State of the art “tools” to investigate soft matter under flow Aaron P. R. Eberlea, Paul D. Butlera a NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, 20899, USA Carlos R. Lopez-Barronb, A. Kate Gurnonb, Norman J. Wagnerb b University of Delaware Department of Chemical and Biomolecular Engineering, Newark, DE, 19716, USA Lionel Porcarc c Large Scale Structure Group, Institut Laue Langevin, Grenoble, France The study of flow in soft matter is a multidisciplinary and active research area that encompasses, but is not limited to, colloids, polymers, foams, gels, liquids, and biological systems and has direct relevance and application in academia and industry alike. Scientific interest lies in both fundamental and practical applications and can be categorized as an attempt to understand: i) the fluid structural reorganization as a result of flow, and/or ii) the relation between flow and stress governing the bulk rheological properties of a system. In truth, the two are intimately related as the rheology of a complex fluid is directly linked to the hierarchy of structures or structural states that occur when a system is forced out of equilibrium. Hence, there is a clear advantage to directly correlating the rheological response of a fluid to the internal structure on all relevant length scales. Within this article we will demonstrate how the ILL has invested in the development of state-of-the-art sample environments and precision data acquisition modules focused on combining small-angle neutron scattering (SANS) with flow (flow-SANS). In addition, we will demonstrate the potential for impact by giving a brief overview of experimental results obtained for three model systems: 1) a polymeric micellar crystal, 2) a worm-like micellar solution (WLM), and 3) a concentrated colloidal suspension. Flow-SANS measurements have been performed for decades via specialized “in-house” flow cells that have a limited accessibility and functionality [Eberle and Porcar (2012)]. In an attempt to standardize these measurements and broaden the use to the general user community the ILL has recently purchased a stress-controlled rheometer from Anton Paar (Paar Physica MCR 501) with a custom-made quartz Couette geometry that allows for the simultaneous measure of the rheological material functions and scattering patterns, termed rheo-SANS. A picture of the rheometer mounted on the D22 SANS beam-line can be seen in Fig. 1a. The standard vertical Couette geometry allows for measurements in two planes of flow: radial geometry, when the incident neutron beam is perpendicular to the flow- and neutral-directions (1-3 plane), and the tangential geometry when the incident beam is perpendicular to the velocity gradient and neutral direction (2-3 plane) [Porcar et al. (2011)]. A cartoon of the vertical Couette cell and accessible shear planes can be seen in Fig. 2a. The flow-velocity gradient plane (1-2 plane), defining the third orthogonal plane in Cartesian coordinates, is not accessible in the vertical Couette geometry and is arguably the most relevant for understanding the relationship between structure/property relationships. In a recent collaborative effort through a long-term proposal between the ILL, the University of Delaware and the NIST Center for Neutron Research (Gaithersburg, MD, USA) and the Center for Neutron Science at the University of Delaware (Newark, DE, USA) a horizontal Couette sample environment that allows for SANS measurements along the 1-2 plane was

successfully designed, fabricated, and tested on a variety of systems [Helgeson et al. (2010); Liberatore et. al. (2006)]. A picture of the 1-2 plane shear cell can be seen in Fig. 1b and a schematic of the horizontal Couette geometry and scattering plane can be seen in Fig. 2b. Additionally, these new sample geometries have been coupled with time-resolved scattering methods available at the ILL on the D22 beamline to investigate time dependent microstructure development and/or rearrangement during transient shear tests such as start-up, cessation, and oscillatory shear. This advanced data acquisition system allows any transient complex fluid structural rearrangement to be studied on the time scale important to the nonlinear responses in transient rheological material properties. Polymeric micellar crystal. Flow-induced order, melting, and recrystallization of a micellar cubic phase during steady shear and large amplitude oscillatory shear (LAOS) flow were studied with Rheo-SANS and time-resolved oscillatory rheo-SANS (tOrSANS). The model system used for this study was a Pluronic block copolymer (F127) solution in ethylammonium nitrate (EAN, a room temperature ionic liquid). F127/EAN solutions form spherical micelles that, at concentrations above 22 wt%, self-assemble into cubic lattices. Under shear, they form layers with hexagonal symmetry (evidenced by scattering spots with six-fold symmetry, see Fig. 3a). At large shear rates > ~ 300 s-1 the crystalline order is lost and the system melts. With this work we studied the fundamentals of flow induced ordering and melting subject to LAOS deformations [Lopez-Barron et al., 2012]. Fig. 3b shows the Lissajous plots (stress vs. strain) along with SANS profiles at different points in the oscillatory cycle. Clearly melting (formation of isotropic SANS spectra) and recrystallization (appearance of Bragg peaks) occurs reversibly within a single LAOS cycle. The square shape of the Lissajous curve indicates shear thinning during two quarters of the cycle. The disappearance of the Bragg peaks coincides with the maximum in shear rate, indicating shear-induced melting. Finally, by performing gap-resolved SANS in the 1-2 plane of flow we found that the melting process does not occur homogeneously throughout the sample. The layers close to the (rotating) inner cylinder experience a higher stress for a given applied shear rate and thus, melt while those close the (stationary) outer cylinder remain ordered (see Figure 3c). These measurements provide important new insight on the physics of melting and recrystallization in soft colloids. Details of this work can be found in Lopez-Barron et al., 2012. Worm-like micelles. WLMs are self assembled flexible rod-like structures that are of technological interest across a broad range of applications from enhanced oil recovery to many consumer products. In addition, they are commonly used as models for understanding polymer physics. Of particular interest to commercial applications, WLMs will display a flow instability known as shear banding. Prior investigations have worked to explain aspects of shear banding but a complete physical interpretation of the connection between the rheology and local structure is lacking in the literature. Utilizing the new Paar Physica MCR501 rheometer we studied shear banding within a well-characterized solution consisting of 6% (2:1 molar ratio) cetylpyridinium chloride (CPCL)/sodium salicylate (NaSal) in 0.5M NaCl/D2O that has been previously studied by Berret, et al. (1997), Rehage and Hoffmann (1988), Rehage and Hoffmann (1991) and many others. Figure 4a shows the characteristic shear banding (stress plateau as a function of shear rate) of this WLM system undergoing steady shear and 2D SANS scattering patterns in the 1-3 plane. It is apparent that an increased alignment of the WLMs is observed with increasing rates of shear. In addition, the 1-2 shear cell was utilized to make spatially resolved microstructure measurements along the shear gradient direction this allowed us to observe changes in the alignment and orientation of the WLMs with respect to the shear direction as a function of shear rate and position across the shear gradient. These results provide

insight into the macroscopic rheological properties and their dependence on the underlying microstructure of WLMs solutions. Details of this work can be found in Gurnon et al. (2012). LAOS measurements were performed on the same sample to further characterize the connection between the bulk rheology and microstructure. Figure 4b shows the result from tOrSANS, on the CPCl WLM solution, made in the 1-3 plane of shear at the ILL. The transition in the microstructure from a highly entangled (isotropic scattering pattern) to an aligned state of the WLM (anisotropic pattern) is evidence for the microstructure origin of the stress overshoot. We observe the points of highest orientation occur at the highest shear rates achieved in the oscillation cycle whereas the isotropic scattering pattern, indicating no flow-induced structure, occurs at zero shear rate. Also, a spatially resolved investigation of these WLMs in the 1-2 plane of shear during LAOS was conducted and proved to be crucial toward understanding the coupling between the orientation of the aligned micelle and resulting bulk stress response (Figure 4c). Concentrated colloidal suspension. While many fluids display a shear thinning response to deformation characterized by a decrease in viscosity at increasing shear rates, concentrated colloidal dispersions can exhibit a shear-thickening response or increase in viscosity. Also known as shear thickening fluids (STF) these suspensions have been exploited in dynamic applications mainly for their extraordinary ability to dissipate energy under dynamic deformations [Helber et al. (1990), Fischer et al. (2006, 2007), Zhang et al. (2008)]. Here, we study the connection between shear thickening and the local nanostructure using the 1-2 plane shear cell on a model system composed of silica particles (radius = 60 nm, volume fraction, φ = 0.40) suspended in d-EG/PEG-600 (70/30% by volume). Figure 5 shows an isotropic scattering pattern at equilibrium (0 s-1) and an anisotropic pattern providing evidence of hydroclusters at high shear rates (627 s-1). These results confirm the theoretical prediction that shear thickening is caused by the formation of hydroclusters along the compression axis of flow [Kalman and Wagner, (2009)]. In addition, LAOS measurements were performed in the 1-2 plane to elucidate the structural states formed during oscillatory deformations. These recent results show the transient formation of hydroclusters during LAOS correspond to the microstructure attained at comparable states during steady shear flow. These results will be leveraged to develop predictive tools for formulation of STFs for specific applications. Details of these experiments can be found in Gurnon et al. (2012) and Eberle and Porcar (2012). The AntonPaar MCR 501 rheometer and the 1-2 plane shear cell sample environments recently commissioned at the ILL take full advantage of the high flux neutron source in order to perform unique Rheo- and flow-SANS measurements. Combined with an advance data acquisition, time resolved studies are now routine. Future efforts are directed toward adding rheometer capabilities onto the 1-2 plane of shear cell. These improvements are in progress and will make this 1-2 Rheo-SANS shear cell the first of its kind thereby keeping the Institut Laue Langevin at the leading fore front in scientific discovery within the field of soft matter. Finally, we note that these geometries are now available to the broader user community for use at the ILL and the results summarized here will be presented at the International Congress on Rheology (ICR) this August in Lisbon, Portugal in four separate presentations.

Figure 1. (a) Paar Physica rheometer and (b) 1-2 shear cell mounted on D22 SANS instrument

Figure 2. Flow cell geometries with accessible scattering planes: a) Couette cell allows scattering in the 1-3 and 2-3 planes (radial and tangential configuration, respectively). b) 1-2 shear cell allows scattering in the 1-2 plane. The 1-, 2-, and 3- directions are defined as the velocity (v), velocity gradient (Δv), and vorticity (ω) directions, respectively adapted from Helgeson et al. (2010).

Figure 3. Stress response and 1-3 flow-SANS profiles during (a) steady shear and (b) LAOS (ω = 1 rad/s and γ0 = 100(%)) of a 24 wt% Pluronic F127/EAN solution at 40 °C. The arrows indicate the direction of oscillation. (c) Gap resolved two-dimensional 1–2 flow-SANS patterns at the applied shear rates (coordinate axis) and normalized gap positions (ordinate axis) indicated.

Figure 4: Stress response and 1-3 flow-SANS profiles during (a) steady shear and (b) LAOS (ω = 0.089 rad/s and γ0 = 1000%) of a 6 wt% CPCl/NaSal aqueous solution with molar ratio [NaSal]/[CpCl]=0.5 at 25 °C. The arrows indicate the direction of oscillation. c) Gap resolved two-dimensional 1–2 flow-SANS patterns at the applied shear rates (coordinate axis) and normalized gap positions (ordinate axis) indicated.

Figure 5: 1-2 plane SANS profiles of a concentrated colloidal suspension (φ = 0.40 silica particles in d-EG/PEG-600) during steady shear at (a) 0 s-1 and (b) 627 s-1

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