Hydrogels display extremely complex frictional behavior with surprisingly slippery surfaces. We measure the sliding behavior of hydrogels submerged in water using a custom-made tribotool. Samples ...with an imposed surface roughness give two distinct frictional regimes. Friction coefficients in the first regime change with asperity sizes and Young's moduli. Under increased normal force, a second frictional regime emerges likely due to smoothening of asperities. Friction coefficients in the second regime remain constant across length scales of roughness and appear to be material specific. The hydrogel polymer network also directly influences the surface topography, and with that, the frictional behavior of hydrogels. We highlight the tribological importance of surface roughness at different length scales, which provides potential to engineer functional frictional behavior.
•Lubricated hydrogel-hydrogel friction was measured with a custom-made tribotool.•Hydrogels display two normal force dependent frictional regimes.•Friction in the first regime changes with hydrogel surface roughness.•Friction in the second regime remains near constant across length scales of roughness.
Strongly confined flow of particulate fluids is encountered in applications ranging from three-dimensional (3D) printing to the spreading of foods and cosmetics into thin layers. When flowing in ...constrictions with gap sizes, w, within 102 times the mean size of particles or aggregates, d, structured fluids experience enhanced bulk velocities and inhomogeneous viscosities, as a result of so-called cooperative, or nonlocal, particle interactions. Correctly predicting cooperative flow for a wide range of complex fluids requires high-resolution flow imaging modalities applicable in situ to even optically opaque fluids. To this goal, we here developed a pressure-driven high-field magnetic resonance imaging (MRI) velocimetry platform, comprising a pressure controller connected to a capillary. Wall properties and diameter could be modified respectively as hydrophobic/hydrophilic, or within w ∼ 100–540 μm. By achieving a high spatial resolution of 9 μm, flow cooperativity length scales, ξ, down to 15 μm in Carbopol with d ∼ 2 μm could be quantified by means of established physical models with an accuracy of 13%. The same approach was adopted for a heterogeneous fat crystal dispersion (FCD) with d and ξ values up to an order of magnitude higher than those for Carbopol. We found that for strongly confined flow of Carbopol in the 100 μm capillary, ξ is independent of flow conditions. For the FCD, ξ increases with gap size and applied pressures over 0.25–1 bar. In both samples, nonlocal interactions span domains up to about 5–8 particles but, at the highest confinement degree explored, ∼8% for FCD, domains of only ∼2 particles contribute to cooperative flow. The developed flow-MRI platform is easily scalable to ultrahigh field MRI conditions for chemically resolved velocimetric measurements of, e.g., complex fluids with anisotropic particles undergoing alignment. Future potential applications of the platform encompass imaging extrusion under confinement during the 3D printing of complex dispersions or in in vitro vascular and perfusion studies.
We establish that the rheological curve of dry granular media is nonmonotonic, both in the presence and absence of external mechanical agitations. In the presence of weak vibrations, the nonmonotonic ...flow curves govern a hysteretic transition between slow but steady and fast, inertial flows. In the absence of vibrations, the nonmonotonic flow curve governs the yielding behavior of granular media. Finally, we show that nonmonotonic flow curves can be seen in at least two different flow geometries and for several granular materials.
We introduce a new Magnetic Resonance Imaging technique to study the geometry of shear zones of soft, low-frictional and hard, frictional granular materials and their mixtures. Hydrogel spheres serve ...as the soft, low-frictional material component, while mustard seeds represent rigid, frictional grains. Some of the hydrogel spheres are doped with
CuSO
4
salt to serve as tracers. A cylindrical split-bottom cell is sheared stepwise and the shear profiles are determined from the differences of tomograms after successive shear steps, using Particle Imaging Velocimetry and Particle Tracking Velocimetry. We find that the shear zone geometry differs considerably between soft grains submersed in water and the same material without the embedding fluid.
Graphic abstract
Thin liquid films in a funnel Lin, T.-S.; Dijksman, J.A.; Kondic, L.
Journal of fluid mechanics,
10/2021, Letnik:
924
Journal Article
Recenzirano
We explore flow of a completely wetting fluid in a funnel, with particular focus on contact line instabilities at the fluid front. While the flow in a funnel may be related to a number of other flow ...configurations as limiting cases, understanding its stability is complicated due to the presence of additional azimuthal curvature, as well as due to convergent flow effects imposed by the geometry. The convergent nature of the flow leads to thickening of the film, therefore influencing its stability properties. In this work, we analyse these stability properties by combining physical experiments, asymptotic modelling, self-similar type of analysis and numerical simulations. We show that an appropriate long-wave-based model, supported by the input from experiments, simulations and linear stability analysis that originates from the flow down an incline plane, provides a basic insight allowing an understanding of the development of contact line instability and emerging length scales.
•Mechanical properties of extracellular polymeric substances (EPS) were investigated.•There is an optimal calcium concentration with respect to increasing mechanical strength.•EPS gels show strain ...hardening behaviour.•EPS gels show synaeresis as a function of calcium concentration.
Bacterial alginate-like exopolymers (ALE) gels have been used in this work as a model for the extracellular polymeric matrix of biofilms. Aim was to relate the mechanical properties and strength of this matrix that make biofilms as persistent to cleaning as they are, to the complex cohesive molecular interactions involved. Mechanical properties of the gels as a function of CaCO3 concentration were investigated using dynamic and static rheology. Gels with relatively low CaCO3 concentrations, between 100 μmol and 300 μmol per g ALE, were found to exhibit similar viscoelastic behaviour as real biofilms, with elastic moduli between 50 Pa and 100 Pa and dissipation factors between 0.2 and 0.3. Increasing CaCO3 concentrations resulted in an increase of the elastic modulus up to 250 Pa, accompanied by an increase in brittleness. At a CaCO3 concentration of 1250 μmol per g ALE this trend stopped, probably due to disturbance of the continuous ALE network by precipitation of salts. Therefore, overdosing of Ca salts can be an adequate approach for the removal of biofouling. All gels exhibited permanent strain hardening under medium strain, and their mechanical properties showed dependency on their strain history. Even after application of an oscillatory strain with 200% amplitude that caused the gel structure to collapse, the gels recovered 65 to 90% of their original shear modulus, for the major part within the first 20 s. Recovery was slightly less for gels with high CaCO3 concentration. In creep tests fitted with a Burgers model with multiple Kelvin elements at least three different interactions in the ALE gels could be distinguished with characteristic retardation times in the range of 10, 100 and 1000 s. Further identification of the mechanisms underlying the gel mechanics will allow the development of targeted strategies to undermine the mechanical strength of biofouling and aid the cleaning process.
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•Recent frictional measurements with commercial and homebuilt soft-solid tribometers have led to new tribology perspectives.•Molecular-scale surface and lubricant interactions caused ...by saliva strongly influence friction coefficients.•Experiments with rough soft surfaces and complex sliding trajectories will advance the field of soft (oral) tribology.•New techniques to visualize soft surfaces in contact will provide better understanding of frictional mechanisms.
The last few decades have witnessed exciting progress in the understanding of soft material mechanics. Many of these advances have been inspired by, and have broad ramifications in the field of food science. One particular aim of food science is to get a better understanding of the physico-chemical mechanisms that are relevant in sensory perception and oral processing. It is recognized that not only rheological properties but also frictional properties are relevant in these processes. The frictional phenomena relevant for sensory perception can be understood by means of tribological measurements. The foods assessed are typically soft, hydrated and heterogeneous; measuring and understanding frictional properties of such materials is a challenge. Yet, also in the field of soft solid tribology, significant steps forward have been made, which now make it possible to do well controlled studies of even realistic food tribology scenarios. In this brief review, we provide a summary of recently developed experimental methods. We discuss challenges including the system dependence of a frictional measurement, and opportunities, such as mimicking in-mouth conditions by including human saliva and using tribo-pairs with similar properties to the oral surfaces. These advances lead to progress on the path towards a complete understanding of oral processing and sensory perception.
The introduction of dynamic covalent bonds into cross-linked polymer networks enables the development of strong and tough materials that can still be recycled or repurposed in a sustainable manner. ...To achieve the full potential of these covalent adaptable networks (CANs), it is essential to understandand controlthe underlying chemistry and physics of the dynamic covalent bonds that undergo bond exchange reactions in the network. In particular, understanding the structure of the network architecture that is assembled dynamically in a CAN is crucial, as exchange processes within this network will dictate the dynamic-mechanical material properties. In this context, the introduction of phase separation in different network hierarchies has been proposed as a useful handle to control or improve the material properties of CANs. Here we reportfor the first timehow Raman confocal microscopy can be used to visualize phase separation in imine-based CANs on the scale of several micrometers. Independently, atomic force microscopy (AFM) confirmed the phase-separated domains inside the polymer. Remarkably, the materials were found to undergo phase separation despite being built up from miscible monomers, which arguably should yield homogeneous materials. We found that the phase separation not only affected the appearance of the material butmore notablyalso had a noticeable effect on the thermal-mechanical properties of the material: CANs (of equal aliphatic/aromatic monomer composition) that displayed phase separation had both a higher crossover temperature (T cross, where tan(δ) = 1, and where the material transits from a rubbery to a viscous state) and an increased elastic modulus (G′). By modifying the CAN architecture, we were able to either suppress or enhance the phase separation, and we propose that the phase separation is driven by favorable π–π interactions between the aromatic components. Our work further shows the importance of phase separation in CANs, including in networks built from miscible components, and provides a handle to control the dynamic material properties. Moreover, our work underlines the suitability of Raman imaging as a method to visualize phase separation in CANs.
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