![]() The deformation of the structure factor in these directions is related to the non-linear flow properties of these suspensions, such as shear thinning 45 or shear-thickening 46,47 behaviors. 43 In particular, under simple shear flow, one may define compression and extension directions 44 along which particles are pushed one against each other or pulled apart. ![]() However, under flow, the structure factor is anisotropically deformed along any direction in the ( v, ∇ v) plane. It does not allow us neither to vary the modulus of q (equal to 33 μm −1) nor to change the orientation of q relative to the flow. A wide-angle light scattering setup, which detects scattered light at a constant diffusion angle, θ = 170°, has been recently developed, 42 with q being close to the gradient of velocity. 41 Nevertheless, light detection at small angles is restricted to the study of systems that exhibit large scattering length scales. On the other hand, small-angle light scattering 35–40 instruments under flow have been developed and used to measure structure factors of colloidal suspensions under flow. Two light scattering techniques have been widely used under flow: Diffusing Wave Spectroscopy (DWS), 28,29 which allows us to study the dynamics of very turbid systems, has been used to probe flow non-linearities, 30,31 reversibility, 32,33 and spatial heterogeneities, 34 but it does not allow us to access the sample structure under flow as q-dependent scattering information is averaged out by multiple scattering. Light scattering in the visible range is thus an ideal candidate for the measure of the structure and the dynamics of soft matter systems. For instance, in plate/plate shear flows, one has access to the q ∈ ( v, ∇ ∧ v) plane 25 only, while directions of q along v and ∇ v may be approximately reached using Couette geometry. 25,26 The associated wavelength being several orders of magnitude smaller than structural length scales ( λ = 0.154 nm 22,26), small-angle scattering is used (typically between 10 −2 and 1°), which allows us to access characteristic length scales up to ≈100 nm, but orienting the scattering vector q relative to the flow field is challenging. 18 Small-Angle X-ray Scattering (SAXS) and Small-Angle Neutron Scattering (SANS) 19–21 allow us to probe the structure of samples at low q-range, and recently, coherent x-ray scattering has been developed 22–24 and used to study the dynamics of colloidal systems under shear. They lead to direct measurement of the structure factor, S( q), related to the fluctuations of the compressibility of the system. We present a theoretical treatment of light scattering under flow that takes into account the Gaussian character of the illumination and detection optical paths, in the case where the scattering volume extension is smaller than the gap of the flow cell, and compare with experimental measurements.Īs a consequence, scattering techniques are instruments of choice for probing the structure and dynamics of large ensembles of particles with high temporal resolution. We demonstrate several capabilities of our apparatus: a measurement of the evolution with shear of the first peak of the structure factor of a concentrated suspension of spherical particles, both in the compression and extension quadrants of the shear flow, and the measurement of the velocity profile in dynamic light scattering. The shear device consists in two parallel plates, and the optical setup allows us to perform light scattering measurements in any plane that contains the gradient of the velocity field direction. ![]() The apparatus is suitable for the study of the structure and the dynamics of soft materials systems with a sub-micron characteristic length scale. We develop and characterize a wide angle static and dynamic light scattering under shear setup.
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