A stress-controlled shear cell for small-angle light scattering and microscopy

Abstract

We develop and test a stress-controlled, parallel plates shear cell that can be coupled to an optical microscope or a small angle light scattering setup, for simultaneous investigation of the rheological response and the microscopic structure of soft materials under an imposed shear stress. In order to minimize friction, the cell is based on an air bearing linear stage, the stress is applied through a contactless magnetic actuator, and the strain is measured through optical sensors. We discuss the contributions of inertia and of the small residual friction to the measured signal and demonstrate the performance of our device in both oscillating and step stress experiments on a variety of viscoelastic materials.

Computing the linear viscoelastic properties of soft gels using an optimally windowed chirp protocol

“…Their highly adaptive and tunable rheological response is of interest for novel technologies and smart material design, but distinguishing the role of different microstructural features over different lengthscales and timescales in order to fully understand and control the wide relaxation spectrum of these soft materials is extremely difficult. Recent advancements in experimental techniques have enabled accurate and efficient determination of the rheological response of soft materials across a broad range of linear and non-linear deformations [8][9][10][11][12][13][14][15] and the combination of such approaches with imaging, ultrasound velocimetry or spectroscopy provides unique opportunities to bridge the gap between the macroscopic rheological behavior of a material and its micro-and even nano-scale structure/dynamics [16][17][18][19][20][21][22][23][24][25][26] . Nevertheless, constitutive models that capture the link between the microstructure and the mechanical response are still funa) mb1853@georgetown.edu b) bavand@mit.edu; *Contributed equally c) mgeri@mit.edu d) divoux@crpp-bordeaux.cnrs.fr e) ed610@georgetown.edu f) gareth@mit.edu damentally lacking, and this limits quantitative interpretation of the rheological measurements.…”

Probing shear-induced rearrangements in Fourier space. I. Dynamic light scattering

“…A commercial linear stage (Linear Stage UMR8.25A, by Newport) coupled to a stepper motor (Newport Precision Motorized Actuator LTA-HS) with a nominal precision of 1 µm was used to impose a controlled displacement in theû x direction. The actual displacement was measured to within an accuracy of about 60 nm using the speckle imaging technique described in 40,41 .…”

Microscopic dynamics and failure precursors of a gel under mechanical load

“…Here, we address these questions by studying the microscopic dynamics of a soft solid submitted to a constant shear stress, using a unique custom-made apparatus (36,37) that couples stress-controlled rheology to small-angle static and dynamic light scattering (see the SI for details on the sample and setup geometry). We focus on a gel made of attractive colloidal particles, a model system for network-forming soft solids, which are ubiquitous in soft matter (38) and in biological materials (39).…”

“…To investigate the relationship between the sudden macroscopic failure of the gel and its microscopic evolution, we inspect static and dynamic light scattering data collected si-multaneously to the rheology measurements (see Fig. 1b (36)). Light scattering probes density fluctuations as a function of wavevector q : I(q) ∝ j,k exp[−iq · (rj − r k )], with I the scattered intensity, rj the position of the j-th particle, and q the scattering vector (see Methods).…”

“…One glass plate is fixed, the other one can slide along theê direction. The shear stress is applied by means of an electromagnetic actuator; strain is measured by an optical, contactless sensor, as described in [Aime S, et al (2016) A stress-controlled shear cell for small-angle light scattering and microscopy. Review of Scientific Instruments 87(12):123907.].…”