Fingering to fracturing transition in a transient gel

Fracture processes are ubiquitous in soft materials, even in complex fluids, subjected to stresses. To investigate these processes in a simple geometry, we use a model self-assembled transient gel and study the instability patterns obtained in a radial Hele-Shaw cell when a low viscosity oil pushes the more viscous transient gel. Thanks to an analysis of the morphology of the patterns, we find a discontinuous transition between the standard Saffman-Taylor fingering instability and a fracturing instability as the oil injection rate increases. Our data suggest that the flow properties of the gel ahead of the finger tip controls the transition towards fracturing. By analyzing the displacement field of the gel in the vicinity of the fingers and cracks, we show that in the fingering regime, the oil gently pushes the gel, whereas in the fracturing regime, the crack tears apart the gel, resulting in a strong drop of the gel velocity ahead of the crack tip as compared to the tip velocity. We find a unique behavior for the whole displacement field of a gel around a crack, which is drastically different from that around a finger, and reveals the solid-like behavior of the gel at short time. Our experiments and analysis provide quantitative yet simple tools to unambiguously discriminate a finger from a crack in a visco-elastic material.Comment: to appear in Soft Matte

Elastowetting of Soft Hydrogel Spheres

When a soft hydrogel sphere is placed on a rigid hydrophilic substrate, it undergoes arrested spreading by forming an axisymmetric foot near the contact line, while conserving its global spherical shape. In contrast, liquid water (that constitutes greater than 90% of the hydrogel's volume) spreads into a thin film on the same surface. We study systematically this elastowetting of gel spheres on substrates of different surface energies, and find that their contact angle increases as the work of adhesion between the gel and the substrate decreases, as one would observe for drops of pure water - albeit being larger than in the latter case. This difference in the contact angles of gel and water appears to be due to the elastic shear stresses that develop in the gel and oppose its spreading. Indeed, by increasing the elastic modulus of the gel spheres, we find that their contact angle also increases. In addition, the length of the contact foot increases with the work of adhesion and sphere size, while it decreases when the elastic modulus of the gel is increased. We discuss those experimental results in light of a minimal analysis based on energy minimization, volume conservation, and scaling arguments

Polybutadiene Cross-Linked With Various Diols – Effect On Thermal Stability

The relationship between cross-linking and thermal stability as related to polybutadiene is the focus of current research. Cross-linked polybutadienes have been prepared using various diols as the cross-linking agent. Cross-linked polymers have been characterized by gel content, swelling ratios, infrared spectroscopy, and thermal analysis. These polymers are not highly cross-linked, as seen by gel content and swelling ratios, and cross-linking does not have a large effect on the onset temperature of the degradation. Nonetheless, extensive formation of a non-volatile residue occurs

First order isotropic - smectic-A transition in liquid crystal-aerosil gels

The short-range order which remains when the isotropic to smectic-A transition is perturbed by a gel of silica nanoparticles (aerosils) has been studied using high-resolution synchrotron x-ray diffraction. The gels have been created \textit{in situ} in decylcyanobiphenyl (10CB), which has a strongly first-order isotropic to smectic-A transition. The effects are determined by detailed analysis of the temperature and gel density dependence of the smectic structure factor. In previous studies of the continuous nematic to smectic-A transition in a variety of thermotropic liquid crystals the aerosil gel appeared to pin, at random, the phase of the smectic density modulation. For the isotropic to smectic-A transition the same gel perturbation yields different results. The smectic correlation length decreases more slowly with increasing random field variance in good quantitative agreement with the effect of a random pinning field at a transition from a uniform phase directly to a phase with one-dimensional translational order. We thus compare the influence of random fields on a \textit{freezing} transition with and without an intervening orientationally ordered phase.Comment: 8 pages, 8 figure

Real-life effectiveness of once-daily calcipotriol and betamethasone dipropionate gel vs. ointment formulations in psoriasis vulgaris: final analysis of the 52-week PRO-long study

Background: Topical therapies are the mainstay of treatment for psoriasis vulgaris. The fixed combination of calcipotriol (Cal) 50 mu g/g plus betamethasone 0.5 mg/g (as dipropionate; BD) is a first-line topical treatment and available as a gel or ointment. The use of these fixed combination products was compared in PRO-long, a long-term noninterventional study, for which interim results (4 and 12 weeks) have previously been reported. Objective: To describe and compare patients' perspectives on the fixed combination gel and ointment formulations; to include efficacy, adherence behaviour, treatment satisfaction and health-related quality of life (HRQoL) aspects during long-term real-life psoriasis management. Methods: PRO-long was a multicentre, prospective, observational, 52-week study of patients prescribed fixed combination Cal/BD gel or ointment in clinical practice. For final analysis the following were assessed at weeks 24, 36 and 52: differences in the proportion of patients with 'mild'/'very mild' disease according to patient's global assessment of disease severity, adherence behaviour, treatment satisfaction (nine-item treatment satisfaction questionnaire for medication) and HRQoL (Skindex-29). Results: Patients (n = 328) were prescribed once-daily Cal/BD gel (n = 152) or ointment (n = 176). At week 52, a higher proportion of patients reported that the severity of their psoriasis was 'mild'/'very mild' vs. baseline (gel: 60.2 vs. 47.1%; ointment: 58.8 vs. 42.4%), with greater treatment satisfaction reported in patients using gel vs. those using ointment. A higher proportion of patients found the gel 'easy' to use compared with the ointment (66.7 vs. 45.2%). Daily application of treatment took <= 5 min for 86.1% of patients using gel and 71.0% of patients using ointment. Conclusion: This real-life study has demonstrated similar effectiveness between the Cal/BD formulations. However, over a 52-week treatment period, patients reported greater treatment satisfaction with the gel, which was considered easier to use, faster to apply and overall a more convenient product

Power and limitations of electrophoretic separations in proteomics strategies

Proteomics can be defined as the large-scale analysis of proteins. Due to the complexity of biological systems, it is required to concatenate various separation techniques prior to mass spectrometry. These techniques, dealing with proteins or peptides, can rely on chromatography or electrophoresis. In this review, the electrophoretic techniques are under scrutiny. Their principles are recalled, and their applications for peptide and protein separations are presented and critically discussed. In addition, the features that are specific to gel electrophoresis and that interplay with mass spectrometry (i.e., protein detection after electrophoresis, and the process leading from a gel piece to a solution of peptides) are also discussed