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Hard and soft proteins at fluid interfaces

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Ref Arxiv: 1810.12199
DOI: 10.1007/s10404-019-2195-0
WoS: 000457868900001
Ref. & Cit.: NASA ADS
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Secondary flows are ubiquitous in channel flows, where small velocity components perpendicular to the main velocity appear due to the complexity of the channel geometry and/or that of the flow itself such as from inertial or non-Newtonian effects, etc. We investigate here the inertialess secondary flow of viscoelastic fluids in curved microchan-nels of rectangular cross-section and constant but alternating curvature: the so-called "serpentine channel" geometry. Numerical calculations (Poole et al, 2013) have shown that in this geometry, in the absence of elastic instabilities, a steady secondary flow develops that takes the shape of two counter-rotating vortices in the plane of the channel cross-section. We present the first experimental visualization evidence and characterization of these steady secondary flows, using a complementarity of µPIV in the plane of the channel, and L. Ducloué · L. Casanellas · A. Lindner confocal visualisation of dye-stream transport in the cross-sectional plane. We show that the measured streamlines and the relative velocity magnitude of the secondary flows are in qualitative agreement with the numerical results. In addition to our techniques being broadly applicable to the character-isation of three-dimensional flow structures in microchan-nels, our results are important for understanding the onset of instability in serpentine viscoelastic flows.

Ref HAL: hal-02281709_v1
DOI: 10.1021/acs.langmuir.9b02091
WoS: 000484644000011
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Ref HAL: hal-02281697_v1
DOI: 10.1021/acs.macromol.9b00188
WoS: 000480827200043
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Ref HAL: hal-02267857_v1
DOI: 10.1039/C9SM00966C
WoS: 000477986500011
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We investigate by time-resolved synchrotron ultra-small X-ray scattering the dynamics of liquid–liquid phase-separation (LLPS) of gluten protein suspensions following a temperature quench. Samples at a fixed concentration (237 mg ml−1) but with different protein compositions are investigated. In our experimental conditions, we show that fluid viscoelastic samples depleted in polymeric glutenin phase-separate following a spinodal decomposition process. We quantitatively probe the late stage coarsening that results from a competition between thermodynamics that speeds up the coarsening rate as the quench depth increases and transport that slows down the rate. For even deeper quenches, the even higher viscoelasticity of the continuous phase leads to a “quasi” arrested phase separation. Anomalous phase-separation dynamics is by contrast measured for a gel sample rich in glutenin, due to elastic constraints. This work illustrates the role of viscoelasticity in the dynamics of LLPS in protein dispersions.

Ref HAL: hal-02188382_v1
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Natural protein sources often display a huge complexity, being composed of a blend of polypeptides of various molecular weight, pHi and charge density. Gluten, the proteins extracted from wheat flour is one of such. Gluten is widely used for its viscoelastic properties as an improver of cereal products (bread, pastry, etc.). It is composed of two classes of proteins, named gliadin and glutenin, similar in their amounts in glutamine (30%) and proline (10%). The more than 25 different polypeptides belonging to the gliadin class are hard to fractionate into individual components because of high redundancy in the primary sequences. Glutenin are in the form of polymers made from several distinct polypeptides concatenated through inter-chain disulfide bonds. Their molecular weights are evenly distributed from 100 kg/mol to 7,000 kg/mol.While it is well established that gliadin confers viscosity to gluten whereas glutenin polymers are at the origin of its elastic resistance, the interactions existing between both classes of gluten protein remain unknown. We previously showed by SLS and multi-angle DLS that gluten proteins suspended in ethanol/water (50/50, v/v), a theta solvent, includes large proteins assemblies (26,000 kg/mol, Rh 100nm) displaying an internal dynamic1. To get a better insight of the composition of those assemblies, we combined biochemical and physicochemical approaches. On the one hand, gluten proteins suspended in ethanol/water were fractionated by Asymmetrical- Flow-Field-Flow Fractionation (A4F) coupled to UV, SLS and QELS detectors. On the other hand, gluten proteins were partitioned by liquid-phase decomposition in respect with temperature. Protein composition of partitioned phases and eluting fractions recovered from A4F were characterized by size-exclusion chromatography. A specific interaction between omega-gliadin and glutenin polymers was evidenced.This result strengthens a very recent study that demonstrates a significant positive correlation between glutenin polymeric proteins, the ω‐gliadin fraction, and dough properties.

Ref HAL: hal-02180931_v1
DOI: 10.1039/C9SM00625G
WoS: 000477949700001
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Deformations of heavy elastic cylinders with their axis in the direction of earth's gravity field are investigated. The specimens, made of polyacrylamide hydrogels, are attached from their top circular cross section to a rigid plate. An equilibrium configuration results from the interplay between gravity that tends to deform the cylinders downwards under their own weight, and elasticity that resists these distortions. The corresponding steady state exhibits fascinating shapes which are measured with lab-based micro-tomography. For any given initial radius to height ratio, the deformed cylinders are no longer axially symmetric beyond a critical value of a control parameter that depends on the volume force, the height and the elastic modulus: self-similar wrinkling hierarchies develop, and dimples appear at the bottom surface of the shallowest samples. We show that these patterns are the consequences of elastic instabilities.