Ran Tao would like to thank funding from the National Institute of Standards and Technology, U.S. Department of Commerce under grant 70NANB15H112. Funding for Aaron M. Forster was provided through congressional appropriations to the National Institute of Standards and Technology.

<ol>
	<li>Macosko, C. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Rheology: principles, measurements, and applications. , (1994).</li><li>Larson, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The Structure and Rheology of Complex Fluids. , (1999).</li><li>Vermant, J., Moldenaers, P., Mewis, J., Ellis, M., Garritano, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Orthogonal+superposition+measurements+using+a+rheometer+equipped+with+a+force+rebalanced+transducer.">Orthogonal superposition measurements using a rheometer equipped with a force rebalanced transducer.</a> Review of Scientific Instruments. 68 (11), 4090-4096 (1997).</li><li>Ferry, J. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Viscoelastic Properties of Polymers. , (1980).</li><li>Yamamoto, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rate-dependent+relaxation+spectra+and+their+determination.">Rate-dependent relaxation spectra and their determination.</a> Transactions of the Society of Rheology. 15 (2), 331-344 (1971).</li><li>Simmons, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+servo-controlled+rheometer+for+measurement+of+the+dynamic+modulus+of+viscoelastic+liquids.">A servo-controlled rheometer for measurement of the dynamic modulus of viscoelastic liquids.</a> Journal of Scientific Instruments. 43 (12), 887-892 (1966).</li><li>Tanner, R. I., Williams, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+orthogonal+superposition+of+simple+shearing+and+small-strain+oscillatory+motions.">On the orthogonal superposition of simple shearing and small-strain oscillatory motions.</a> Rheologica Acta. 10 (4), 528-538 (1971).</li><li>Schoukens, G., Mewis, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nonlinear+rheological+behaviour+and+shear-dependent+structure+in+colloidal+dispersions.">Nonlinear rheological behaviour and shear-dependent structure in colloidal dispersions.</a> Journal of Rheology. 22 (4), 381-394 (1978).</li><li>Zeegers, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+sensitive+dynamic+viscometer+for+measuring+the+complex+shear+modulus+in+a+steady+shear+flow+using+the+method+of+orthogonal+superposition.">A sensitive dynamic viscometer for measuring the complex shear modulus in a steady shear flow using the method of orthogonal superposition.</a> Rheologica Acta. 34 (6), 606-621 (1995).</li><li>Mewis, J., Schoukens, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+spectroscopy+of+colloidal+dispersions.">Mechanical spectroscopy of colloidal dispersions.</a> Faraday Discussions of the Chemical Society. 65, 58-64 (1978).</li><li>Lin, N. Y. C., Ness, C., Cates, M. E., Sun, J., Cohen, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tunable+shear+thickening+in+suspensions.">Tunable shear thickening in suspensions.</a> Proceedings of the National Academy of Sciences. 113 (39), 10774-10778 (2016).</li><li>Gracia-Fern&#225;ndez, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Simultaneous+application+of+electro+and+orthogonal+superposition+rheology+on+a+starch/silicone+oil+suspension.">Simultaneous application of electro and orthogonal superposition rheology on a starch/silicone oil suspension.</a> Journal of Rheology. 60 (1), 121-127 (2015).</li><li>Sung, S. H., Kim, S., Hendricks, J., Clasen, C., Ahn, K. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Orthogonal+superposition+rheometry+of+colloidal+gels:+time-shear+rate+superposition.">Orthogonal superposition rheometry of colloidal gels: time-shear rate superposition.</a> Soft Matter. 14 (42), 8651-8659 (2018).</li><li>Colombo, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superposition+rheology+and+anisotropy+in+rheological+properties+of+sheared+colloidal+gels.">Superposition rheology and anisotropy in rheological properties of sheared colloidal gels.</a> Journal of Rheology. 61 (5), 1035-1048 (2017).</li><li>Jacob, A. R., Poulos, A. S., Kim, S., Vermant, J., Petekidis, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Convective+Cage+Release+in+Model+Colloidal+Glasses.">Convective Cage Release in Model Colloidal Glasses.</a> Physical Review Letters. 115 (21), 218301 (2015).</li><li>Jacob, A. R., Poulos, A. S., Semenov, A. N., Vermant, J., Petekidis, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow+dynamics+of+concentrated+starlike+micelles:+A+superposition+rheometry+investigation+into+relaxation+mechanisms.">Flow dynamics of concentrated starlike micelles: A superposition rheometry investigation into relaxation mechanisms.</a> Journal of Rheology. 63 (4), 641-653 (2019).</li><li>Moghimi, E., Vermant, J., Petekidis, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Orthogonal+superposition+rheometry+of+model+colloidal+glasses+with+short-ranged+attractions.">Orthogonal superposition rheometry of model colloidal glasses with short-ranged attractions.</a> Journal of Rheology. 63 (4), 533-546 (2019).</li><li>Tao, R., Forster, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=End+effect+correction+for+orthogonal+small+strain+oscillatory+shear+in+a+rotational+shear+rheometer.">End effect correction for orthogonal small strain oscillatory shear in a rotational shear rheometer.</a> Rheologica Acta. 59 (2), 95-108 (2020).</li><li>Schrag, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Deviation+of+velocity+gradient+profiles+from+the+"gap+loading"+and+"surface+loading"+limits+in+dynamic+simple+shear+experiments.">Deviation of velocity gradient profiles from the "gap loading" and "surface loading" limits in dynamic simple shear experiments.</a> Transactions of the Society of Rheology. 21 (3), 399-413 (1977).</li><li>Ewoldt, R. H., Johnston, M. T., Caretta, L. M., Spagnolie, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Complex Fluids in Biological Systems. Biological and Medical Physics, Biomedical Engineering. , 207-241 (2015).</li></ol>

The full description of the procedures used in this paper requires the identification of certain commercial products and their suppliers. The inclusion of such information should in no way be construed as indicating that such products or suppliers are endorsed by NIST or are recommended by NIST or that they are necessarily the best materials, instruments, software or suppliers for the purposes described.

In this protocol, we present a detailed experimental procedure for performing viscosity calibration measurements using Newtonian fluids for a commercial orthogonal superposition rheology technique with a double-wall concentric cylinder geometry. The calibration factors, i.e., the primary end-effect factor CL and the orthogonal end-effect factor CLo, are determined independently by conducting steady shear rate sweep and orthogonal frequency sweep tests. After obtai...

Understanding the rheological properties of complex fluids is essential to many industries for the development and manufacture of reliable and reproducible products1. These &#8220;complex fluids&#8221; include suspensions, polymeric liquids, and foams that widely exist in our everyday life, for example, in personal care products, foods, cosmetics, and household products. The rheological or flow properties (e.g., viscosity) are key quantities&#160;of interest in establishing performance metrics for end use and processability, but flow properties are interconnected with the microstructures that exist within complex fluids. One prominent characteristic of complex fluids that distinguishes them from simple liquids is that they possess diverse microstructures spanning multiple length scales2. Those microstructures can be easily affected by different flow conditions, which, in turn, result in changes in their macroscopic properties. Unlocking this structure-property loop via non-linear viscoelastic behavior of complex fluids in response to flow and deformation remains a challenging task for experimental rheologists.
Orthogonal superposition (OSP) rheology3 is a robust technique to address this measurement challenge. In this technique, a small amplitude oscillatory shear flow is superimposed orthogonally to a unidirectional primary steady-shear flow, which enables the simultaneous measurement of a viscoelastic relaxation spectrum under the imposed primary shear flow. To be more specific, the small oscillatory shear perturbation can be analyzed using theories in linear viscoelasticity4, while the non-linear flow condition is achieved by the primary steady-shear flow. As the two flow fields are orthogonal and thus not coupled, the perturbation spectra can be directly related to the variation of the microstructure under the primary non-linear flow5. This advanced measurement technique offers an opportunity to elucidate structure-property-processing relationships in complex fluids to optimize their formulation, processing, and application.
The implementation of modern OSP rheology was not the result of a sudden epiphany; rather, it is based on many decades of development of custom devices. The first custom made OSP apparatus is dated back to 1966 by Simmons6, and many efforts were made thereafter7,8,9,10. Those early custom-built devices suffer from many drawbacks such as alignment issues, the pumping flow effect (due to the axial movement of the bob to provide orthogonal oscillation), and limits to instrument sensitivity. In 1997, Vermant et al.3 modified the force rebalance transducer (FRT) on a commercial separate motor-transducer rheometer, which enabled OSP measurements for fluids with a wider viscosity range than previous devices. This modification enables the normal force rebalance transducer to function as a stress-controlled rheometer, imposing an axial oscillation in addition to a measurement of the axial force. Recently, the geometries required for OSP measurements, after the methodology by Vermant, have been released for a commercial separate motor-transducer rheometer.
Since the advent of commercial OSP rheology, there is a growing interest in applying this technique for the investigation of various complex fluids. Examples include colloidal suspensions11,12, colloidal gels13,14 and glasses15,16,17. While the availability of the commercial instrument promotes OSP research, the complicated OSP geometry requires a deeper understanding of the measurement than other routine rheological techniques. The OSP flow cell is based on a double-wall concentric cylinder (or Couette) geometry. It features an open top and open bottom design to enable fluid to flow back and forth between the annular gaps and the reservoir. Despite the optimization made to the geometry design by the manufacturer, when undergoing OSP operation the fluid experiences an inhomogeneous flow field, geometric end effects, and residual pumping flow, all of which can introduce substantial experimental error. Our previous work18 reported important end-effect correction procedures using Newtonian fluids for this technique. To obtain correct viscosity results, appropriate end-effect factors in both primary and orthogonal directions should be applied. In this protocol, we aim to present a detailed calibration methodology for the OSP rheological technique and provide recommendations for best practices to reduce measurement errors. The procedures delineated in this paper on OSP geometry setup, sample loading, and OSP test settings should be easily adoptable and translated for non-Newtonian fluids measurements. We advise that users utilize the calibration procedures described here to determine the end-effect correction factors for their applications prior to OSP measurements on any fluid classification (Newtonian or Non-Newtonian). We note that the calibration procedures for end factors have not been reported previously. The protocol provided in the present paper also describes step-by-step guide and tips on how to perform accurate rheological measurements in general and the technical resource on the understanding of &#8220;raw&#8221; data versus &#8220;measured&#8221; data, which may be overlooked by rheometer users.

<table><tbody><tr><td>Advanced Peltier System</td><td>TA Instruments</td><td>402500.901</td><td>Enviromental control device</td></tr><tr><td>ARES-G2 Rheometer</td><td>TA Instruments</td><td>401000.501</td><td>Rheometer</td></tr><tr><td>Brookfield Silicone Fluid, 12500cP</td><td>AMTEK Brookfield</td><td>12500 cps</td><td>Viscosity standard liquid</td></tr><tr><td>OSP Slotted Bob, 33 mm</td><td>TA Instruments</td><td>402796.902</td><td>Bob, upper geometry</td></tr><tr><td>OSP Slotted Double Gap Cup, 34 mm</td><td>TA Instruments</td><td>402782.901</td><td>Double wall cup, lower geometry</td></tr><tr><td>Pipette (1 &amp;ndash; 10 mL)</td><td>Eppendorf</td><td>3120000089</td><td>To load test materials</td></tr><tr><td>Pipette (100 &amp;ndash; 1,000 &amp;micro;L)</td><td>Eppendorf</td><td>3123000063</td><td>To load test materials</td></tr><tr><td>Pipette Tips (0.5 &amp;ndash; 10 mL)</td><td>Eppendorf</td><td>022492098</td><td>To load test materials</td></tr><tr><td>Pipette Tips (50 &amp;ndash; 1,000 &amp;micro;L)</td><td>Eppendorf</td><td>022491555</td><td>To load test materials</td></tr><tr><td>Spatula</td><td>VWR</td><td>82027-532</td><td>To load test materials</td></tr><tr><td>TRIOS</td><td>TA Instruments</td><td>v4.3.1.39215</td><td>Rheometer software</td></tr></tbody></table>

calibration procedures for orthogonal superposition rheology

Orthogonal superposition (OSP) rheology is an advanced rheological technique that involves superimposing a small-amplitude oscillatory shear deformation orthogonal to a primary shear flow. This technique allows the measurement of structural dynamics of complex fluids under non-linear flow conditions, which is important for the understanding and prediction of the performance of a wide range of complex fluids. The OSP rheological technique has a long history of development since the 1960s, mainly through the custom-built devices that highlighted the power of this technique. The OSP technique is now commercially available to the rheology community. Given the complicated design of the OSP geometry and the non-ideal flow field, users should understand the magnitude and sources of measurement error. This study presents calibration procedures using Newtonian fluids that includes recommendations for best practices to reduce measurement errors. Specifically, detailed information on the end-effect factor determination method, sample filling procedure, and identification of the appropriate measurement range (e.g., shear rate, frequency, etc.) are provided.

Representative results from the viscosity calibration measurements on a 12.2 Pa s silicone viscosity standard are represented in Figure 5 and Figure 6. Note that the primary end-effect factor and the orthogonal end-effect factor are both set to 1.00 for the calibration runs. Figure 5 shows the steady shear viscosity and the torque as a function of shear rate on a double y-axis plot. The silicone liquid is a Newtonian fluid; as expec...

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Calibration Procedures for Orthogonal Superposition Rheology

We present a detailed calibration protocol for a commercial orthogonal superposition rheology technique using Newtonian fluids including end-effect correction factor determination methods and recommendations for best practices to reduce experimental error.

Orthogonal superposition (OSP) rheology is an advanced rheological technique that involves superimposing a small-amplitude oscillatory shear deformation ...

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2.0K Views.  National Institute of Standards and Technology. We present a detailed calibration protocol for a commercial orthogonal superposition rheology technique using Newtonian fluids including end-effect correction factor determination methods and recommendations for best practices to reduce experimental error.

Video: Calibration Procedures for Orthogonal Superposition Rheology

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The system comprises a high-pressure rheometer which is connected to a circulation loop. The rheometer has a rotational flow-through measurement cell with two alternative geometries: coaxial cylinder and double gap. The circulation loop contains a mixer, to bring the crude oil sample into equilibrium with CO2, and a gear pump that transports the mixture from the mixer to the rheometer and recycles it back to the mixer. The CO2 and crude oil are brought to equilibrium by stirring and circulation and the rheology of the saturated mixture is measured by the rheometer. The system is used to measure the rheological properties of Zuata crude oil (and its toluene dilution) in equilibrium with CO2 at elevated pressures up to 220 bar and a temperature of 50 &#176;C. The results show that CO2 addition changes the oil rheology significantly, initially reducing the viscosity as the CO2 pressure is increased and then increasing the viscosity above a threshold pressure. The non-Newtonian response of the crude is also seen to change with the addition of CO2.

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A method for measuring the rheology of crude oil in equilibrium with carbon dioxide at reservoir conditions is presented.

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The ...

Environment

Dielectric RheoSANS &#8212; Simultaneous Interrogation of Impedance, Rheology and Small Angle Neutron Scattering of Complex Fluids

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and microstructural properties of complex fluids is presented. The instrument consists of a Couette geometry contained within a modified forced convection oven mounted on a commercial rheometer. This instrument is available for use on the small angle neutron scattering (SANS) beamlines at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). The Couette geometry is machined to be transparent to neutrons and provides for measurement of the electrical properties and microstructural properties of a sample confined between titanium cylinders while the sample undergoes arbitrary deformation. Synchronization of these measurements is enabled through the use of a customizable program that monitors and controls the execution of predetermined experimental protocols. Described here is a protocol to perform a flow sweep experiment where the shear rate is logarithmically stepped from a maximum value to a minimum value holding at each step for a specified period of time while frequency dependent dielectric measurements are made. Representative results are shown from a sample consisting of a gel composed of carbon black aggregates dispersed in propylene carbonate. As the gel undergoes steady shear, the carbon black network is mechanically deformed, which causes an initial decrease in conductivity associated with the breaking of bonds comprising the carbon black network. However, at higher shear rates, the conductivity recovers associated with the onset of shear thickening. Overall, these results demonstrate the utility of the simultaneous measurement of the rheo-electro-microstructural properties of these suspensions using the dielectric RheoSANS geometry.

Here, we present a procedure for the measurement of simultaneous impedance, rheology and neutron scattering from soft matter materials under shear flow.

A procedure for the operation of a new dielectric RheoSANS instrument capable of simultaneous interrogation of the electrical, mechanical and ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

Longitudinal Measurement of Extracellular Matrix Rigidity in 3D Tumor Models Using Particle-tracking Microrheology

The mechanical microenvironment has been shown to act as a crucial regulator of tumor growth behavior and signaling, which is itself remodeled and modified as part of a set of complex, two-way mechanosensitive interactions. While the development of biologically-relevant 3D tumor models have facilitated mechanistic studies on the impact of matrix rheology on tumor growth, the inverse problem of mapping changes in the mechanical environment induced by tumors remains challenging. Here, we describe the implementation of particle-tracking microrheology (PTM) in conjunction with 3D models of pancreatic cancer as part of a robust and viable approach for longitudinally monitoring physical changes in the tumor microenvironment, in situ. The methodology described here integrates a system of preparing in vitro 3D models embedded in a model extracellular matrix (ECM) scaffold of Type I collagen with fluorescently labeled probes uniformly distributed for position- and time-dependent microrheology measurements throughout the specimen. In vitro tumors are plated and probed in parallel conditions using multiwell imaging plates. Drawing on established methods, videos of tracer probe movements are transformed via the Generalized Stokes Einstein Relation (GSER) to report the complex frequency-dependent viscoelastic shear modulus, G*(&#969;). Because this approach is imaging-based, mechanical characterization is also mapped onto large transmitted-light spatial fields to simultaneously report qualitative changes in 3D tumor size and phenotype. Representative results showing contrasting mechanical response in sub-regions associated with localized invasion-induced matrix degradation as well as system calibration, validation data are presented. Undesirable outcomes from common experimental errors and troubleshooting of these issues are also presented. The 96-well 3D culture plating format implemented in this protocol is conducive to correlation of microrheology measurements with therapeutic screening assays or molecular imaging to gain new insights into impact of treatments or biochemical stimuli on the mechanical microenvironment.

Particle-tracking microrheology can be used to non-destructively quantify and spatially map changes in extracellular matrix mechanical properties in 3D tumor models.

The mechanical microenvironment has been shown to act as a crucial regulator of tumor growth behavior and signaling, which is itself remodeled and ...

Bioengineering

Combining Microfluidics and Microrheology to Determine Rheological Properties of Soft Matter during Repeated Phase Transitions

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement during previous phase changes and applied shear. To determine the extent of these changes, we have developed a microfluidic device that enables repeated phase transitions induced by exchange of the surrounding fluid and microrheological characterization while limiting shear on the sample. This technique is &#181;2rheology, the combination of microfluidics and microrheology. The microfluidic device is a two-layer design with symmetric inlet streams entering a sample chamber that traps the gel sample in place during fluid exchange. Suction can be applied far away from the sample chamber to pull fluids into the sample chamber. Material rheological properties are characterized using multiple particle tracking microrheology (MPT). In MPT, fluorescent probe particles are embedded into the material and the Brownian motion of the probes is recorded using video microscopy. The movement of the particles is tracked and the mean-squared displacement (MSD) is calculated. The MSD is related to macroscopic rheological properties, using the Generalized Stokes-Einstein Relation. The phase of the material is identified by comparison to the critical relaxation exponent, determined using time-cure superposition. Measurements of a fibrous colloidal gel illustrate the utility of the technique. This gel has a delicate structure that can be irreversibly changed when shear is applied. &#181;2rheology data shows that the material repeatedly equilibrates to the same rheological properties after each phase transition, indicating that phase transitions do not play a role in microstructural changes. To determine the role of shear, samples can be sheared prior to injection into our microfluidic device. &#181;2rheology is a widely applicable technique for the characterization of soft matter enabling the determination of rheological properties of delicate microstructures in a single sample during phase transitions in response to repeated changes in the surrounding environmental conditions.

We demonstrate the fabrication and use of a microfluidic device that enables multiple particle tracking microrheology measurements to study the rheological effects of repeated phase transitions on soft matter.

The microstructure of soft matter directly impacts macroscopic rheological properties and can be changed by factors including colloidal rearrangement ...

Studying Large Amplitude Oscillatory Shear Response of Soft Materials

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl sulfoxide (DMSO) and xanthan gum in water &#8212; two concentrated polymer solutions used as viscosifiers in foods, enhanced oil recovery, and soil remediation. Understanding the nonlinear rheological behavior of soft materials is important in the design and controlled manufacturing of many consumer products. It is shown how the response to LAOS of these polymer solutions can be interpreted in terms of a clear transition from linear viscoelasticity to viscoplastic deformation and back again during a period. The LAOS results are analyzed via the fully quantitative Sequence of Physical Processes (SPP) technique, using free MATLAB-based software. A detailed protocol of performing a LAOS measurement with a commercial rheometer, analyzing nonlinear stress responses with the freeware, and interpreting physical processes under LAOS is presented. It is further shown that, within the SPP framework, a LAOS response contains information regarding the linear viscoelasticity, the transient flow curves, and the critical strain responsible for the onset of nonlinearity.

We present a detailed protocol outlining how to perform nonlinear oscillatory shear rheology on soft materials, and how to run the SPP-LAOS analysis to understand the responses as a sequence of physical processes.

We investigate the sequence of physical processes exhibited during large amplitude oscillatory shearing (LAOS) of polyethylene oxide (PEO) in dimethyl ...

Macro-Rheology Characterization of Gill Raker Mucus in the Silver Carp, Hypophthalmichthys molitrix

The silver carp, Hypophthalmichthys molitrix, is an invasive planktivorous filter feeder fish that infested the natural waterways of the upper Mississippi River basin due to its highly efficient filter feeding mechanism. The characteristic organs called gill rakers (GRs), found in many such filter feeders, facilitate the efficient filtration of food particles such as phytoplankton that are of a few microns in size.
The motivation to investigate the rheology of the GR mucus stems from our desire to understand its role in aiding the filter feeding process in the silver carp. The mucus-rich fluid, in a &#8216;thick and sticky&#8217; state may facilitate the adhesion of food particulates. The permeation and transport through the GR membrane are&#160;facilitated by the action of external shear forces that induce varying shear strain rates. Therefore, mucus rheology can provide a vital clue to the tremendous outcompeting nature of the silver carp within the pool of filter feeding fish. Based on this it was posited that GR mucus may provide an adhesive function to food particles and act as a transport vehicle to assist in the filter feeding process.
The main objective of the protocol is to determine the yield stress of the mucus, attributed to the minimum shear stress required to initiate flow at which irreversible plastic deformation is first observed across a structured viscoelastic material. Accordingly, rheological properties of the GR mucus, i.e., viscosity, storage, and loss moduli, were investigated for its non-Newtonian, shear-thinning nature using a rotational rheometer. &#160;
A protocol presented here is employed to analyze the rheological properties of mucus extracted from the gill rakers of a silver carp, fished at Hart Creek location of the Missouri River. The protocol aims to develop an effective strategy for rheological testing and material characterization of mucus assumed to be a structured viscoelastic material.

Macro-Rheology Characterization of Gill Raker Mucus in the Silver Carp, Hypophthalmichthys molitrix

This protocol presents a method to perform rheology characterization of mucus that resides on gill rakers (GRs) of the silver carp. Viscoelastic characteristics of GR-mucus, obtained by measuring viscosity, storage and loss moduli, are evaluated for the apparent yield stress to understand the filter feeding mechanism in GRs.

The silver carp, Hypophthalmichthys molitrix, is an invasive planktivorous filter feeder fish that infested the natural waterways of the upper Mississippi ...

Microtensiometer for Confocal Microscopy Visualization of Dynamic Interfaces

Adsorption of surface-active molecules to fluid-fluid interfaces is ubiquitous in nature. Characterizing these interfaces requires measuring surfactant adsorption rates, evaluating equilibrium surface tensions as a function of bulk surfactant concentration, and relating how surface tension changes with changes in the interfacial area following equilibration. Simultaneous visualization of the interface using fluorescence imaging with a high-speed confocal microscope allows the direct evaluation of structure-function relationships. In the capillary pressure microtensiometer (CPM), a hemispherical air bubble is pinned at the end of the capillary in a 1 mL volume liquid reservoir. The capillary pressure across the bubble interface is controlled via a commercial microfluidic flow controller that allows for model-based pressure, bubble curvature, or bubble area control based on the Laplace equation. Compared to previous techniques such as the Langmuir trough and pendant drop, the measurement and control precision and response time are greatly enhanced; capillary pressure variations can be applied and controlled in milliseconds. The dynamic response of the bubble interface is visualized via a second optical lens as the bubble expands and contracts. The bubble contour is fit to a circular profile to determine the bubble curvature radius, R, as well as any deviations from circularity that would invalidate the results. The Laplace equation is used to determine the dynamic surface tension of the interface. Following equilibration, small pressure oscillations can be imposed by the computer-controlled microfluidic pump to oscillate the bubble radius (frequencies of 0.001-100 cycles/min) to determine the dilatational modulus The overall dimensions of the system are sufficiently small that the microtensiometer fits under the lens of a high-speed confocal microscope allowing fluorescently tagged chemical species to be quantitatively tracked with submicron lateral resolution.

This manuscript describes the design and operation of a microtensiometer/confocal microscope to do simultaneous measurements of interfacial tension and surface dilatational rheology while visualizing the interfacial morphology. This provides the real-time construction of structure-property relationships of interfaces important in technology and physiology.

Adsorption of surface-active molecules to fluid-fluid interfaces is ubiquitous in nature. Characterizing these interfaces requires measuring surfactant ...

Characterizing Multiscale Mechanical Properties of Brain Tissue Using Atomic Force Microscopy, Impact Indentation, and Rheometry

To design and engineer materials inspired by the properties of the brain, whether for mechanical simulants or for tissue regeneration studies, the brain tissue itself must be well characterized at various length and time scales. Like many biological tissues, brain tissue exhibits a complex, hierarchical structure. However, in contrast to most other tissues, brain is of very low mechanical stiffness, with Young&#39;s elastic moduli E on the order of 100s of Pa. This low stiffness can present challenges to experimental characterization of key mechanical properties. Here, we demonstrate several mechanical characterization techniques that have been adapted to measure the elastic and viscoelastic properties of hydrated, compliant biological materials such as brain tissue, at different length scales and loading rates. At the microscale, we conduct creep-compliance and force relaxation experiments using atomic force microscope-enabled indentation. At the mesoscale, we perform impact indentation experiments using a pendulum-based instrumented indenter. At the macroscale, we conduct parallel plate rheometry to quantify the frequency dependent shear elastic moduli. We also discuss the challenges and limitations associated with each method. Together these techniques enable an in-depth mechanical characterization of brain tissue that can be used to better understand the structure of brain and to engineer bio-inspired materials.

We present a set of techniques to characterize the viscoelastic mechanical properties of brain at the micro-, meso-, and macro-scales.

To design and engineer materials inspired by the properties of the brain, whether for mechanical simulants or for tissue regeneration studies, the brain ...

Neuroscience

Rapid Viscoelastic Characterization of Airway Mucus Using a Benchtop Rheometer

In muco-obstructive lung diseases (e.g., asthma, chronic obstructive pulmonary disease, cystic fibrosis) and other respiratory conditions (e.g., viral/bacterial infections), mucus biophysical properties are altered by goblet cell hypersecretion, airway dehydration, oxidative stress, and the presence of extracellular DNA. Previous studies showed that sputum viscoelasticity correlated with pulmonary function and that treatments affecting sputum rheology (e.g., mucolytics) can result in remarkable clinical benefits. In general, rheological measurements of non-Newtonian fluids employ elaborate, time-consuming approaches (e.g., parallel/cone-plate rheometers and/or microbead particle tracking) that require extensive training to perform the assay and interpret the data. This study tested the reliability, reproducibility, and sensitivity of Rheomuco, a user-friendly benchtop device that is designed to perform rapid measurements using dynamic oscillation with a shear-strain sweep to provide linear viscoelastic moduli (G&#39;, G&#34;, G*, and tan &#948;) and gel point characteristics (&#947;c&#160;and &#963;c) for clinical samples within 5 min. Device performance was validated using different concentrations of a mucus simulant, 8 MDa polyethylene oxide (PEO), and against traditional bulk rheology measurements. A clinical isolate harvested from an intubated patient with status asthmaticus (SA) was then assessed in triplicate measurements and the coefficient of variation between measurements is &#60;10%. Ex vivo use of a potent mucus reducing agent, TCEP, on SA mucus resulted in a five-fold decrease in elastic modulus and a change toward a more &#34;liquid-like&#34; behavior overall (e.g., higher tan &#948;). Together, these results demonstrate that the tested benchtop rheometer can make reliable measures of mucus viscoelasticity in clinical and research settings. In summary, the described protocol could be used to explore the effects of mucoactive drugs (e.g., rhDNase, N-acetyl cysteine) onsite to adapt treatment on a case-by-case basis, or in preclinical studies of novel compounds.

The viscoelastic properties of mucus play a critical role in mucociliary clearance. However, traditional mucus rheological techniques require complex and time-consuming approaches. This study provides a detailed protocol for the use of a benchtop rheometer that can rapidly and reliably perform viscoelastic measurements.

Calibration Procedures for Orthogonal Superposition Rheology

Summary

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Microtensiometer for Confocal Microscopy Visualization of Dynamic Interfaces

Characterizing Multiscale Mechanical Properties of Brain Tissue Using Atomic Force Microscopy, Impact Indentation, and Rheometry

Rapid Viscoelastic Characterization of Airway Mucus Using a Benchtop Rheometer