediated Programming

Stress represents a critical influence on motor system function and has been shown to impair movement performance. We hypothesized that stress-induced motor impairments are due to brain-specific changes in miRNA and protein-encoding gene expression. Here we show a causal link between stress-induced motor impairment and associated genetic and epigenetic responses in relevant central motor areas in a rat model. Exposure to two weeks of mild restraint stress altered the expression of 39 genes and nine miRNAs in the cerebellum. In line with persistent behavioural impairments, some changes in gene and miRNA expression were resistant to recovery from stress. Interestingly, stress up-regulated the expression of Adipoq and prolactin receptor mRNAs in the cerebellum. Stress also altered the expression of Prlr, miR-186, and miR-709 in hippocampus and prefrontal cortex. In addition, our findings demonstrate that miR-186 targets the gene Eps15. Furthermore, we found an age-dependent increase in EphrinB3 and GabaA4 receptors. These data show that even mild stress results in substantial genomic and epigenomic changes involving miRNA expression and associated gene targets in the motor system. These findings suggest a central role of miRNA-regulated gene expression in the stress response and in associated neurological function

Measurement of the cross-section ratio sigma(psi(2S))/sigma(J/psi(1S)) in deep inelastic exclusive ep scattering at HERA

The exclusive deep inelastic electroproduction of ψ(2S) and J/ψ(1S) at an ep centre-of-mass energy of 317 GeV has been studied with the ZEUS detector at HERA in the kinematic range 2<Q2<80 GeV2, 30<W<210 GeV and |t|<1 GeV2, where Q2 is the photon virtuality, W is the photon–proton centre-of-mass energy and t is the squared four-momentum transfer at the proton vertex. The data for 2<Q2<5 GeV2 were taken in the HERA I running period and correspond to an integrated luminosity of 114 pb−1. The data for 5<Q2<80 GeV2 are from both HERA I and HERA II periods and correspond to an integrated luminosity of 468 pb−1. The decay modes analysed were μ+μ− and View the MathML source for the ψ(2S) and μ+μ− for the J/ψ(1S). The cross-section ratio σψ(2S)/σJ/ψ(1S) has been measured as a function of View the MathML source  and t. The results are compared to predictions of QCD-inspired models of exclusive vector-meson production

Bulk advection and interfacial flows in the binary coalescence of surfactant-laden and surfactant-free drops

This work focuses on the study of bulk flows accompanying the coalescence of two aqueous drops, one containing surfactant and the other surfactant-free, in silicone oils of various viscosities. It is observed that the surfactant-free drop intrudes into the surfactant-laden drop in the form of a penetrating jet whose speed increases and average radius decreases with increasing outer phase viscosity. Mixing patterns within the coalescing drops are due to the force imbalance caused by capillary pressure difference and surfactant-induced Marangoni stresses. The driving force for mixing associated with the difference in interfacial tension between the drops is considerably stronger than that related to the drop size. The long timescale mixing of the drops is driven by rapid interior convection, and the subsequent, slow, diffusive process. Three-dimensional numerical simulations show excellent qualitative and quantitative agreement with the experimental results. The implications of our results to formulation strategies of complex microstructures in practical applications are also discussed