Image1_Order and density fluctuations near the boundary in sheared dense suspensions.JPEG

We introduce a novel approach to reveal ordering fluctuations in sheared dense suspensions, using line scanning in a combined rheometer and laser scanning confocal microscope. We validate the technique with a moderately dense suspension, observing modest shear-induced ordering and a nearly linear flow profile. At high concentration (ϕ = 0.55) and applied stress just below shear thickening, we report ordering fluctuations with high temporal resolution, and directly measure a decrease in order with distance from the suspension’s bottom boundary as well as a direct correlation between order and particle concentration. Higher applied stress produces shear thickening with large fluctuations in boundary stress which we find are accompanied by dramatic fluctuations in suspension flow speeds. The peak flow rates are independent of distance from the suspension boundary, indicating that they likely arise from transient jamming that creates solid-like aggregates of particles moving together, but only briefly because the high speed fluctuations are interspersed with regions flowing much more slowly, suggesting that shear thickening suspensions possess complex internal structural dynamics, even in relatively simple geometries.</p

Image2_Order and density fluctuations near the boundary in sheared dense suspensions.JPEG

We introduce a novel approach to reveal ordering fluctuations in sheared dense suspensions, using line scanning in a combined rheometer and laser scanning confocal microscope. We validate the technique with a moderately dense suspension, observing modest shear-induced ordering and a nearly linear flow profile. At high concentration (ϕ = 0.55) and applied stress just below shear thickening, we report ordering fluctuations with high temporal resolution, and directly measure a decrease in order with distance from the suspension’s bottom boundary as well as a direct correlation between order and particle concentration. Higher applied stress produces shear thickening with large fluctuations in boundary stress which we find are accompanied by dramatic fluctuations in suspension flow speeds. The peak flow rates are independent of distance from the suspension boundary, indicating that they likely arise from transient jamming that creates solid-like aggregates of particles moving together, but only briefly because the high speed fluctuations are interspersed with regions flowing much more slowly, suggesting that shear thickening suspensions possess complex internal structural dynamics, even in relatively simple geometries.</p

DataSheet1_Order and density fluctuations near the boundary in sheared dense suspensions.PDF

We introduce a novel approach to reveal ordering fluctuations in sheared dense suspensions, using line scanning in a combined rheometer and laser scanning confocal microscope. We validate the technique with a moderately dense suspension, observing modest shear-induced ordering and a nearly linear flow profile. At high concentration (ϕ = 0.55) and applied stress just below shear thickening, we report ordering fluctuations with high temporal resolution, and directly measure a decrease in order with distance from the suspension’s bottom boundary as well as a direct correlation between order and particle concentration. Higher applied stress produces shear thickening with large fluctuations in boundary stress which we find are accompanied by dramatic fluctuations in suspension flow speeds. The peak flow rates are independent of distance from the suspension boundary, indicating that they likely arise from transient jamming that creates solid-like aggregates of particles moving together, but only briefly because the high speed fluctuations are interspersed with regions flowing much more slowly, suggesting that shear thickening suspensions possess complex internal structural dynamics, even in relatively simple geometries.</p

DataSheet2_Order and density fluctuations near the boundary in sheared dense suspensions.pdf

We introduce a novel approach to reveal ordering fluctuations in sheared dense suspensions, using line scanning in a combined rheometer and laser scanning confocal microscope. We validate the technique with a moderately dense suspension, observing modest shear-induced ordering and a nearly linear flow profile. At high concentration (ϕ = 0.55) and applied stress just below shear thickening, we report ordering fluctuations with high temporal resolution, and directly measure a decrease in order with distance from the suspension’s bottom boundary as well as a direct correlation between order and particle concentration. Higher applied stress produces shear thickening with large fluctuations in boundary stress which we find are accompanied by dramatic fluctuations in suspension flow speeds. The peak flow rates are independent of distance from the suspension boundary, indicating that they likely arise from transient jamming that creates solid-like aggregates of particles moving together, but only briefly because the high speed fluctuations are interspersed with regions flowing much more slowly, suggesting that shear thickening suspensions possess complex internal structural dynamics, even in relatively simple geometries.</p

Image3_Order and density fluctuations near the boundary in sheared dense suspensions.JPEG

We introduce a novel approach to reveal ordering fluctuations in sheared dense suspensions, using line scanning in a combined rheometer and laser scanning confocal microscope. We validate the technique with a moderately dense suspension, observing modest shear-induced ordering and a nearly linear flow profile. At high concentration (ϕ = 0.55) and applied stress just below shear thickening, we report ordering fluctuations with high temporal resolution, and directly measure a decrease in order with distance from the suspension’s bottom boundary as well as a direct correlation between order and particle concentration. Higher applied stress produces shear thickening with large fluctuations in boundary stress which we find are accompanied by dramatic fluctuations in suspension flow speeds. The peak flow rates are independent of distance from the suspension boundary, indicating that they likely arise from transient jamming that creates solid-like aggregates of particles moving together, but only briefly because the high speed fluctuations are interspersed with regions flowing much more slowly, suggesting that shear thickening suspensions possess complex internal structural dynamics, even in relatively simple geometries.</p

Robust Organogels from Nitrogen-Containing Derivatives of (<i>R</i>)-12-Hydroxystearic Acid as Gelators: Comparisons with Gels from Stearic Acid Derivatives^†

Thirteen members of a new class of low molecular-mass organogelators (LMOGs), amides, and amines based on (R)-12-hydroxystearic acid (HSA; i.e., (R)-12-hydroxyoctadecanoic acid) and the properties of their gels have been investigated by a variety of structural and thermal techniques. The abilities of these LMOGs, molecules with primary and secondary amide and amine groups and the ammonium carbamate salt of 1-aminooctadecan-12-ol, to gelate a wide range of organic liquids have been ascertained. Their gelating efficiencies are compared with those of HSA and the corresponding nitrogen-containing molecules derived from stearic acid (i.e., HSA that lacks a 12-hydroxyl group). Several of the HSA-derived molecules are exceedingly efficient LMOGs, with much less than 1 wt % being necessary to gelate several organic liquids at room temperature. Generally, the self-assembled fibrillar networks of the gels consist of spherulitic objects whose dimensions depend on the protocol employed to cool the precursor sol phases. X-ray studies indicate that the LMOG molecules are packed in lamellae within the fibers that constitute the spherulites. In addition, some of the organogels exhibit unusual thixotropic properties: they recover a large part of their viscoelasticity within seconds of being destroyed by excessive strain shearing. This recovery is at least an order of magnitude faster than for any other organogel with a crystalline fibrillar network reported to date. Correlations of these LMOG structures (as well as with those that lack a hydroxyl group along the n-alkyl chain, a headgroup at its end, or both) with the properties of their gels, coupled with the unusual rheological properties of these systems, point to new directions for designing LMOGs and organogels

Confocal Rheology system and <i>Type-I</i> collagen morphology for all concentraions.

<p>(A) An illustration of a collagen network (<i>red</i>) adhered to a PAA gel substrate (<i>tan</i>) embedded with fluorescent microspheres (<i>green</i>) enclosed in a confocal-rheometer (<i>gray</i>). Microspheres embedded at the interface (<i>dotted circle</i>) are imaged and tracked for calculation of boundary stresses. (Figure not drawn to scale.) A portion of the confocal volume (<i>dotted box</i>) is shown in (B), with fluorescently-labeled collagen (concentration <i>c</i> = 0.25 mg/mL) and vectors representing the measured microsphere displacements (<i>γ</i>_<i>R</i> = 36%.) Scale bar = 25 <i>μ</i>m. (C)-(G) Confocal fluorescence images of collagen networks for concentrations <i>c</i> = {0.25, 0.50, 0.75, 1.0, 1.5}mg/mL and mesh sizes <i>ξ</i> = {12.1 ± 1.0, 6.9 ± 0.9, 4.8 ± 1.0, 3.8 ± 1.0, 2.2 ± 0.7} <i>μ</i>m. Each image is a 10-<i>μ</i>m-thick maximum-projection along the optical axis of the collagen network ≈ 20 <i>μ</i>m above the PAA gel surface. Scale bar = 25 <i>μ</i>m.</p

Reversibly Cross-Linking Amino-Polysiloxanes by Simple Triatomic Molecules. Facile Methods for Tuning Thermal, Rheological, and Adhesive Properties

The physical properties of five siloxane polymers with different types and frequencies of amino functional groups along the polymer side chains have been manipulated from flowing liquids to gels and to rubber-like materials by the simple addition or subtraction of a neutral triatomic molecule, CO2 or CS2, at room temperature. The chemical changes, formation of ammonium carbamates and ammonium dithiocarbamates, create materials whose properties are totally different from those of the parent polymers as a result of the introduction of ionic cross-links. These materials can be returned to their original forms by heating (in the case of the CO2 adducts) or to their protonated original forms by treatment with an acid (in the case of the CO2 and CS2 adducts). Heating the ammonium dithiocarbamates leads to loss of H2S and permanent (covalent) thiourea cross-links between the polymer chains. The new materials adhere strongly to other surfaces and can be swelled to several times their original volumes by different liquids. The rheological, swelling, and physical properties of the new materials have been correlated with the structures of the original polymers to provide a comprehensive picture of how changes at the nanometric length scale are translated to macroscopic changes. At least for the polysiloxanes examined here, the properties of the adducts do not correlate with the molecular weights of the original polymers, but do with the frequency of amino groups. The results demonstrate a simple, new method to cross-link polysiloxanes (and, in principle, a wide range of other polymers), transforming them into materials with totally different and potentially commercially useful properties

Stress Heterogeneities in Sheared <i>Type-I</i> Collagen Networks Revealed by Boundary Stress Microscopy

<div><p>Disordered fiber networks provide structural support to a wide range of important materials, and the combination of spatial and dynamic complexity may produce large inhomogeneities in mechanical properties, an effect that is largely unexplored experimentally. In this work, we introduce <i>Boundary Stress Microscopy</i> to quantify the non-uniform surface stresses in sheared collagen gels. We find local stresses exceeding average stresses by an order of magnitude, with variations over length scales much larger than the network mesh size. The strain stiffening behavior observed over a wide range of network mesh sizes can be parameterized by a single characteristic strain and associated stress, which describes both the strain stiffening regime and network yielding. The characteristic stress is approximately proportional to network density, but the peak boundary stress at both the characteristic strain and at yielding are remarkably insensitive to concentration.</p></div

Boundary stresses for all concentrations.

<p>(A)-(E) Boundary stresses at <math><mrow><msub><mi>γ</mi><mi>L</mi></msub><mo>/</mo><msubsup><mi>γ</mi><mn>0</mn><mi>L</mi></msubsup><mo>=</mo><mn>1</mn><mo>.</mo><mn>5</mn></mrow></math> for concentrations c = {0.25, 0.50, 0.75, 1.0, 1.5} mg/mL, respectively, with colormaps normalized by the peak stress in each case. Scale bar = 25 <i>μ</i>m.</p