![]() ![]() The aperture in the back cover for the outgoing neutron beam subtends an angle of ±40° relative to the axis of the neutron beam. There are no optical windows on the enclosure. Neither of the last two apertures exposes the direct or reflected laser beam. The only apertures in the cover are for the passage of the neutron beam (also these are normally covered by aluminum foil to preserve light-tightness), a cut-out on the left side where the fluid hoses emanate from, and a cut-out on the front wall permitting access to a connector panel. The clear disadvantage of SANS, of course, is that (at present) it can only be performed at large-scale facilities.Ī rigid black plastic enclosure (350 mm high), but with an aluminum back wall, fits over and locates on the breadboard, completely enclosing everything inside (see Fig. There is, therefore, a clear scientific benefit to performing simultaneous DLS-SANS measurements on soft matter systems arising from the synergy between the two techniques. This “contrast variation” approach is possible with light and x-rays (by varying the refractive index or electron density, respectively) but, in practical terms, is extremely difficult to execute. On the other hand, the great advantage of neutrons is that they permit one to selectively highlight, or suppress, the scattering from one or more components in a complex soft matter system by deuteration. The best laboratory SAXS or SANS instruments might achieve a time resolution of around 100 ms in an optimal system (although sub-millisecond time resolution has been demonstrated with a rare form of SANS called TISANE 22), while even synchrotron SAXS would be hard-pressed to achieve better than 100 μs. 19–21 However, compared to DLS, these techniques are only capable of studying the time-averaged structure in the system. Techniques such as (Ultra-)Small-Angle Scattering, whether performed with x-rays or neutrons, are capable of probing soft matter on comparable to somewhat complementary length scales as light scattering, typically from a few nm to hundreds of nm. These two conditions typically characterize a wide range of soft matter systems. 16–18 All this is possible because the technique studies spatial perturbations of the order of a micrometer or less (a typical scattering vector is of the order of Q = 10 −2 nm −1) but also probes time scales in the range of nanoseconds and above. ![]() 5,6 However, the autocorrelation function itself is especially useful for investigating the microscopic dynamics of soft matter systems, including colloids, 7–9 polymers, 10 microgels, 11,12 proteins, 13 and vesicles, 14 and for characterizing phenomena such as sol–gel transitions 15 or the formation of arrested states through changing concentration or waiting time (aging). This information can then be used to derive the hydrodynamic size of objects in suspension. 4 DLS measures the stochastic temporal variations in the scattered laser light, resulting in a time autocorrelation function describing the timescales of mutual diffusive motion of the scattering objects. Light scattering has long been used for the characterization of soft matter and dynamic light scattering (DLS) has demonstrated itself to be a crucial and indispensable technique to investigate the dynamics. ![]()
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